78 (1): 133 – 170
2020© Senckenberg Gesellschaft für Naturforschung, 2020.

Comparative analysis of worker head anatomy of Formica
and Brachyponera (Hymenoptera: Formicidae)
Adrian Richter 1, 2, *, Francisco Hita Garcia 2, Roberto A. Keller 2, 3,
Johan Billen 4, Evan P. Economo 2, # & Rolf Georg Beutel 1, #Institut für Zoologie and Evolutionsforschung, FSU Jena, 07743 Jena, Germany; Adrian Richter [adrian.richter@uni-jena.de]; Rolf Georg
Beutel [rolf.beutel@uni-jena.de] — 2 Biodiversity and Biocomplexity Unit, Okinawa Institute of Science and Technology Graduate University, Onna-son, Okinawa, 904-0495, Japan; Francisco Hita Garcia [fhitagarcia@gmail.com]; Evan P. Economo [evaneconomo@gmail.
com] — 3 Museu Nacional de Historia Natural e da Ciência and Centre for Ecology, Evolution and Environmental Changes, Universidade
de Lisboa, 1749-016 Lisbon, Portugal; Roberto A. Keller [roberto.kellerperez@gmail.com] — 4 Ecological Networks, Zoological Institute KU
Leuven, 3000 Leuven, Belgium; Johan Billen [johan.billen@kuleuven.be] — * Corresponding author; # joint supervision1

Accepted on May 8, 2020.
Published online at www.senckenberg.de/arthropod-systematics on May 26, 2020.
Editor in charge: Klaus-Dieter Klass
Abstract. An organism’s morphology plays a crucial role in its interactions with its environment. Therefore, comparative anatomical
analysis is a critical basis to understanding the ecology, behavior, and evolution. While our knowledge of ant internal anatomy has considerably improved in recent years, it is still highly fragmentary, and many evolutionary questions remain unsolved. The current work is acontribution of a series of studies with the larger goal to increase our knowledge of ant head morphology and reconstruct the evolution ofthis tagma across the ant phylogeny. We investigated the head anatomy of “generalized” ant species from phylogenetically distant clades
to establish a very solid basis for future works on the formicid head and its transformations. To achieve a multifaceted documentation, weused a broad array of techniques, including microphotography, scanning electron microscopy, µCT-scan based 3D-reconstructions, andhistological sections. This enabled us to show many anatomical features in unprecedented clarity and detail. Our results outline considerable conservation of the main structural features across the ant tree of life, but they also reveal many details that could prove phylogenetically informative and/or functionally important. The cephalic digestive tract with its sclerotization, associated musculature, and glands ismore diverse than previously reported. More work will be necessary to clarify the functional and systematic significance of the observed
differences. The cephalic endoskeleton, especially the tentorium and torular apodeme, is identified as a second structural complex of highpotential. This previously neglected character system is apparently functionally important and very likely phylogenetically informative.
Our results improve the basis for reconstructing the groundplan of the formicid head and evolutionary transformations in the stem groupand crown group. Future studies focusing on functional aspects and evolutionary changes of different elements of the head will help tocreate a complete picture of the evolution of this highly successful group of insects.
Key words. Ants, head, anatomy, skeletomusculatur system, 3D-reconstruction, µCT-scan, homology.

1.

Introduction

Formicidae is one of the most dominant groups of insects
in nearly all terrestrial ecosystems outside polar regions(e.g. Lach et al. 2010). Like other organisms, ants interact
with their environment using morphological structures
such as mouthparts or locomotor organs. Consequently,
comprehensive knowledge of structure and function of
the body parts of extant and extinct species is necessary
to understand their success and the evolution on the phenotypic level. With the ant phylogeny resolving in ever-

ISSN 1863-7221 (print)

increasing accuracy and resolution (e.g. Branstetter et
al. 2017; Borowiec et al. 2019), we have a basis to understand how the ant phenotype arose and was successively
remodeled and diversified over time.
The current study presents a comparative analysis ofhead structures among two major subfamilies, extendinga previous analysis of the cephalic morphology of workers from the subfamily Myrmicinae (Richter et al. 2019).
These, combined with forthcoming studies on other sub-| eISSN 1864-8312 (online) | DOI: 10.26049/ASP78-1-2020-06

133Richter et al.: Head anatomy of Formicidae

families, will form a basis for a broad analysis of how
the anatomy of the ant head has evolved across the phylogeny, facilitating or at least accompanying the stunning
diversification of ant morphology, ecology, and behavior.
As the head is the body region where sensory (acquisition, processing) and feeding functions are concentrated,
it plays a central role in the way insects interact with theirenvironment. Its morphology is apparently linked withthe particular ecology and behavior displayed by the organism.
The external morphology of ants has been well studied within the framework of phylogenetic and taxonomic
studies (e.g. Baroni Urbani et al. 1992; Bolton 2003;
Keller 2011). However, with the exception of specificstructures such as the glandular system (e.g Billen 1993;
Billen et al. 2013; Billen & Al-Khalifa 2015; 2016),
the richness of internal character systems remains to beevaluated in a phylogenetic context. Key aspects of internal ant morphology such as the head musculature, its
endoskeleton, and the cephalic digestive tract have neverbeen studied in detail and only documented for very few
species (e.g. Janet 1923; Lillico-Ouachour et al. 2018),
making it presently impossible to evaluate them in thecontext of the phylogeny of Formicidae, thus preventing
the development of meaningful hypotheses on their evolution. The first complete description of the head of a formicid species with modern methods was done by Richteret al. (2019) for Wasmannia affinis Santschi, 1929 (Myrmicinae), revealing many novel aspects of ant anatomy
including a modified dorsal mandibular articulation (secondary mandibular joint with respect to the groundplanof ectognathous insects), and the presence of a previously
undescribed prepharyngeal muscle originating on a torular apodeme. That study also raised questions about theevolution of structures that have only rarely been documented in the literature, such as the cephalic digestive
tract and cranial endoskeleton (see for instance Kubota etal. 2019 and Yamada et al. 2020 on the tentorium). Our
ultimate goal is thus to provide a detailed and systematic
documentation of external and internal head structures
across the whole family, incorporating most lineages andseveral representatives for larger clades to achieve a solid
assessment of the character evolution based on current
phylogenetic hypotheses.
The present work represents a further step towards thisgoal, documenting the cephalic anatomy of Formica rufa
Linnaeus, 1761 (type species of Formicidae, Formicinae),
and Brachyponera luteipes (Mayr, 1862) (Ponerinae).
Formica rufa feeds mainly on honeydew of plant sucking arthropods and additionally uses species of different
arthropod groups as prey (e.g. Domisch et al. 2009). In
contrast, Brachyponera luteipes is primarily predacious,
like most members of the subfamily Ponerinae (Schmidt& Shattuck 2014) but relies on seeds as additional foodsource (Zhou et al. 2007). We also included in our anatomical investigation the closely related Brachyponera
chinensis (Emery 1895), an invasive species in North
America from its native Asian range (Guénard & Dunn
2010), which accepts a diverse array of different food134

substrates, including sugar and lipids (Mo 2013). Bothspecies are rather “generalized” forms, thus serving as
good representatives of the two major ant clades formicoids and poneroids (informal ending -oid does not imply
superfamily rank in studies on ants), an appropriate starting point to investigate the diversity of anatomical structures in ants as a whole. We chose generalized species
to get an initial idea of the anatomical diversity acrossthe ant phylogenetic tree, and to assess which structures
may contain phylogenetic signal or may be of interest
from a functional perspective, and thus provide new insights in the evolution of ants. While comparing the observed structures and discussing potential phylogenetic
and functional implications, another aim is to provide thegroundwork for the clarification of persisting homology
problems. Well-established homology hypotheses are
crucial for understanding the evolution on the phenotypiclevel of any group of organsism. However, this aspect has
been rather neglected in ant morphology and taxonomy
in recent decades, with few exceptions, such as Keller’smorphology-based phylogeny of ants (Keller 2011), thedetailed treatment of male genitalia by Boudinot (2013),
or Richter et al. (2019) addressing the homology of sections of the cephalic digestive tract for the first time. Related to the problem of homology is the use of a consistent, ontologized terminology as the basis for comparative
morphological work, as recently discussed by Silva &Feitosa (2019). Even though progress has been made,many issues of homology and terminology persist.

2.

Material and methods

2.1. Material
Adult workers of Formica rufa, Brachyponera chinen­
sis, and Brachyponera luteipes were used in this study.
Formica rufa shows a slight worker size polymorphismthat does not affect our qualitative anatomical results. We
used workers with a head width (measured as maximum
head width in full face view) ranging from ca. 1.2 mm to
ca. 2.2 mm. Brachyponera luteipes workers are monomorphic with a representative head width of ca. 0.79 –0.81 mm (n = 3). For CT scanning specimens from thecollection of OIST were chosen. The specimens of F. rufa
with the CASENT numbers 0790267 and 0709411 wereused, and of B. luteipes the individuals with the CASENT
numbers 0709409 and 0709409. The specimens used
for histological sectioning were collected in Belgium(F. rufa) and Taiwan (B. chinensis and luteipes). For the
SEM images, specimens of F. rufa collected in Belgium
and B. luteipes collected in Japan (Okinawa) were used.

2.2. Scanning electron microscopy
Workers of F. rufa were fixed in 70% ethanol. The headof several specimens was severed and after removal ofARTHROPOD SYSTEMATICS & PHYLOGENY — 78 (1) 2020

the antennae with forceps macerated in 10% KOH overnight. Afterwards, the maxillolabial complex was either
already in an extended position, or extended by slightpressure onto the head. The mouthparts of some heads
were dissected using Dumont forceps and minute needles attached to a hobby knife handle. All samples weretransferred to 100% ethanol (70, 80, 90, 95, 100%) anddried at the critical point in liquid CO2 with an Emitech K
850 Critical Point Dryer (Sample Preparation Division,
Quorum Technologies Ltd., Ashford, England). The samples were glued on the tip of minute needles and sputter
coated with gold using an Emitech K 500 (Sample Preparation Division, Quorum Technologies Ltd., Ashford,
England). Using a rotable specimen holder (Pohl 2010),
SEM micrographs were taken with a Philips ESEM XL30
(Philips, Amsterdam, Netherlands) equipped with Scandium FIVE software (Olympus, Münster, Germany).
Workers of B. luteipes with an extended maxillolabial
complex were killed and fixed in 70% ethanol. The headof one specimen was slightly compressed with forceps
to fully inflate the glossa (see Fig. 8). The elastic cuticleof the head was not damaged in this process. The headof several specimens was severed with Dumont No. 5
forceps and the antennae removed. Some of the heads
were immersed in 10% KOH solution and macerated forsix hours at room temperature. They were then washedin water twice for five minutes and subsequently transferred to 70% ethanol. The mouthparts were removed
using Dumont forceps and minute needles attached to ahobby knife handle. All samples of B. luteipes (removed
mouthparts, entire heads, heads without mouthparts, andheads without antennae) were dehydrated in a gradedethanol series (70, 80, 90, 95, 100%) and subsequently
transferred to t-butyl (2 × for 10 minutes). In t-butyl, thesamples were put into a Hitachi ES-2030 freeze dryer(Hitachi, Chiyoda, Japan) and vacuum dried overnight.
The samples were then glued onto SEM stubs with double-sided adhesive tape, employing a dog hair glued toa glass pipette to manipulate and clean them. Samples
were sputter coated with gold (Model: VE3030CVD ofthe OIST imaging section). SEM images were taken witha JEOL JSM-7900F (JEOL Ltd, Tokyo, Japan), using the
manufacturer software. Two heads of B. luteipes without
antennae and a few dissected mouthparts of this species
were glued on minute needles and documented as described above for Formica.

2.3. Photomicrography
Before heads of F. rufa and B. luteipes were sputter
coated with gold as described above, photos were takenwith a Canon EOS 7 D Mark II equipped with a Canon
MP-E65 macro lense, in combination with an adjustable
extension bellows. The samples were illuminated by two
flashes through a transparent plastic cylinder for soft
light. Zerene Stacker (Zerene Systems LLC, Richland,
USA) was used to combine image stacks with a different
focus.

2.4. Histological section series
The heads were removed from the body with a transverse
cut behind the compound eyes, thus creating a large posterior opening for the penetration of chemicals used during tissue processing. The heads were fixed in 2% cold
glutaraldehyde in a buffer of 50 mM Na-cacodylate and
150 mM saccharose. Postfixation took place in 2% osmium tetroxide in the same buffer, and was followed bydehydration in a graded acetone series. Tissues were embedded in Araldite® and sectioned with a Leica EM UC6
ultramicrotome (Wetzlar, Germany). Transverse, longitudinal, and frontal serial semithin sections were madefor both species twith a thickness of 1 µm. A 0.1% solution of methylene blue and thionin was used for staining. Sections were viewed under an Olympus BX-51
microscope (Tokyo, Japan), equipped with an Olympus
Camedia C-3040 Zoom digital camera (Tokyo, Japan).
Pictures were taken at 10 µm intervals employing a 10 ×
objective and used for anatomical comparisons and descriptions. Some additional images of anatomical details
were taken with a 40 × objective. Selected section images were mounted as image plates as described below.

2.5. Micro-computed tomography scanningµCT-scanning was performed using a Zeiss Xradia 510
Versa 3D X-ray microscope operated with the Zeiss
Scout-and-Scan Control System software (version
11.1.6411.17883) at the Okinawa Institute of Scienceand Technology Graduate University, Japan. Specimens
fixed in 100% ethanol were immersed in a 2 M iodine
solution for four days, except for one specimen of F. rufa
(CASENT0709411) which was accidentally kept in thesolution for several months. All specimens were washedin 100% ethanol for one hour before mounting them within a sealed pipette tip of appropriate size. 1601 projection
images were taken for a full 360° rotation. Scan settings
were selected in order to yield optimum scan quality.
Specimens of B. luteipes were scanned at 40 kv and 3 W
and voxel sizes of 1,1557 µm3 (CASENT0709409) and
1,2244 µm3 (CASENT0709419) were achieved. Specimens of F. rufa were scanned with the same energy values, and voxel sizes of 2.5527 µm3 (CASENT0709411)
and 2,8337 µm3 (CASENT0790267) were achieved. 3D
reconstructions of the resulting scan projection data weredone with the Zeiss Scout-and-Scan Control System
Reconstructor (version 11.1.6411.17883) and saved in
DICOM file format.

2.6. 3D modelling and volume calculation
Scans of CASENT0790267 (F. rufa) and CASENT­0709409(B. luteipes) were segmented. As the F. rufa individual
showed an abnormal muscle in the region of the mandibular musculature (see Results section), the mandibles
and associated structures were also segmented for the135Richter et al.: Head anatomy of Formicidae

scan of CASENT0709411 (F. rufa). Segmentation was
performed in Amira 6.5 (Visage Imaging GmbH, Berlin,
Germany). Large structures (cuticle, mandibles, brain,large muscles, pharyngeal gland) were pre-segmented bymanually segmenting every 30th slice, and subsequently
semiautomatically segmented using the online applica­tion biomedisa (Lösel & Heuveline 2016) and the watershed function of Amira. The resulting segmentations
were compared to assess the anatomical accuracy. The
ones with less spilling of segmented materials into
wrong structures were selected for post-processing (biomedisa for CASENT0709409 and CASENT0709411,
watershed for CASENT0790267) and then cleaned up
and completed by adding the remaining structures with
the interpolation function of amira. Finally, segmented
materials were exported with the plugin script “multiExport” (Engelkes et al. 2018) in Amira 6.2 as Tiff image
stacks. The image series were then imported in VG-Studio 3.2.5 (Volume Graphics GmbH, Heidelberg, Germany) to create volume renders (Phong) of individual
structures. To calculate the volumina of different structures, a label analysis was run in Amira. To obtain thevolume of the head capsule, it was labelled as one material. As the antennae were not completely present on thescans, only the scape was labelled. The volumes of individual materials from the label analysis were divided by
the head volume in Microsoft Excel (2016) to assess thepercentage of the volume compared to the entire head. It
is important to note that only one specimen per species
was investigated in this way. Only selected values arereported within the manuscript, but the whole table ofvolume measurements is available in the supplementarymaterial (File 2).

2.7. Data availability
The µCT scans used in this study are available at the online repository Zenodo as DICOM files under the DOI
10.5281/zenodo.3786977. Additionally, videos of 3Dvolume renders of all the main anatomical structures areavailable there.

2.8. Image processing
Image plates were arranged in Adobe Photoshop® CS6(Adobe System Incorporated, San Jose, USA). All images were subjected to limited levels adjustment and smart
sharpen (30%). Labels for the image plates were createdin Adobe Illustrator® CS6 (Adobe Systems Incorporated, San Jose, USA).

2.9. Terminology
The terminology largely follows Richter et al. (2019),
but several adjustments were made. We recognize theincreasing demand for morphological data, which are
136

computer-parsable and suitable for automated computation (see e.g. Vogt 2019). Even though it would not
be possible to present our descriptions as “knowledge
graph” without losing a lot of detailed information, we
follow the suggestion of this author to link the used terms
to the Hymenoptera Anatomy Ontology (HAO, Yoder et
al. 2010) wherever possible. This revealed many gaps in
HAO concerning structures in ant morphology, without
an established general designation. It is also apparent thatsome of the current definitions in HAO are not perfectly
applicable to ants, for example if structures used in thedefinition are missing or the positional relationships aredifferent. However, in many cases older alternative definitions, also deposited in HAO, reflect the meaning of
the terms better when applied to Formicidae, making an
assignment of HAO terms possible. For the musculature
we do not use HAO terms, but instead follow the nomenclatures of v. Kéler (1963) and Wipfler et al. (2011),
which are commonly used in entomology, and were alsopreviously applied to ants (Richter et al. 2019) and othergroups of Hymenoptera (e.g. Zimmermann & Vilhelmsen 2016). As separate regions of the head capsule, e.g.
“frons”, “vertex” etc., are not separated by any sutures orother unambiguous markings, we refer to them instead as“frontal region”, “vertexal region” etc.
As indicated in Richter et al. (2019), M. 41a (0hy1a)(described for the first time by the authors) is reinterpreted
as M. 47 / 0hy2 (v. Kéler 1963; Wipfler et al. 2011). The
terminology of the processes/arms of the sitophore plate
has been rather inconsistent in previous publications andno label exists for them in the HAO yet. Therefore, they
will consistently be designated as “oral arms” following
Zimmermann & Vilhelmsen (2016). “Torular apodeme”
is introduced as a term to describe the “phragma of theantennal acetabulum” of Richter et al. (2019). The term
torulus is used here as encompassing the whole sclerite
forming the antennal insertion, including the acetabulum,external torular rims/ lobes as well as the internal torular
apodeme, following Keller (2011).
We introduce here the term “atala” (plural, “atalae”)(áh-tah-lah, from the Arabic
for crowbar) for theprominent process on the lateral side of the mandibularbase between the dorsal and ventral articulations of themandible. This process receives the tendon of the abductor muscle, and the distance of its tip from the axis ofmandibular rotation mechanically facilitates the openingof the mandibles, comparable with the use of a crowbar,
providing an elongated lever arm for improved force
transduction. A mandibular atala is universally present
across all ants. Previous names for this structure include
abductor apodeme (Keller 2011), abductor swelling(Richter et al. 2019), and lateral articular process of themandible (Silva & Feitosa 2019). We prefer our new
term because this structure is obviously of high functional (biomechanics of the abductor) and phylogenetic
importance (with considerable variation in aculeate Hymenoptera, Keller unpubl. data), which justifies a specific term, instead of an ambiguous (abductor apodeme)or composite one.

ARTHROPOD SYSTEMATICS & PHYLOGENY — 78 (1) 2020

3.

Results

A full description is provided for each of the documented
cephalic elements of Formica rufa, whereas the treatment of head structures of Brachyponera luteipes focuses on the differences. When structures are not mentioned
explicitly, it is implied that they are very similar to what
was described for Formica. A table summarizing structural congruities and differences between the two genera
is presented in Supplementary File 3. The structural differences between the examined Brachyponera luteipes
and B. chinensis are negligible.

3.1. Head capsule
Formica rufa
The prognathous head (HAO_0000397) (moderately declined in working posture, i.e. subprognathous in general
insect terminology; e.g. Beutel et al. 2014) of workers
is rectangular with rounded corners, appearing almost
cubic in dorsal and ventral view (Figs. 1A,C, 2A,C)(heads of smaller specimens are slightly less wide anteriorly and more rounded) (Figs. 1A, 2A); it reaches its
maximum width immediately behind the eyes (ce, Figs.
1B, 2B) (HAO_0000217); in lateral view it appears oval
with a slightly flattened ventral side. The cervical articulation between the prothorax (HAO_0000874) and themargin of the posteroventrally located occipital foramen(HAO_0000347) is narrow; the area immediately surrounding it is countersunk into a concavity of the occipital region (HAO_0000658) (Figs. 1C, 2C); the strongly
sclerotized postocciput (HAO_0000790) forms a collarlike enclosure of the foramen, which appears hourglassshaped due to the presence of mesally directed postoccipital condyles (HAO_0000654). The fissure-shaped
posterior tentorial pits (HAO_0000768) are visible at thelateral edge of the postocciput (ptp, Fig. 2C). The postgenal bridge (HAO_0000777) is extensive and each sideof it is slightly convex (pgb, Figs. 1C, 2C); the internal
postgenal ridge (HAO_0001104) is externally marked
by a thin sulcus (pgr, Figs. 1C, 2C). The large clypeus(HAO_0000212) appears roughly diamond-shaped in
dorsal view; its main middle portion is slightly raised
above the surrounding areas, together with the anterior
frontal region between the antennae (cl, Figs. 1A,B,
2A,B); its anterior margin is strongly inflected (cli, Fig.
5B) and the inflected portion is thickened. The epistomal
sulcus (es, Figs. 1A, 2A) (HAO_0000306) runs from thedorsal mandibular articulation (dma, Figs. 1B, 2B) diagonally to the anterior tentorial pits (HAO_0000126)(atp, Figs. 1A, 2A), from where it forms an arch in frontof the antennal articulations; from the posterior, almost
straight portion at the level of the scapal base, a pair of
diagonal lines run posteromesad, forming a triangularsupraclypeal area (HAO_0001821) (sca, Figs. 1A, 2A)
with different cuticular sculpture; from the top corner of
the triangle a thin frontal line is marked by a smoothersurface sculpture compared to the surrounding cuticle (fl,Figs. 1A, 2A). The frontal carinae (HAO_0001533) areshort and do not form distinct “frontal lobes” (frc, Figs.
1A,B, 2A,B); the toruli, which are thus uncovered dorsally, face dorsolaterad due to the raised frontal region
between them and have a relatively simple circular rim(HAO_0000103) (to, Figs. 1A,B, 2A,B); a small lobe
emerges from the median arch but does not cover the bulbus of the scape (tol, Fig. 3F). The compound eyes are
well-developed (565 ommatidia in the smallest investigated worker), oval and located dorsolaterally on the posterior portion of the head. Additionally, three small ocelli(HAO_0000661) (oc, Figs. 1A, 2A) are located in shallow depressions on the vertexal region (HAO_0001077).
The ventrally visible hypostoma (HAO_0000411) is
the hypostomal carina (HAO_0000413) (hysc, Figs.
1C, 2C, 11A); it has a median indentation and a lateral
inward twist (Fig. 11A); together with the main partof the hypostoma it forms the deep hypostomal cavity(HAO_0001316) (hyc, Fig. 11A), which contains thebase of the maxillolabial complex; the hypostoma forms
large triangular processes projecting into the oral foramen (hysp, Fig. 11A); the concave anterior surface of theprocesses (*, Fig. 11A) receives the lateral margin of thestipes (HAO_0000958), whereas their lateral margin encloses the mandibles (HAO_0000506) on the mesal side;a much smaller additional rounded projection (white arrowhead, Fig. 11A) is present mesad the socket (pleurostomal fossa HAO_0000732) (vma, Fig. 11A) that receives the ventral mandibular condyle (HAO_0000508);
this rounded projection fits into a depression of the closed
mandible (*, Fig. 4B). The cuticle is largely smooth, witha fine reticular microstructure and a short pubescence.

Brachyponera luteipes
The head is rectangular with rounded corners and distinctly longer than broad in dorsal view; the maximum widthis reached in the middle region; in profile it appears alsoroughly rectangular but wedge-shaped anteriorly (Figs.
1D – F, 2D – F). The occipital region appears concave
in ventral view; the occipital foramen is located further
posterior and is narrower than in Formica; the concave
area around it is more extended; a low carina delimits
it along its dorsal edge (occipital carina HAO_0000653)(oca, Figs. 1F, 2F); the carina is almost completely flattened laterad the occipital region but continues into shortdistinct diagonal portions ventrally (white arrowhead,Figs. 1F, 2F). The postgenal bridge is raised in the middle region rather than depressed (pgb, Figs. 1F, 2F); thepostgenal ridge is not externally marked by a different
cuticular structure or a sulcus. The clypeus is posteriorly
narrowed by the anteromesally shifted antennal insertions (cl, Figs. 1D,E, 2D,E); the area laterad the anteriortentorial pits is similarly shaped as in Formica, whereas
rounded concavities for the antennal insertions are present mesad of them; the posterior median clypeal portion extends between the antennal insertions as a long,thin strip (mcl, Figs. 1A, 2A); it bears a mesal carina directly between the antennal sockets; the mesal clypeal
137

Richter et al.: Head anatomy of Formicidae

Fig. 1. Photomicrographs of heads of Formica rufa (A – C) and Brachyponera luteipes (D – F). Note the slight difference in shape between asmall (A,B) and a larger head (C). A,D: Dorsal view. B,E: Lateral view. C,F: Ventral view. — Abbreviations: ce – compound eye; cl – cly­
peus; dma – dorsal mandibular articulation; es – epistomal sulcus; fr – frontal area; frc – frontal carina; fl – frontal line; ga – galea; glo –glossa; hysc – hypostomal carina; hyst – elongated tip of hypostomal carina; lbr – labrum; md – mandible; oc – ocelli; oca – occipital carina;
occ – occipital region; pgb – postgenal bridge; pgr – postgenal ridge (visible through the head capsule); plb – labial palp; pmx – maxillary
palp; pm – prementum; pocc – postocciput; sca – supraclypeal area; st – stipes; to – torulus; tol – torular lobe; vma – ventral mandibular articulation; vt – area of the vertex. — Symbols: white arrowhead – ventral carina on the postgena; black arrowhead – groove on lateral clypeus.

area including the region between the antennal sockets(which consists of the anterior frontal region in Formica)
is raised much more steeply than in Formica, which results in an almost vertical orientation of the anterior cly­
peal portion (Figs. 1E, 2E); distinct clypeal grooves arepresent directly laterad the anterior tentorial pits (blackarrowhead, Figs. 1D, 2D). The clypeal inflection (cli,Fig. 5D) is much shorter; its posterior edge bears a flatridge on the dorsal and ventral side. The epistomal sulcus
is represented by an external furrow that is deeper andnarrower in the part mesad the anterior tentorial pit (es,Figs. 1D,E, 2D,E). A supraclypeal area is presented asa small, elongated oval depression directly posterad theclypeus (sca, Figs. 1D, 2D). The frontal carinae are more
approximated due to the mesal shift of the antennal insertions, but they are also short (frc, Figs. 1D,E, 2D,E); as in
Formica, the frontal carinae do not form “frontal lobes”;
they are slightly curved and anteriorly end above the very138

large torular lobes (tol Figs. 1D,E, 2D,E, 3E), which almost completely cover the torular acetabula (ac, Fig. 3E)
in dorsal view. The lateral arch of the torolus (trl, Fig.
3E) is short and barrel-shaped, which results in a morehorizontal orientation of the antennal base. The compound eye is slightly smaller than in Formica and has
fewer ommatidia (63, n = 1); it is also slightly less ovoid
and located only a short distance behind the dorsal mandibular articulation, close to the anterior head margin.
Ocelli are missing. The hypostomal carina (hysc, Figs.
1F, 2F) is broader than in Formica and straight, withouta twist in its lateral part; its anterolateral edges are distinctly projecting above the mesal mandibular base (hye,Figs. 1F, 2F); the triangular processes are more rounded
and thicker (hysp, Fig. 11B); they are not concave anteriorly and the lateral stipital margin consequently inserts
along the edge between the process and the remaining
hypostoma (*, Fig. 11B). An additional process forming

ARTHROPOD SYSTEMATICS & PHYLOGENY — 78 (1) 2020

Fig. 2. SEM micrographs of heads of Formica rufa (A – C) and Brachyponera luteipes (D – F). Note the slight difference in shape between
a small (A,B) and a larger head (C) in Formica rufa. A,D: Dorsal view. B,E: Lateral view. C,F: Ventral view. — Abbreviations: al – atala;
atp – anterior tentorial pit; ce – compound eye; cl – clypeus; dma – dorsal mandibular articulation; es – epistomal sulcus; fr – frontal area;
frc – frontal carina; fl – frontal line; ga – galea; glo – glossa; hysc – hypostomal carina; hyst – elongated tip of hypostomal carina; lbr –labrum; md – mandible; oc – ocelli; oca – occipital carina; occ – occipital region; pgb – postgenal bridge; pgr – postgenal ridge (visible asline of smooth cuticle); plb – labial palp; pmx – maxillary palp; pm – prementum; pocc – postocciput; sca – supraclypeal area; st – stipes;
to – torulus; tol – torular lobe; vma – ventral mandibular articulation; vt – area of vertex. — Symbols: white arrowhead – ventral continuation of occipital carina; black arrowhead – groove on lateral clypeus.

the mesal arch of the pleurostomal fossa is missing (compare Fig. 11A,B). The cuticle is largely smooth but covered with small pores with minute setae inserted in them.

3.2. Endoskeleton
Formica rufa
The long, tube-like anterior tentorial arms (HAO_0001454)(ata, Figs. 5E,F, 9A,B, 11A, 13B) are oval to round incross section. They extend through the head with a diagonal orientation. The median lamellae form short broad
lobes along the middle third of the anterior arms (ml,Figs. 5E,F, 9A,B, 11A); they appear slightly sinuate insagittal view (Fig. 9B). Additional lateral lamellae arepresent between the proximal limit of the median lamellae and the anterior tentorial pits (ll, Figs. 9A,B, 13B);
they are dorsally oriented at their posterior origin andventrolaterally at their anterior end, forming a relatively even curve between these points. The posterior arms
139

Richter et al.: Head anatomy of Formicidae(HAO_0001343) are very short (pta, Figs. 5E,F, 9A,B,
11A). The tentorial bridge (HAO_0000998), locatedshortly before the occipital foramen, is slightly curved
upwards (tb, Figs. 5E,F, 9A,B, 11A); it bears a distinct
anteromedian process (tba, Fig. 11A) (HAO_0002479)
and a small anterior strengthening ridge. Dorsal tentorial arms (HAO_0000275) are present as thin outgrowths
shortly anterad the posterior end of the lamellae (dta,Figs. 5E,F, 9A,B, 11A); they are anterodorsally orientated and do not reach the head capsule; the distal end isslightly inflated. A secondary tentorial bridge is missing.
Posterior tentorial processes are present as short tubes in
the lateroventral postoccipital region (ppt, Figs. 5B,F,
9B,F). The well-developed postgenal ridge extends along
the entire ventral midline of the head; its inflated dorsaledge forms a rod-like margin (pgr, Figs. 5F, 9F); posteriorly it splits into two flat ridges, which extend towards
the posterior tentorial pits without reaching them. The
torolus bears a finger-like apodeme extending into the
cephalic lumen (toa, Figs. 5B,F, S1A) (phragma of theantennal acetabulum of Richter et al. 2019/ torular apo­deme). The epistomal ridge (HAO_0000305) is strongly
developed internally (esr, Fig. 5B). Two short ridges
originating at the posterior clypeal margin between theantennal foramina (see Fig. S1A) demarcate the triangular supraclypeal area.

Brachyponera luteipes
The posterior position of the occipital foramen and theelongate shape of the head result in a more horizontal
orientation of the tentorium (Figs. 5G,H, 9C,D, 11B,
13H). The median lamellae are present along almost theentire length of the anterior arms (ml, Figs. 9C,D, 11B); a
posteriorly deepening concavity forms the area of originof the extrinsic antennal muscles (Fig. 5H). The laterallamellae (ll, Fig. 9C,D) are much broader than in For­mica; anteriorly they end only shortly before the medianlamellae; they are dorsolaterally oriented over most of
their length, twisting only slightly dorsad posteriorly andlaterad anteriorly. The anterior arms are sharply bent dorsad at their posterior end; the vertical portion is fused tothe posterior head capsule (ata, Figs. 5H, 9D); it is connected with the tentorial bridge (tb, Figs. 5G,H, 9C,D),
which is shaped like a rounded arch and strengthened
by a ridge on its ventral side. The posterior arms (pta,Figs. 9C,D, 11B) are very short and fused with the anterior arms, and thus almost unrecognizable as individual
endoskeletal elements. The obsolete dorsal arms, minuteshort tubes originating at the posterior end of the medianlamellae, run anteriorly directly ventrad the brain (notvisible in µCT-data; Fig. S2F). Tube-like posterior processes in the postoccipital region are missing. The torular
apodeme, a long and flat cuticular extension, reaches into
the head capsule; its distal part is bent towards the anterior cephalic margin (toa, Fig. 5C). The epistomal ridge
is firmly connected with the closely approximated antennal sockets (esr, Fig. 5D); the left and right branches
diverge posterior to the antennal foramina, before flattening and converging in the sagittal plane; in one specimen
140of B. chinensis the left and right branches of the ridge
ridge fuse in front of the antennal insertions; the division of the two branches remains as a Y-shaped tip of theridge (Fig. 14E); the base of the Y-shaped ridge shortens
posterad, resulting in a new separation of the right andleft branches, which then converge towards the midline
before obliterating completely. The supraclypeal area is
not marked by internal ridges.

3.3. Antennae
Formica rufa
The geniculate 12-segmented antennae (HAO_0000101)
are inserted directly behind the clypeus; the distance
between the foramina (HAO_0001022) is only slightly shorter than their distance to the lateral marginof the head (Figs. 1A, 2A). The semisperical bulbus(HAO_0000889, radicle) (bb, Fig. 3B) of the scapus(HAO_0000908) (sc, Fig. 3B) is almost completely visible as it rests on the shallow torular acetabulum (ac, Fig.
3F); its lateral margin articulates with a stout, peg-like
antennifer (HAO_0001431). The bulbus is connected tothe distal main part of the scape by a short, straight constriction (bulbus neck, bbn, Fig. 3B), located on its posterolateral surface. The scapus is almost half as long as theentire antenna. The cylindrical pedicel (HAO_0000706)(pd, Fig. 3B) is straight except for a basal angle resulting in the geniculate antennal shape. The straight andcylindrical flagellomeres (HAO_0000342) are longer
than wide, and decrease in length towards the antennal
apex. They bear a dense vestiture of thin articulated hairsinterspersed with thicker setae (see Walther 1979 fora more detailed documentation). Short straight setae on
the bulbus including its neck and the proximal pedicellar region, presumably prorpioreceptors, are only indistinctly visible (Fig. 3B).
Musculature (Figs. 5E,F, 12): The four extrinsic muscles are of similar size (smallest ca. 0.1% and largest ca.
0.15% of the head volume). All of them insert on thintendons (the precise insertion sites are not shown in the
3D-reconstructions; the tendons were not recognizable inthe data set but visible on histological sections, see Fig.
12E). M. tentorioscapalis anterior (M. 1/ 0an1): origin(= O): dorsal surface of the anterior tentorial arm, laterallamella and posteriorly also on mesal lamella; Insertion(= I): on a tendon originating anterolaterally on the bulbus. M. tentorioscapalis posterior (M. 2/ 0an2): smallest of the four muscles, O: mesally on the mesal lamella,posterior to 0an4; I: on a tendon originating posteromesally on the bulbus. M. tentorioscapalis lateralis (M. 3/0an3): largest of the four muscles, O: anterior tentorialarm, posterior to the other three muscles, partly on theposterior ends of the lateral and mesal lamellae; I: on atendon originating laterally/ventrally (as the bulbus is
more vertically than horizontally oriented) on the bulbus.
M. tentorioscapalis medialis (M. 4/ 0an4): O: anteriortentorial arm and on the anterior mesal and lateral lamellae, anteromesad 0an1; I: on a tendon originating mesal-

ARTHROPOD SYSTEMATICS & PHYLOGENY — 78 (1) 2020

Fig. 3. SEM micrographs of the antennae and antennal sockets of Formica rufa (B,D,F) and Brachyponera luteipes (A,C,E). A – D: Dorsal
view. E,F: Lateral view. — Abbreviations: ac – torular acetabulum; bb – bulbus; bbn – bulbus neck; es – epistomal sulcus; frc – frontal
carina; pd – pedicellus; sc – scapus; tol – torular lobe; trl – lateral torular arch.

ly/dorsally on the anterior region of the bulbus. Intrinsic
muscles, one of them bipartite. M. scapopedicellaris lateralis (M. 5/ 0an6), O: laterodorsally on the scapus at
the level of the distal bundle of 0an7; I: on a short tendon
originating from the dorsolateral base of the pedicellus.
M. scapopedicellaris medialis (M. 6/ 0an7): two distinct
bundles O: mesoventrally on the distal half of the scapus;

I: on a long tendon originating from the mesoventral baseof the pedicellus.

Brachyponera luteipes
The antennal sockets are closely approximated medially,
and also closer to the anterior cephalic margin (Figs. 1D,
2D). The bulbus is sunk into the deeper acetabulum of141

Richter et al.: Head anatomy of Formicidae

the torolus and completely concealed dorsally by the medial torular lobe. The bulbus neck is shifted anteromesad
compared to Formica; it is sharply bent posterad (Fig.
3A). The flagellomeres are about as long as wide andincrease in size towards the antennal apex; the preapical flagellomere is wider than long, the apical one distinctly longer than wide. Like in Formica the vestiture
is composed of fine and larger setae (see also Siddiqui
et al. 2010); the densitiy of the latter increases towards
the apical flagellomere; irregularly dispersed pores on theantenna are likely sockets of missing setae (Fig. 3C); a
group of setae on the bulbus neck is much longer thanany of the other proprioceptors (Fig. 3A).
Musculature (Figs. 5G,H, 12): The four extrinsic muscles differ distinctly in size (smallest ca. 0.1% and largest ca. 0.28% of the head volume). All of them insert
with tendons (precise insertion sites not shown in 3Dreconstructions but visible in histological sections, seeFig. 12F). M. tentorioscapalis anterior (M. 1/ 0an1): O:
dorsal surface of the anterior tentorial arm, mesal lamella
and anteriorly on lateral lamella; I: anterolaterally on thebulbus. M. tentorioscapalis posterior (M. 2/ 0an2): O:
very long area along the deep concavity of the mesal lamella; I: posteromesally on the bulbus. M. tentorioscapalis lateralis (M. 3/ 0an3): by far the largest extrinsic
muscle, O: anterior arm and its lateral lamella, posterolaterad the other muscles; I: laterally/ventrally on thebulbus. M. tentorioscapalis medialis (M. 4/ 0an4): O:
only mesally on the anterior region of the mesal lamella,mesad 0an1; I: mesally/dorsally on the anterior region of
the bulbus. Intrinsic muscles both undivided. M. scapopedicellaris lateralis (M. 5/ 0an6), O: laterodorsally on
the distal third of the scapus; I: with two short tendonson the laterodorsal pedicellar base. M. scapopedicellaris
medialis (M. 6/ 0an7): single bundle, O: mesoventrally
on the distal third of the scapus, slightly proximad 0an6;
I: with a short tendon on the mesoventral base of thepedicellus.

3.4. Mandibles
Formica rufa
The large mandibles (HAO_0000506) are divided into a
thick basal stem and a distal blade, the latter appearing
roughly triangular in dorsal view (Fig. 4A,B). The blade
is bent mesad and torqued relative to the stem; its masticatory margin forms an angle of approximately 45° withthe main axis of mandibular movement; it bears a seriesof seven to eight teeth, a large apical one followed by aslightly smaller preapical tooth, then much smaller onesof about equal size; the basal margin is straight and forms
an angle of almost 90° with the masticatory margin; thebasal margin ends dorsally on the mesal side of the mandibular base; short carinae extend from the ventral/external mandibular margin to the apical and preapical tooth,respectively (mdc, Fig. 4B). An indistinct carina, subtended proximally by a row of sensilla, is present along
the row of teeth following the preapical one (Fig. 4B).
142

The mandibular base forms the articulatory surfacesof the primary (ventral) and secondary (dorsal) joint;
dorsally it bears the membranous mandalus (ma, Fig.
4A), which ends in the fissure-shaped opening of themandibular gland (mgo Fig. 4A); a minute cuticular process originating on the proximomesal edge of the mandalus extends above the mandibular gland duct; a flat,
approximately crescent-shaped groove is present distad
the mandalus (white arrowhead, Fig. 4A). The extensive, dorsoventrally elongated articulatory surface of thedorsal (secondary) mandibular joint (mandibular acetabulum, HAO_0001391) (dma, Fig. 4A) articulates with
an elongated laterodistal clypeal condyle (dma, Figs.
1A, 2A, 11A); the surface of both elements of the joint
is smooth. The ventral basal margin forms the condyleof the primary mandibular joint (HAO_0000508) (vma,Fig. 4B), which is in the form of a stout knob fitting into
the small pleurostomal fossa (HAO_0000732) (vma,Fig. 11A); a distinct smooth depression mesad/ dorsad
the condyle (*, Fig. 4B) fits with the small rounded projection of the hypostoma (see “head capsule”, white arrowhead, Fig. 11A). A flat depression with a wrinkled
surface (mmg, Fig. 4B) is present on the mesal mandibular surface; the larger triangular hypostomal processcontacts the ventral part of the mandibular base at theproximal margin of this depression. The atala, a process
on the lateral side of the mandibular base of similar sizeas the ventral condyle, is located between the dorsal andventral articulations (al, Fig. 4A); it fits into a lateralconcavity of the oral foramen (ala, Fig. 11A); its distal
surface displays a scale-like pattern, whereas the proximal surface contacting the head capsule is very smooth;the tendon of M. craniomandibularis externus inserts
internally on the atala.
Musculature (Fig. 5A,B): M. craniomandibularis internus (M. 11/ 0md1): largest cephalic muscle, O: entire
posterior cephalic area and most of the lateral internalsurface, also on extensive dorsal and ventral regions;most of the attachment areas are connected, but a large
ventral region is separated from extensive lateral anddorsal attachment sites enclosing the eye; a smaller, dorsomesal occipital bundle is also isolated from the rest;
I: most fibers insert on thin cuticular fibrillae of the adductor tendon, the main part of which is a sclerotized,
massive, rectangular plate (mda, Fig. 5A,B); the plateis connected to the mesal mandibular base with a lessstrongly sclerotized broad ligament; posteriorly, the sclerotized plate divides into two extensive sheet-like mainbranches (mdalb, mdacb, Fig. 5B) and a thin and shortmesal branch (mdamb, Fig. 5B); the proximal part of thelateral edge of the main central branch is bent upwards
at about 90°, partly connecting the two main branches;a dorsal side branch originating from the central branch(mdaab, Fig. 5B) receives the fibers of the dorsomesal
isolated bundle; two fiber bundles connect directly to themain body of the apodeme and the mesal branch, one on
the lateral side and one mesally. M. craniomandibularis
externus (M. 12/ 0md3): distinctly smaller than 0md1
and appearing flattened in lateral view, O: ventrally on

ARTHROPOD SYSTEMATICS & PHYLOGENY — 78 (1) 2020

Fig. 4. SEM micrographs of the mandibles of Formica rufa (A,B) and Brachyponera luteipes (C – F). A,C: Dorsal view. B,D: Ventral view.
E,F: Dorsolateral view. — Abbreviations: al – atala; at – apical tooth; bm – basal margin; dma – dorsal mandibular articulation; ma – mandalus; mdp – mandibular pit; mdc – mandibular carinae; mgo – mandibular gland opening; mmg – mesal mandibular groove. — Symbols:
white arrowhead – groove distad mandalus; * – groove mesodorsad ventral mandibular articulation; black arrows – minute tubercles; whitearrows – minute pores; white outlines – area of mandalus.

the anterior head capsule, reaching about half-way between the hypostoma and the occipital foramen, and on
the anterior postgenal ridge. I: thin, short tendon originating from the atala. M. hypopharyngo-mandibularis(M. 13) / M. tentorio-mandibularis medialis inferior(0md8): extremely thin, two fibers, O: ventrolaterally
from the lateral lamella of the anterior tentorial arm next
to the insertion of 0mx3; I: dorsomesally on the proximal
inner surface of the mandible. M. X (Fig. S1C): an aberrant muscle only present in one side of the head of one of
the scanned Formica individuals connects the dorsal andventral walls of the posterior head capsule and displaces
some of the fibers of 0md1 in that region (Fig. S1).

Brachyponera luteipes
The mandibles are longer and appear less curved thanin Formica in dorsal view (Fig. 4C). The blade is longer and less strongly bent mesad, although the masticatory margin curves further apicomesad; the masticatory
margin is slightly closer to a horizontal orientation andforms an angle of ca. 30° with the mandibular axis ofrotation. Nine teeth (“incisivi”) are present, with an alternating pattern of larger and smaller ones over the entire

length of the edge following the mandibular apex; thebasal margin ends further laterally, at about mid-lengthof the dorsal side of the basal mandibular stem (Fig. 4C).
The basal margin is in contact with a ventral projectionof the internal clypeal edge (clf, Fig. 11B) over its entire
length. A carina running from the basal tooth towards thepreapical tooth is missing, whereas one extending towards the apical one is present (mdc, Fig. 4D). The veryfine curved fissure of the mandalus extends around thedorsal articulatory surface (mgo, Fig. 4C). A shelf-likesurface laterad the mandalus bears few small setae. The
mandibular acetabulum is deeper than in Formica (dma,Fig. 5C). The ventral (primary) mandibular condyle appears more slender than in Formica (vma, Fig. 5D); thedepression mesad/dorsad the condyle (*, Fig. 4D) is lessconcave. The surface of the flat depression on the mesal mandibular side is smooth, entirely lacking wrinkles(mmg, Fig. 4D); the surface of the atala is also smooth.
A relatively flat pit on the proximolateral mandibular
region displays small tubercles and minute pores on its
surface (mdp, Fig. 4E,F).
Musculature (Fig. 5C,D): M. craniomandibularis internus (M. 11/ 0md1): largest muscle of the head, O:
143

Richter et al.: Head anatomy of Formicidae

Fig. 5. Volume renderings of head of Formica rufa (CASENT0709411: A,B; CASENT0790267: E,F) and Brachyponera luteipes(CASENT0709409: C,D,G,H). A,C,E,G: Upper part dorsal view, lower part ventral view. B,D,F,H: Sagittal view. A – D: Mandibular
musculature and mandibular gland. E – H: Antennal musculature, in F and H parts of antennal torulus transparent to show muscles andbulbus. — Abbreviations: 0an1 – M. tentorioscapalis anterior; 0an2 – M. tentorioscapalis posterior; 0an3 – M. tentorioscapalis lateralis;
0an4 – M. tentorioscapalis medialis; 0an6 – M. scapopedicellaris lateralis; 0an7 – M. scapopedicellaris medialis; 0md1 – M. cranio­
mandibularis internus; 0md3 – M. craniomandibularis externus; 0md8 – M. tentoriomandibularis; al – atala; amp – antennal ampulla;

144

ARTHROPOD SYSTEMATICS & PHYLOGENY — 78 (1) 2020

posterior bundles mainly on the occipital region, otherparts on large areas of the lateral surface, the dorsolateralareas, and most of the surface of the ventral wall of thehead capsule. As in Formica, a large lateral/dorsolateralarea and a large ventral area are separated from each other, and a mesal bundle inserting above the occipital foramen is also isolated; I: most of the fibers attach directlyon the apodeme; its main body (mda, Fig. 5C,D) divides
into three parts of very similar size, a central (mdacb, Fig.
5B), a dorsolateral (mdalb, Fig. 5B) and a ventromesal(mdamb, Fig. 5B) branch, all three extensive and sheetlike; the central one is straight, whereas the others areconcave; a smaller branch divided from the mesal one(mdaab, Fig. 5B) receives the isolated muscle bundle as
in Formica; in addition to the fibers of the mesal and lateral bundle inserted close to the main body of the apo­
deme, almost all bundles of the posterior head regions are
directly attached; only the portions of the muscle closest
to lateral, mesal and anteroventral margins are attachedby cuticular threads, which are shorter than those of For­mica. M. craniomandibularis externus (M. 12/ 0md3):
very similar to that of Formica. O: more fibers on the
postgenal ridge than on the ventral wall of the head capsule. I: upon a tendon that is shorter than in Formica.
M. hypopharyngo-mandibularis (M. 13)/ M. tentoriomandibularis medialis inferior (0md8): similar to its
homologue in Formica, but with its origin placed further
anterad, at the anterior edge of the lateral lamella.

3.5. Maxillae
Formica rufa
The maxillae (HAO_0000513) are closely associatedwith the labium (HAO_0000453), both forming themaxillolabial complex (Figs. 1B,C, 2B,C). The cardines(HAO_0000187) are small elongated rods, T-shaped on
their proximal end, with the lateral extension much longer than the mesal one (cd, Fig. 6A,B), which articulateswith a small condyle of the head capsule (HAO_0002074)(cc, Fig. 11); a notch in the distal hypostomal margin, laterad the condyle, receives the lateral extension and theproximal cardinal shaft; the distal cardinal tip is strongly
bent anterad/dorsad; a membrane at the V-shaped articulatory area connects it to the stipes (cardinostipital hinge,
HAO_0002076); a stipital process extends beyond the
distal cardinal margin internally, additionally reinforcing the cardinostipital articulation; ventrally, the cardo iscovered with minute spines and also bears a number ofshort and thin setae (Fig. 6B). The cardo is largely enclosed in the hypostomal cavity, with only the tip vis-ible externally, even in the fully extended position of themaxillolabial complex; in the retracted position it faces
downward, whereas the stipes (HAO_0000958) is dorsally oriented with an angle of almost 90° relative to the longitudinal cephalic axis. The external surface of the stipes
is sclerotized (external stipital sclerite, HAO_0002098)(st, Fig. 6A); in the retracted position it closes the oral
foramen (HAO_0000670), together with the prementum(HAO_0000804) and the labrum (HAO_0000456); thestipes appears triangular in cross section and its mesal
face is also sclerotized (HAO_0002096) (Fig. 13B), especially the proximal internal edge, which forms a stabilizing sclerite (stis, Fig. S2E); the mesal face bears an
irregular row of setae on its distal two thirds (Fig. 6B);
proximally, a membranous area adjacent with the stipes
connects with the membrane closing the oral foramen;
the internal sclerite is connected to the stipito-premental conjunctivum (HAO_0002125) (spc, Figs. 7B, S1D,
S2D,E), a weakly sclerotized band of cuticle linking thestipes, the premental arm (HAO_0002155), and the hypopharynx at the lateroventral edges of the infrabuccal
pouch (HAO_0001563); the external stipital face appears
roughly rectangular, but with the distal end tapering towards the mesal side; a small and rounded proximomesal
stipital lobe covers the membrane connecting the preand postmentum (HAO_0000785) (Figs. 1C, 2C). The
lateral stipital edge is deeply inserted into the concave
anterior surface of the triangular hypostomal process(Fig. 11A; white arrowhead, S2A); the mesal edge isin contact with the premental ditches (HAO_0002227)
when the mouthparts are retracted. The six-segmented
maxillary palp inserts distally on the mesal stipital edge
(HAO_0000515) (pmx, Fig. 6A,B); palpomere 1 is shortand stout, and its distal margin appears slanted; the following palpomeres are elongate and each of them is thinner than the preceding one; all palpomeres, especiallythe distal ones, are densely covered with setae; distinctly different types of hair-shaped sensilla are present, asubdecumbent pubescence and distinct sets of elongate,
thicker setae on the dorsal side of palpomeres 1 – 5 andthe ventral margins of the distal portions of palpomere
3 – 5. The galeolacinial complex, formed by the closely
connected endite lobes (ga, Fig. 6A,B, HAO_0000368;
lc, Fig. 6A,B, HAO_0000457), is broadly connected tothe distal internal edge of the stipes; the entire structure is bent mesad above the labium and hypopharynx(Fig. 13A). The galea is sclerotized on its dorsal andventral side; its anterior half is bent downwards in theretracted position, and bears a row of setae along theedge of its dorsal sclerotized area; additional setae areinserted along the entire dorsal surface of the inflectedata – anterior tentorial arm; cli – clypeal inflection, ventral wall; dta – dorsal tentorial arm; esr – epistomal ridge; hyc – hypostomal cavity; hysp – hypostomal triangular process; md – mandible; mda – mandibular apodeme; mdaab – mandibular apodeme, accessory branch;mdacb – mandibular apodeme, central branch; mdalb – mandibular apodeme, lateral branch; mdamb – mandibular apodeme, mesal branch;
mdc – mandibular carinae; mdg – mandibular gland; ml – mesal lamella; pgr – postgenal ridge; ppt – posterior process of tentorium; pta –posterior tentorial arm; tb – tentorial bridge; toa – torular apodeme; vma – ventral mandibular articulation. — Colors: beige / brown –mouthparts; grey – cuticle; orange / red – muscles; pink – antennal ampullae; purple – glands.

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Fig. 6. SEM micrographs of the maxillae of Formica rufa (A,B) and Brachyponera luteipes (C – F). A: Dorsal view, slightly shifted frontolaterad. B: Mesoposterior view. E,H: Close view of the paraglossa. C,D,F: Ventral view. E: Dorsolateral view. — Abbreviations: 0mx1 –M. craniocardinalis externus; cd – cardo; csga – conical sensilla of galea; dlsst – distal lateral stipital surface; ga – galea; gacr – galealcrown; lc – lacinia; lcc – lacinial comb; mtga – galeal microtrichia; mxc – maxillary / galeal crown; pmx – palpus maxillaris; st – stipes.
— Symbols: white arrowheads – fields of microtrichia on lacinia; black arrowheads – extended distal lateral edge of stipes.

part (galeal crown HAO_0002132) (gacr, Fig. 6A,B),
and also on the straight part of the galea close to thestipes; ventrally, the galea bears a long comb of setae(galeal comb, HAO_0002243) (mxc, Fig. 6B) close to
its outer margin; the area between the comb and the outer margin is densely covered with minute microtrichia.
The lacinia also bears a row of setae along its edge (lacinial comb, HAO_0002124) (lcc, Fig. 6A,B); close to146

the galea, its ventral side displays a large field of small
scale-like combs of microtrichia; a smaller field of longer, hair-like microtrichia, originating in pairs or tripletsfrom scale like bases, are visible on the posterior endof the lacinial lobe (white arrowheads, Fig. 6B); thebasal lacinial sclerite (HAO_0002093) (lcs, Fig. S2A) is
mesally connected to the stipes and well sclerotized; itis more or less continuous with the internal stipital scle-

ARTHROPOD SYSTEMATICS & PHYLOGENY — 78 (1) 2020

rite, but can be recognized on cross sections as distinctly
thickened region, where it receives the stipitolacinialmuscle (0mx6).
Musculature (Fig. 9B,F): M. craniocardinalis externus(M. 15/ 0mx1): long muscle composed of many thin fibers; O: posteroventral head capsule anterad the depression around the occipital foramen, and partly on theposterior postgenal ridge; I: on a relatively short tendon
originating on the lateral proximal extension of the cardo. M. tentoriocardinalis (M. 17/ 0mx3) or M. tentoriostipitalis posterior (0mx5): O: anteriorly on the anteriortentorial arm, partly on its lateral lamella (directly behindinsertion of 0md8); I: on a thin tendon originating on aninternal process of the base of the external stipital sclerite, which is part of the cardinostipital hinge. M. tentoriostipitalis (M.18)/ M. tentoriostipitalis anterior (0mx4):
two distinct bundles; O: anterior bundle ventrally on theanterior tentorial arm directly behind 0mx3; posterior
bundle ventrally on the lobe-like mesal lamella; I: on flat,
merging tendons inserted on the mesal stipital wall (internal stipital sclerite) close to the stipito-premental conjunctivum. M. stipitolacinialis (M. 20/ 0mx6): very flatmuscle; O: distally on the lateral stipital margin; I: on thebasal lacinial sclerite. M. stipitogalealis (M. 21/ 0mx7):
O: along almost the entire length of the internal surfaceof the external stipital sclerite; I: on a tendon attached tothe inner base of the galea. M. stipitopalpalis externus(M. 22/ 0mx8): well-developed muscle, similar in size to0mx7. O: external stipital sclerite laterad 0mx7; I: baseof first palpomere. M. palpopalpalis maxillae primus,
secundus, tertius? quartus? (M. 24, 25, 26, 27/ 0mx12,
13, 14, 15): intrinsic muscles of the maxillary palp;
the two proximal ones (primus/secundus) are distinct,
whereas the following muscles in the thin palpomeres are
not clearly recognizable on µCT images and histologicalsections. O: on the base of the palpomere; I: base of thefollowing palpomere.

Brachyponera luteipes
The proximal end of the cardines appears rather L- than
T-shaped, with a scarcely visible mesal extension (Fig.
6E,F); the V-shaped articulatory area formed with thestipes is deeper and narrow. In cross section the stipes
appears more rectangular proximally and triangular distally. The external stipital sclerite is of similar shape, butwith its distal margin tapering more towards its middle
region, rather than mesally; the distal part of the lateral
stipital margin forms a broad shoulder region (dlsst,
Fig. 6); it is distally expanded as a wedge-shaped tip ofthe external face (black arrowhead, Fig. 6), which isexternally pressed against the lateral labral process in
the retracted position; a notch is present posteromesadthe extension (Fig. S1F); its mesal part receives the first
palpomere in retracted position, providing space for the
palpus to protrude from the retracted complex; furthermore, the lateral labral process reaches into the lateralpart of the groove (Fig. S1F); the mesal stipital edge
interacts more closely with the premental ditches thanin Formica, almost over the entire length of the stipes;

the lateral stipital edge is in contact with the angle between the hypostoma and the hypostomal/ postgenal
triangular process (*, Fig. 11B). The three-segmented
maxillary palp inserts mesad the notch described above(pmx, Fig. 6E); palpomere 1 has a straight distal margin;
the following two are both long and thin; palpomere 2
is narrowed proximally and widens distally, 3 appears
slightly club-shaped; only palpomere 3 bears two setae apically; no other types of sensilla were identified.
The maxillary endite lobes are of similar shape, but
the posterior margin of the lacinia is more pointed (lc,Fig. 6E,F); the stout, apically pointed setae of the galealcrown are interspersed by few slightly shorter and thinner setae (gacr, Fig. 6E); the setae on the dorsal galeal
surface are rather long and less dense than in Formica; asecond field of much shorter setae is present on the posterior region of the galea, close to the lacinia (Fig. 6E);
the maxillary comb is formed by very closely arranged
blunt setae that decrease in length anteriorly (mxc, Fig.
6C,D,F); a well-developed row of cone-like sensillacoeloconica is present distad the comb (csga, Fig. 6C),
it is surrounded by small hair-like microtrichia (mtga,
Fig. 6C); the posterior ventral galeal surface proximad
the comb and close to the lacinia is scale-like (Fig. 6F);
the lacinial comb is formed by a row of thick, spinelike cuticular extensions instead of setae (lcc, Fig. 6); a
conspicuous longitudinal fold is present on the ventral
surface of the lacinia (Fig. 6C,F); its surface is smooth,
except for the area close to the anterior margin, whichbears a small field of scale-like microtrichial combs
with some surrounding individual hair-like microtrichia,
similar to the structures found in Formica (white arrowhead, Fig. 6D,F). In contrast to Formica the basal
lacinial sclerite is not thickened.
Musculature (Fig. 9C,D): M. craniocardinalis externus(M. 15/ 0mx1): relatively thin muscle; O: similar to theorigin in Formica, but with less fibers on the ventral headcapsule and more on the postgenal ridge; I: on a broad,anteriorly narrowing tendon attached to the lateral cardinal base. M. tentoriocardinalis (M. 17/ 0mx3) or M.
tentoriostipitalis posterior (0mx5): O: similar to the origin in Formica, but partly on the mesal lamella insteadof the lateral one; I: like in Formica, with the tendon running very close to the cardo. M. tentoriostipitalis (M.18)/
M. tentoriostipitalis anterior (0mx4): only one distinctly
developed bundle; O: ventrally on the broadest part ofthe mesal lamella; I: with two merging tendons as in
Formica and on the same insertion site. M. stipitolacinialis (M. 20/ 0mx6): slightly more compact but otherwise
similar to its equivalent in Formica. M. stipitogalealis(M. 21/ 0mx7): O: further distad than in Formica; I: witha very short tendon on the inner base of the galea. M.
stipitopalpalis externus (M. 22/ 0mx8): much smaller
than 0mx7, otherwise similar to its equivalent in For­mica. M. palpopalpalis maxillae primus, secundus? (M.
24, 25/ 0mx12, 13): intrinsic muscles of the maxillary
palp; only the proximal one (primus) is clearly recognizable on µCT images and histological sections. O: on thebase palpomere 1; I: base of palpomere 2.
147

Richter et al.: Head anatomy of Formicidae

Fig. 7. SEM micrographs of the labium of Formica rufa (A – E) and Brachyponera luteipes (F – H). A,C,D: Dorsal view. B,E,H: Lateralview. F,G: Dorsolateral view. E,H: Close view of the paraglossa. — Abbreviations: bpb – basiparaglossal brush; dhy – distal hypo­pharynx; glam – anterior glossal margin; glds – dorsal glossal sclerite; glo – glossa; ibp – infrabuccal pouch; pgl – paraglossa; plb – palpus
labialis; pmd – premental ditch; spc – stipito-premental conjunctivum; svo – salivary opening.

3.6. Labium and distal hypopharynx
Formica rufa
A membrane, which is folded inwards and thus not visible externally, connects the proximal part of the labium(HAO_0000453) with the maxillary base; the stipitopremental conjunctivum connects the entire maxillolabial
complex with the infrabuccal pouch (HAO_0001563)(spc, Figs. 7B, S1D, S2D,E). The labium is also completely fused to the distal hypopharynx (HAO_0001575),
which forms the dorsal surface of the proximal part of theentire structure (the labium and the distal hypopharynge148

al part including the infrabuccal pouch [hypopharyngeal
wall, HAO_0000409] are described together here); theventral labial surface is formed by the small postmentum(psm, Fig. 7B, HAO_0000785) and the large prementum(Fig. 7B, HAO_0000804); the anterior upper surface
bears the fused glossae (simply referred to as “glossa”
in the following, HAO_0000376) (glo, Fig. 7A – C) andparaglossae (HAO_0000686) (pgl, Fig. 7A,B,E). The
postmentum, a relatively broad horseshoe-shaped sclerite, is connected to the prementum by a membrane; thedistal region of the sclerite and the proximal membrane
are covered with small, spine-like microtrichia. The pre-

ARTHROPOD SYSTEMATICS & PHYLOGENY — 78 (1) 2020

mentum is strongly sclerotized; it is divided into a lateral(HAO_0002152) and a ventral face (HAO_0002156) byshallow premental ditches (HAO_0002227) (pmd, Fig.
7B); the distal margin of the oval ventral face appears
slightly truncated in ventral view (Figs. 1C, 2C); the lateral premental face bears relatively short, stout premental
arms (HAO_0002155) (pma, Fig. 13B) as proximolateralextensions; they are connected to long crescent-shaped
lateral hypopharyngeal sclerites (hypopharyngeal rods,
HAO_0000408) (hyr, Fig. 13B), which stabilize the distal hypopharynx. The distal hypopharyngeal part (dhy,Fig. 7A,B) is relatively short, broad and raised highabove the prementum; the lateral sclerites are anteriorly
continuous with a broad, weakly sclerotized conjunctivum, which is bent downwards into the distal tip of the
hypopharynx, thus forming the massive hypopharyngeal
buttons (HAO_0002234) (hyb, Fig. 13A); these structures are ventrodistally continuous with the salivarial
sclerite (HAO_0001682) (sv, Figs. 9F, 12A). The lateral hypopharyngeal surface is mostly smooth, whereas
the dorsal surface bears a dense cover of long, hair-likemicrotrichia (Fig. 7A,B); its anterior apical region has
a scale-like surface structure with each scale bearing asingle hair-like microtrichium; the distal hypopharynx is
limited by the salivary opening (HAO_0000906) (svo,Fig. 7C); proximally it is continuous with the large,
bulbous, hemispherical infrabuccal pouch (ibp, Figs.
7A,B,D, 11E, 12A, 13B); only a narrow opening is visible, especially when the ventral mouthparts are retracted;
the upper and lower lip of the pouch opening bear minutemicrotrichia (Fig. 12C); the inner wall is glabrous andhas a honeycombed surface (Fig. 9D); from the upper
lip of the pouch opening the hypopharyngeal wall continues towards the prepharynx (described in the section“cephalic digestive tract”). The well-developed basiparaglossal brushes (HAO_0002199) (bpb, Fig. 7A – C) on
both sides of the salivary opening consist of several rowsof blunt setae which are shortest laterally; the lateral baseof the brushes is covered with rows of minute finger-likemicrotrichia; the brushes are anteriorly continuous withmembranous folds covered with spines, representing thestrongly reduced paraglossae (HAO_0000686) (pgl, Fig.
7A,B,E). The glossa (HAO_0000376) (glo, Fig. 7A – C),
which is inserted between these folds, is stabilized dorsally by the broad dorsal glossal sclerites (anterior glossal
plate, HAO_0000112) (glds, Fig. 7A – C) and ventrally
by the ventral glossal sclerite (posterior glossal sclerite,
HAO_0000748) (glvs, Fig. 12A); the middle region of thelatter is thick and two lateral arms project anteriorly into
the ventral glossal wall; the membranous wall of the glossa
is densely covered with hair-like microtrichia (Fig. 7B,C);
the distal edge of the glossa (HAO_0002206) (glam, Fig.
7B) and of the prementum each bear rows of long setae.
The four-segmented labial palps (HAO_0000450) (plb,
Fig. 7A,B) insert immediately proximad the paraglossae;
the palpomeres are similar in length and diameter, except
for the slightly less wide apical palpomere; palpomere 1
is glabrous except for its distal margin, whereas the entire
surface of the other palpomeres is covered with long, thinsetae; this fine pubescence is interspersed with thicker,
longer, more erect hair-like sensilla on the dorsal surfaceof especially palpomere 3 and 4, around the distal margins of palpomeres 2 and 3 and the tip of palpomere 5;
the articulatory membranes between the palpomeres arecovered with small denticles.
Musculature (Figs. 9E,F, 12, 13): M. tentoriopraementalis (M. 29/ 0la5): O: posterior postgena close toposterior tentorial pit, ventrad 0hy3; I: the tendons of
the paired muscle merge medially as a broad unpaired
structure which extends over the dorsal margin of the hypostomal cavity and inserts on the posterior premental
margin. M. praementoparaglossalis (M. 31/ 0la11): O:
median premental region; I: middle region of the ventral
glossal sclerite. M. praementoglossalis (M. 32/ 0la12):
O: on the prementum promixad 0la11; I: base of the dorsal glossal sclerites. M. praementopalpalis externus (M.
34/ 0la14): small muscle; O: anteriorly on the premental
arms; I: base of palpomere 1. M. palpopalpalis labii primus/ secundus (M. 35/36 0la16/ 17): intrinsic muscles ofthe labial palp; O: base of palpomere 1/2; I: base of palpomere 2/3. M. tentoriohypopharyngalis (M42/ 0hy3):
O: posterior postgena close to the posterior tentorial pit,
dorsad 0hy3; I: with long tendon on hypopharyngeal button, close to the salivarium.

Brachyponera luteipes
The distal part of the postmentum bears only few small
denticles instead of spine-like microtrichia (psm, Fig.
8D). The premental ditches are slightly deeper and narrower (pmd, Fig. 8D) than in Formica. The ventral premental face is more oval; (pm, Fig. 8D) the premental
arms are thinner and rather short. The distal hypopharynx
is also raised high above the prementum; the stabilizing
lateral sclerites (hypopharyngeal rods) are very long andthin (hyr, Fig. 14C); the lateral surface of the distal hypopharynx (dhy, Fig. 7F) is set with long scales formedby comb-like microtrichia; the dorsal surface is coveredby a complex microtrichial array; the dorsolateral margins are densely set with hair-like microtrichia; the apicalpart of the hypopharynx has a scale-like surface structure like in Formica, with each scale bearing a minutehair-like projection posteriorly; a dorsal smooth triangular depression of the hypopharynx is sparsely set withlong microtrichia; they are more densely arranged proximally; the proximal region of the distal hypopharynx has
a scale-like surface structure formed by combs of shortmicrotrichia; the length of the microtrichia decreases further proximad towards the infrabuccal pouch; the membrane adjacent with the pouch is completely smooth. The
salivary opening lies distally between the basiparaglossal
brushes, directly behind the glossa; a stripe of cuticle withshort microtrichia on its ventral side covers the opening(svo, Fig. 7F,G). The infrabuccal pouch is smaller thanin Formica (ibp, Figs. 11H, 12B); the opening is also
narrow; very distinct folds are present dorsally (similar
position in all examined individuals); a strongly folded
surface is displayed when the pouch is extruded, withwidely opened mouthparts (Fig. 8A,C). The lateral sur149

Richter et al.: Head anatomy of Formicidae

Fig. 8. SEM micrographs of the head and details of mouthparts of Brachyponera luteipes. A – C,E: Anteroventral view. D: Ventral view.
— Abbreviations: al – atala; cl – clypeus; ep – epipharynx; ga – galea; glam – anterior glossal margin; glo – glossa; hysc – hypostomal
carina; lbr – labrum; md – mandible; plb – palpus labialis; pm – prementum (ventral face); pmd – premental ditch; pmx – palpus maxillaris; psm – postmentum; st – stipes; vma – ventral mandibular articulation.

face of the basiparaglossal brushes is covered with rowsof minute finger-like microtrichia. Reduced paraglossae
are likely represented by folds of smooth cuticle with two
minute distal sensilla (pgl, Fig. 7F,H). The broad ventral glossal sclerite is formed by a thin layer of cuticle
close to the distal premental margin; it narrows towards
the glossa, forming a dorsoventral flat sheet that fansout ventrally before it splits into two arms; the area between the distal premental margin and the glossa is larger
than in Formica (Fig. 8D); most of the glossal surface isdensely covered with hook-shaped microtrichia, which
are thinner towards the lateral margins; the distal edgeof the glossa is smooth (glam, Fig. 8D). The labial palps(plb, Figs. 7F, 8A,D) are three-segmented, with a verythin palpomere 2; the apical palpomere is club-shaped;
palpomere 1 bears two ventral setae (Fig. 8D), palpomere
3 three long distal setae, a dorsal seta at about midlength,
and a short terminal cone-like sensillum (Fig. 7F).
Musculature (Figs. 9G,H, 12, 14): M. tentoriopraementalis (M. 29/ 0la5): like in Formica. M. praementopara­

glossalis (M. 31/ 0la11): O: proximolaterally on the pre­mentum; I: further laterad compared to Formica. M. prae­mentoglossalis (M. 32/ 0la12): O: central premental area,
distad 0la11, covering larger area of prementum than in
Formica; I: dorsal glossal sclerites. M. praemento­pal­pa­
lis externus (M. 34/ 0la14): small muscle, very difficult
to distinguish from 0la11 but otherwise like in Formica.
M. palpopalpalis labii primus/ secundus (M. 35/36
0la16/ 17): only the first intrinsic muscle recognizablewith the applied techniques. M. tentoriohypopharyn­
galis (M42/ 0hy3): O: very close to the posterior tentorial pit, laterad 0la5; I: hypopharyngeal button.

3.7. Salivarium and salivary duct
Formica rufa
The wall of the salivary duct (HAO_0002236) is formed
by a thick layer of tissue throughout most of its length,with the diameter decreasing towards the labium (svd,Fig. 9. Volume renderings of the head of Formica rufa (CASENT0790267: A,B,E,F) and Brachyponera luteipes (CASENT0709409:
C,D,G,H). A,C,E,G: Upper part dorsal view, lower part ventral view. B,D,F,H: Sagittal view. A – D: Maxillary musculature and gland.
E – H: Labial musculature and salivary duct. — Abbreviations: 0hy3 – M. tentoriohypopharyngalis; 0hy7 – M. praementosalivarialis;
0hy12 – M. hypopharyngosalivarialis; 0la5 – M. tentoriopraementalis; 0la11 – M. praementoparaglossalis; 0la12 – M. praementoglossalis;
0la14 – M. praementopalpalis externus; 0la16 – M. palpopalpalis labii primus; 0la17 – M. palpopalpalis labii secundus; 0mx1 – M. cranio­

150

ARTHROPOD SYSTEMATICS & PHYLOGENY — 78 (1) 2020

cardinalis externus; 0mx3 – M. tentoriocardinalis; 0mx4 – M. tentoriostipitalis anterior; 0mx6 – M. stipitolacinialis; 0mx7 – M. stipito­
galealis; 0mx8 – M. stipitopalpalis externus; 0mx12 – M. palpopalpalis maxillae primus; 0mx13 – M. palpopalpalis maxillae secundus;
ata – anterior tentorial arm; dta – dorsal tentorial arm; lb – labium; ll – lateral lamella; ml – mesal lamella; mx – maxilla; mxg – maxillary
gland; pta – posterior tentorial arm; ppt – posterior process of tentorium; sv – salivary sclerite; svd – salivary duct; tb – tentorial bridge. —Colors: beige / brown – mouthparts; grey – cuticle; orange / red – muscles; purple – glands and ducts.

151

Richter et al.: Head anatomy of Formicidae

Fig. 10. SEM micrographs of the labrum of Formica rufa (A,B,B’) and Brachyponera luteipes. (C,D,D’). A,B,B’: Frontal view. C,D,D’:
Caudal view. — Abbreviations: ep – epipharyngeal wall; 0lb2 – M. frontoepipharyngalis; ep – epipharynx; lbrp – proximolateral labralprocesses.

Figs. 9E,F, 12A, 13E); the duct forms a loop anterior to
the brain (Fig. 9E,F); it opens into the sclerotized salivarium (HAO_0000906) (sv, Figs. 9F, 13E) between the
distal hypopharynx and the basiparaglossal brushes. The
long, U-shaped and sclerotized salivarium encloses thesalivary duct ventrally; it fuses with the hypopharyngeal
button proximad its opening and to the basiparaglossal
brushes distally.
Musculature (Figs. 9E,F, 12A): M. hypopharyngosalivarialis (M. 37/ 0hy12): two closely adjacent bundles, O:
dorsolaterally from the distal hypopharynx, mostly fromthe stabilizing sclerites (hypopharyngeal rods); I: mesal
bundle dorsally on the distal salivary duct, lateral bundle
laterally on the salivarium. M. praementosalivarialis anterior & (or) posterior (M. 38, 39/ 0hy7): a well-developed muscle; O: prementum proximad 0la12; I: ventrally
on the sclerotized salivarium, and a few fibers ventrally
on the distal salivary duct.

Brachyponera luteipes
The salivary duct is also thick-walled, especially the portion forming the loop (svd, Figs. 9G,H, 12B, 14A,B),
which is largely restricted to the horizontal plane in contrast to Formica, bending only very slightly in vertical direction (Fig. 9H). The salivarium is broader and shorter.
Musculature (Fig. 9G,H): M. hypopharyngosalivarialis(M. 37/ 0hy12): like in Formica. M. praementosalivarialis anterior & (or) posterior (M. 38, 39/ 0hy7): largely
reduced, no fibers originating on the prementum andinserting on the salivarium are visible in B. luteipes or
B. chinensis.
152

3.8. Labrum
Formica rufa
The anterior surface of the trapezoid labrum (HAO_­
0000456) forms a distinct hump (Fig. 10A); the distal
margin is bilobed, with a median notch; proximolateralprocesses are posteriorly directed (not visible in frontal
view, lbrp, Fig. 10A,B), cone-shaped and slightly bent
downwards; the bases of the maxillary palps fit below
the processes when the mouthparts are retracted (Fig.
S1E). The labral surface displays a scale-like cuticularsurface structure, and a minute cone-shaped sensillum
is inserted on the posterior surface of each distal lobe
(Fig. 10B’).
Musculature (Figs. 11C, 13B): M. frontoepipharyngalis (M. 9/ 0bl2): a moderately sized muscle; O: frontal
area shortly posterad the antennal bases, laterad 0bu1; I:
upon a thin, long tendon attached mesad the base of theproximolateral processes.

Brachyponera luteipes
The labrum has an even surface, without an anterior
hump (Fig. 10C). The proximolateral processes are welldeveloped, hook-shaped and project laterad, beyond themargins of the labrum proper (lbrp, Fig. 10C,D); they
fit behind the laterodistal margin of the stipes when the
maxillolabial complex is retracted (Fig. S1F).
Musculature (Figs. 10D, 11F): M. frontoepipharyngalis (M. 9/ 0lb2): O: frontal area shortly posterad the antennal bases, laterad 0bu1, just mesad the frontal carina;
I: like in Formica.

ARTHROPOD SYSTEMATICS & PHYLOGENY — 78 (1) 2020

3.9. Distal epipharynx
Formica rufa
The distal part of the epipharyngeal wall (HAO_0000300),
the unsclerotized inner wall of the labrum and the laterally free portion of the epipharyngeal wall distad the prepharynx, forms the semimembranous upper wall of thelaterally open buccal cavity (ep, Figs. 11E, 12A,C); thesurface is smooth, including the upper lip of the functional mouth opening (Fig. 12C).
Musculature: epipharyngeal muscles function as part ofthe cephalic digestive tract and are treated in that section.

Brachyponera luteipes
The epipharyngeal surface (ep, Figs. 8A,B, 11H, 12B,D)
is covered with slightly wavy rows of short finger-like microtrichia; the dorsal lip of the functional mouth opening
bears a brush of very long hair-like microtrichia (whitearrowhead, Fig. 12D), expanded as a roughly triangular
patch in the middle region of this structure (Fig. 8B).
Musculature: see above.

3.10. Cephalic digestive tract
Formica rufa
The cephalic digestive tract is divided into three functional subunits: the buccal cavity (HAO_0000670), theprepharynx (pph, Figs. 11C – E, 12A, 13A,B) (cibarium,
HAO_0000201), and the pharynx (ph, Figs, 11A,C – E,
12A) (HAO_0001740). The buccal cavity is the spacecontaining the mouthparts. Within this compartment, anarrow pre-oral chamber is limited by the labium anddistal hypopharynx on the ventral side, the maxillae laterally, by the distal epipharynx dorsally, and by the labrum anterodorsally when the mouthparts are retracted(Fig. 11E). The distal hypopharynx posteriorly expands,
forming the infrabuccal pouch (ibp, Figs. 11E, 12A). The
lateral margins of the proximal epi- and hypopharynx
fuse, thus forming an extensive prepharyngeal tube (pph,Figs. 11C – E, 12A, 13A,B), which anteriorly opens into
the buccal cavity via the broad, fissure-shaped functional
mouth (HAO_0000361) (fmo, Figs. 11D,E, 12B); theanteriormost section of the prepharynx, the buccal tube
(bt, Figs. 11D,E, 12A,B), appears like a slightly flattened
crescent in cross section, and is distinctly bent backwards
when the mouthparts are in a retracted position (Fig.
11E); its cuticle is thick both on the anterior (epipharyngeal) and posterior (hypopharyngeal) side, and displaysa peculiar banding pattern (Fig. 12A,C); the epipharyngeal side is densely covered with minute microtrichia,
whereas the hypopharyngeal side display a sparse pattern of much longer hair-like microtrichia (Fig. 12C);
both types of hairs are oriented towards the functional
mouth; the main part of the prepharynx appears broadly
crescent-shaped to oval in cross section (Fig. 13A,B); it
runs parallel to the clypeus and anterior frontal region,
ending posteriorly with the anatomical mouth opening(amo, Fig. 11E), marked by the frontal ganglion (fg, Fig.

11C,E) and the insertion sites of the muscles 0bu2 and0hy1. The anterior roof of the prepharynx is sclerotized,with a broad hump (Figs. 11E, 12A), and appears finshaped in sagittal view (possibly representing the buccal lobe, HAO_0002412); the sclerotized prepharyngeal
floor forms the sitophore plate (HAO_0000939) (sp, Fig.
13A); this structure bears two stabilizing elements, theoral arms (oa, Figs. 11C, 13B, S1G), which originate anterolaterally and are ventrally directed; they form an evencurve towards the dorsal side of the prepharynx and are
dorsally arranged around the anatomical mouth opening,with the dorsal portion reaching into the anterior pharynx; the posterior ends of the arms are bent outwards andform an intricate sheath-like structure which accommodates the openings of the pharyngeal gland. The pharyngeal opening, i.e. the anatomical mouth (amo, Fig. 11E),
is not sclerotized except for the oral arms; at the level ofthe anatomical mouth opening the digestive tube forms
an angle of about 60° (n = 2), with the pharynx runningstraight towards the occipital foramen; its dorsal wall is
deeply folded (Fig. S2C), resulting in a crescent to Yshape in cross section. The width of the posterior pharynx decreases and it is flattened at the attachment area of
0ph2; the pharyngeal wall is finely wrinkled throughout
most of its length, especially in the attachment area of
0bu2 and 0bu3 (Fig. 12A).
Musculature (Figs. 11C,E, 12, 13): M. frontohypo­pha­ryngalis/ M. frontooralis (M. 41/ 0hy1): well-develop­ed
paired muscle (Fig. 11C,E): O: centrally on the frontal
area, laterad 0bu3 (Fig. S1A); I: broadly on the posterior oral arms, on the posterior/dorsal side of the sheathsurrounding the pharyngeal gland opening (Fig. 11C).
M. clypeopalatalis (M. 43/ 0ci1) two subcomponents(Figs. 11E, 12A,E, 13A); M. 43a/ 0ci1a: unpaired, relatively small muscle, O: mesally on the anterior clypeus,
flanked by 0ci1b (Fig. S1A); I: broadly on the anteriorwall of the buccal tube, close to the functional mouth
opening; M. 43b/ 0ci1b: very strongly developed paired
muscle; asymmetric, with stronger bundle on one side;
O: along the midline of almost the entire clypeus, intersecting with 0ci1a (Fig. S1A); I: anterior dorsal prepharyngeal wall, especially on the hump. M. clypeobuccalis (M. 44/ 0bu1): distinctly developed but muchsmaller than 0ci1b; paired muscle (Figs. 11E, 12A, 13B),
O: posterior clypeus close to the epistomal ridge; I: dorsal prepharyngeal wall, behind the hump amd anterior to
the frontal ganglion. M. frontobuccalis anterior (M. 45/0bu2): well-developed, paired but closely adjacent along
the midline (Figs. 11E, 12A), O: frontal area, distinctly
posterad the supraclypeal area (Fig. S1A); I: mesally on
the dorsal side of the pharynx, directly posterior to thefrontal ganglion. M. frontobuccalis posterior (M. 46/0bu3): strongly developed, flat and unpaired muscle with
two subunits (Figs. 11E, 12A), O: anterior subunit directly posterad 0bu2, posterior subunit distinctly separated
from it, shortly anterad the median ocellus (Fig. S1A);
I: dorsomesally on the pharynx between the dorsal longitudinal folds, posterad 0bu2. M. frontobuccalis lateralis/ M. tentoriooralis (M. 47/ 0hy2): a relatively small,
153

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Fig. 11. Volume renderings of heads of Formica rufa (CASENT0790267: A,C – E) and Brachyponera luteipes (CASENT0709409: B,F – H).
A,B: Frontal view of head capsule. C,F: Dorsal view of cephalic digestive tract with glands and parts of the musculature, 0lb2, and thecentral nervous system. D,G: Ventral view of cephalic digestive tract including glands, and the central nervous system. E,H: Sagittal viewof cephalic digestive tract including glands, musculature and buccal cavity, and the central nervous system. — Abbreviations: 0bu1 – M.
clypeobuccalis; 0bu2 – M. frontobuccalis anterior; 0bu3 – M. frontobuccalis posterior; 0bu5 – M. tentoriobuccalis posterior; 0ci1a – M.
clypeopalatalis, unpaired portion; 0ci1b – M. clypeopalatalis, paired portion; 0hy1 – M. frontooralis; 0hy2 – M. tentoriooralis; 0hy9 – M.
oralis transversalis; 0lb2 – M. frontoepipharyngalis; 0ph2 – M. tentoriopharyngalis; ala – atalar acetabulum; amo – anatomical mouth
opening; ata – anterior tentorial arm; atp – anterior tentorial pit; br – brain; bt – buccal tube; clf – ventral clypeal projection / flange;
cc – cardinal condyle; dhy – distal hypopharynx; dta – dorsal tentorial arm; dma – dorsal mandibular articulation; ep – epipharynx; epk –epipharyngeal keel; fc – frontal commissure; fg – frontal ganglion; fmo – functional mouth opening; frc – frontal carina; hyc – hypostomal
cavity; hysc – hypostomal carina; hysp – hypostomal process; ibp – infrabuccal pouch; lb – labium; lbr – labrum; mped – M. pharyngo­
epipharyngalis, dorsal portion; mpel – M. pharyngoepipharyngalis, lateral portion; nan – antennal nerve; no – optical nerve; oa – oral arm;ocn – ocellar nerve; opl – optic lobes; pgr – postgenal ridge; ph – pharynx; phg – pharyngeal gland; pph – prepharynx; pphg – prepharyngeal gland; pta – posterior tentorial arm; sog – suboesophageal ganglion; tb – tentorial bridge; to – torulus; tol – torular lobe; trl – lateral
torular arch; vma – ventral mandibular articulation. — Colors: beige / brown – mouthparts; green – cephalic digestive tract (prepharynx
and pharynx); grey – cuticle; orange / red – muscles; purple – glands; yellow – nervous system. — Symbols: white arrowhead – hypo­stomal hump contacting mandibular groove; * – anterior surface of triangular hypostomal process receiving lateral stipital margin.

paired muscle (Figs. 11C, 13B), O: on the proximal partof the torular apodeme; I: anteriorly on the posterior oralarm, opposite to 0hy1. M. tentoriobuccalis anterior (M.
154

48/ 0bu5) (possibly together with M.50/ 0bu6): strongly developed unpaired muscle, constricted anteriorly
due to the limited space between the large infrabuccal

ARTHROPOD SYSTEMATICS & PHYLOGENY — 78 (1) 2020

pouch and the prepharynx (Figs. 11E, 12A, 13B), O: anterior process of the tentorial bridge with a long tendon.
I: broadly on the sitophore plate. M tentoriopharyngalis (M. 52/ 0ph2): relatively small, paired muscle (Fig.
11E), O: tentorial bridge and posterior tentorial arm; I:
ventral side of the pharynx above and slightly posterad
the suboesophageal ganglion. M. transversalis buccae(M. 67)/ M. oralis transversalis (0hy9): a well-developed layer of transverse muscles between the dorsal
oral arms, directly anterior to the frontal ganglion, andan additional layer of transverse fibers connecting thearms on the ventral side posterior to the frontal ganglion(Figs. 11C,E, 12A, 12B). M. annularis stomadaei (M.
68/ 0st1): a thin layer of ring muscles around the pharynx behind the frontobuccal muscles. M. longitudinalis
stomadaei (M. 69/ 0st2): weakly developed layer of
longitudinal muscles below the ring musculature layer.
M. pharyngoepipharyngalis (Mpe): very strongly developed longitudinal muscles connecting the anterior
pharynx and the epipharynx: a thin unpaired medianbundle on the dorsal side of the prepharynx connecting
the dorsal prepharyngeal hump and the pharynx at theinsertion site of 0bu3 (Mped, Figs. 11C,E, 12A); pairedlarge lateral bundles connect the posterior oral arms withthe epipharynx directly before it connects with the buccal tube (Mpel, Figs. 11C, 13A,B).

Brachyponera luteipes
In its general features the cephalic digestive tract is similar to that of Formica. The bend of buccal tube is less distinct (bt, Fig. 11H), and almost completely straigthened
when the mouthparts are extruded (Fig. 12B). The lumenof the posterior prepharynx and pharynx is narrower(relative volume of cephalic digestive tract 1,2 % in For­mica, 0,5 % in Brachyponera). The microtrichia on the
hypopharyngeal side of the buccal tube are more densely
arranged than in Formica (Fig. 12D). The dorsal sclerotization of the prepharynx is more pronounced and forms
a short sclerotized cuticular keel on top of the more elongated prepharyngeal hump (epk, Figs. 11H; 12B). The
oral arms (oa, Figs. 11F, 12B, 14B) anteriorly originate
on the ventrolateral part of the sitophore plate and are
bent inwards; they are not evenly curved like in Formica
but abruptly bent in front of the frontal ganglion; theirdorsal portions form nearly straight, elongate wall-likestructures (Fig. S1H), which are strongly constricted at
the level of the frontal ganglion (fg, Fig. 11F,H); posterior to this constriction, the posterior ends of the arms arestrongly bent laterad and form a sheath around the openings of the pharyngeal gland as in Formica. The pharynx
forms a slightly less distinct angle (40 – 50°, n = 2) withthe prepharynx (Fig. 11H); its lumen is very narrow andmoderately flattened throughout most of its length.
Musculature (Figs. 11F,H, 12, 14): M. frontohypo­pha­ryngalis/ M. frontooralis (M. 41/ 0hy1): smaller thanin Formica (Figs. 11F, 12B); O: centrally on the frontal
area, slightly posterolaterad 0bu3 (Fig. S1B); I: narrowerand concentrated on the posterior side of the short posterior end of the oral arm (Fig. 11F). M. clypeopalata-lis (M. 43/ 0ci1), two components (Figs. 11H, 12B,C,
14A,D): M43a/ 0ci1a: O: similar to Formica (Fig. S1B),
I: narrower than in Formica. M43b/ 0ci1b: not as asymmetric as in Formica; O: anterior, broad portion of theclypeus, narrowing posteriorly (Fig. S1B): I: only on
the sclerotized keel of the dorsal prepharyngeal wall. M.
clypeobuccalis (M. 44/ 0bu1): very flat but longer (andwith slightly larger relative volume) than in Formica(Figs. 11H, 12B); O: along the thin posterior clypeal portion between the antennal sockets (Fig. S1B). I: dorsal
prepharyngeal wall posterad the sclerotized keel and anterad the frontal ganglion. M. frontobuccalis anterior(M. 45/ 0bu2): overall similar to Formica, but with adifferent attachment angle (Fig. 11H), originating aroundthe middle dorsal cephalic region (Fig. S1B). M. frontobuccalis posterior (M. 46/ 0bu3): not recognizable as aseparate bundle, likely very closely adjacent to 0bu2 or
reduced. M. frontobuccalis lateralis/ M. tentoriooralis(M. 47/ 0hy2): similar to that of Formica, but longer
due to the anteriorly shifted antennal sockets (Figs. 11F,
S1B); insertion area narrower due to the short posterior
ends of the oral arms (Fig. 11F). M. tentoriobuccalis
anterior (M. 48/ 0bu5) (possibly together with M.50/
0bu6): Appears more massive, with much larger relative
volume (0.9% vs. 1.7%); not constricted by the infrabuccal pouch; origin and insertion as in Formica (Figs. 11H,
12B, 14B). M tentoriopharyngalis (M. 52/ 0ph2): very
similar to that of Formica (Fig. 11H). M. transversalis buccae (M. 67)/ M. oralis transversalis (0hy9): awell-developed layer of transverse muscles between thestraight portion of the dorsal arms of the sitophore plate,directly anterior to the frontal ganglion (Fig. 11F,H),and an additional thin layer of transverse fibers connecting the arms on the ventral side posterior to the frontal
ganglion. M. pharyngoepipharyngealis (Mpe): verystrongly developed longitudinal muscle bundles connecting the anterior pharynx and the epipharynx; the median
portion of three thin dorsal bundles (Mped, Figs. 11F,H,
12B) connects the posterior end of the epipharyngeal
sclerotized keel with the insertion site of 0bu2; the lateralbundles attach close to the sclerotized keel; additionally,
two large paired lateral bundles are present (Mpel, Figs.
11F, 14A), one of them originating laterally, the other one
mesally on the dorsal wall formed by the oral arms; themesal bundle inserts on the sclerotized keel and the lateral bundle on the epipharynx, where it bends into the
buccal tube.

3.11. Cephalic glands
Formica rufa
The mandibular gland (HAO_0000509) (mdg, Figs.
5A,B, 13B) is composed of ca. 50 large cells forming
a flat, cup-shaped layer, adjacent to a roughly sphericalreservoir (mdr, Fig. 13B), which is connected to the
mandalus at the dorsal mandibular base by a thin duct.
From the membranous wall of the mandalus, a moderately sclerotized fold reaches towards the duct, result155

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Fig. 12. Histological sections of head of Formica rufa (A,C,E) and Brachyponera luteipes (B,D,F). All sections longitudinal. A,B: Para­sagittal region, overwiew. C,D: Parasagittal region, detail of buccal area. E,F: Details of antennal insertion. — Orientation: ← anterior region in all images except E: ↓ anterior region. — Abbreviations: 0an1 – M. tentorioscapalis anterior; 0an3 – M. tentorioscapalis lateralis;
0an4 – M. tentorioscapalis medialis; 0bu1 – M. clypeobuccalis; 0bu2 – M. frontobuccalis anterior; 0bu3 – M. frontobuccalis posterior;
0bu5 – M. tentoriobuccalis posterior; 0ci1a – M. clypeopalatalis, unpaired portion; 0ci1b – M. clypeopalatalis, paired portion; 0hy1 – M.
frontooralis; 0hy3 – M. tentoriohypopharyngalis; 0hy7 – M. praementosalivarialis; 0hy9 – M. oralis transversalis; 0hy12 – M. hypopha­ryngosalivarialis; 0la5 – M. tentoriopraementalis; 0la11 – M. praementoparaglossalis; 0la12 – M. praementoglossalis; 0md3 – M. cranio­
mandibularis externus; bb – bulbus of scapus; br – brain; bt – buccal tube; dhy – distal hypopharynx; ep – epipharynx; epk – epipharyngeal
keel; fmo – functional mouth opening; ga – galea; gl – glossa; glvs – ventral glossal sclerite; hyb – hypopharyngeal button; ibp – infrabuccal
pouch; lb – labium; lbr – labrum; lc – lacinia; md – mandible; Mped – M. pharyngoepipharyngalis, dorsal portion; oa – oral arm; ph –pharynx; phg – pharyngeal gland; pph – prepharynx; pphg – prepharyngeal gland; sog – suboesophageal ganglion; sv – salivarium; svd –salivary duct; toa – torular apodeme. — Symbols: white arrow – fringe of long microtrichia along functional mouth opening.

ing in an anchor-like shape in cross-section (Fig. 13A).
The flat maxillary gland (HAO_0000514) is formed bya group of about 50 slightly smaller gland cells, each156

of them connected to an individual minute duct. The
ducts collectively open in the membranous area laterad
the cardinal base (mxgo, Fig. 13C). The well-developed

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Fig. 13. Histological sections of head of Formica rufa. A,B,C: Cross sections. D,E: Frontal sections. — Orientation: A,B,C: ↓ ventral; D: ↓anterior region; E: ← anterior region. — Abbreviations: 0an4 – M. tentorioscapalis medialis; 0bu1 – M. clypeobuccalis; 0bu5 – M. tento­
riobuccalis posterior; 0ci1b – M. clypeopalatalis, paired portion; 0hy2 – M. tentoriooralis; 0hy3 – M. tentoriohypopharyngalis; 0hy7 –M. praementosalivarialis; 0hy9 – M. oralis transversalis; 0la5 – M. tentoriopraementalis; 0la11 – M. praementoparaglossalis; 0la12 –M. praementoglossalis; 0la14 – M. praementopalpalis externus; 0md3 – M. craniomandibularis externus; 0mx3 – M. tentoriocardinalis;0mx7 – M. stipitogalealis; 0mx8 – M. stipitopalpalis externus; 0la14 – M. praementopalpalis externus; al – atala; ata – anterior tentorialarm; bpb – basiparaglossal brush; bt – buccal tube; cd – cardo; dhy – distal hypopharynx; ep – epipharynx; hyb – hypopharyngeal button;
hyr – hypopharyngeal rod; ibp – infrabuccal pouch; lc – lacinia; ll – lateral tentorial lamella; md – mandible; mda – mandibular apodeme;
mdgd – mandibular gland duct; mdgr – mandibular gland reservoir; Mpel – M. pharyngoepipharyngalis, lateral portion; mxg – maxillary
gland; mxgo – maxillary gland opening; oa – oral arm; plb – palpus labialis; pma – premental arm; pmx – palpus maxillaris; pph – prepharynx; pphg – prepharyngeal gland; pphgo – prepharyngeal gland opening; sc – scapus; sp – sitophore plate; spc – stipito-premental
conjunctivum; st – stipes; sv – salivarium; svd – salivary duct; svo – salivary opening.

prepharyngeal (/ propharyngeal) gland laterad the prepharynx (pphg, Figs. 11D, 13B,D) is of rather irregularshape; mesally it extends into the space between the buccal tube and the main prepharyngeal part; it opens withnumerous very thin ducts (pphgd, Fig. 13D) on a sieve
plate at the lateral edges of the buccal tube (pphgo, Fig.
13A,D). The pharyngeal (“postpharyngeal”) gland (phg,Figs. 11A – E, 12A), by far the largest in the head (ca.
3.5% of the head volume), opens dorsolaterally into thepharynx at the level of the anatomical mouth; its openings are stabilized by the posterior ends of the oral arms
(phgo, Fig. S2B); it is composed of more than 40 tightly
packed tubes surrounded by thick gland epithelium; itcompletely encloses the brain dorsally and anterolaterally; anteriorly, about four ventrally directed tubes on eachside almost completely enclose the anterior pharynx; thetubes coalesce close to the openings, thus forming more
extensive reservoirs.
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Fig. 14. Histological sections of heads of Brachyponera luteipes (A,B) and Brachyponera chinensis (D,E). All sections transverse. — Orientation: ↓ ventral.— Abbreviations: 0an2 – M. tentorioscapalis posterior; 0an3 – M. tentorioscapalis lateralis; 0bu1 – M. clypeobuc­
calis; 0bu5 – M. tentoriobuccalis posterior; 0ci1b – M. clypeopalatalis, paired portion; 0hy3 – M. tentoriohypopharyngalis; 0la5 – M.
tentoriopraementalis; 0md1 – M. craniomandibularis internus; 0md1sf – M. craniomandibularis internus, direct fibers; 0md3 – M. cra­niomandibularis externus; 0mx3 – M. tentoriocardinalis; ata – anterior tentorial arm; bt – buccal tube; cc – cardinal condyle; cd – cardo;
epk – epipharyngeal keel; esr – epistomal ridge; ll – lateral tentorial lamella; ml – mesal tentorial lamella; md – mandible; mda – mandibular apodeme; Mpel – M. pharyngoepipharyngalis, lateral portion; mxg – maxillary gland; oa – oral arm; ph – pharynx; phg – pharyngeal gland; pgr – postgenal ridge; pph – prepharynx; pphg – prepharyngeal gland; pphgo – prepharyngeal gland opening; sc – scapus;
svd – salivary duct; toa – torular apodeme.

Brachyponera luteipes
The small mandibular gland (mdg, Fig. 5C,D, difficult
to identify on sections) is placed more posteriorly thanits equivalent in Formica, and the duct is slightly bent
several times. The lateral mandibular pit is associatedwith an internal mandibular gland, represented by a thickepithelium below this concavity (mdpg, Fig. S2H). It
is unclear whether the minute pores observed in the pit(Fig. 4F) are associated with the gland. Brachyponera
chinensis lacks the thick epithelium observed in B. lu­
teipes (Fig. S2G), even though the pit looks very similar
externally. The well-developed maxillary gland is morebulbous and formed by less cells (about 35 visible on
sections) (mxg, Figs. 9C,D, 14A). The prepharyngeal (/propharyngeal) gland is smaller and concentrated in thespace between the buccal tube and the remaining prepharynx (pphg, Figs. 11F – G, 12B,D, 14A,C); the cells
are very densely packed. The pharyngeal (“postpharyngeal”) gland is a single sack-like structure with several
flat lobes (phg, Figs. 11F – G, 12B, 14B); the largest, unpaired lobe reaches posterad above half of the length of
the brain; smaller lobes reach anterolaterad close to the158

mandibular insertions below the eyes, and small ventral
lobes are present around the anterior pharynx between
the brain and the infrabuccal pouch; the gland is formed
by a thick epithelial layer surrounding a reservoir space;its surface is more irregular, with densely arranged small
knobs reflecting the shape of the epithelial cells.

3.12. Brain and suboesophageal complex
Formica rufa
The brain (br, Figs. 11D,E, 12A) forms a very compact
unit with the suboesophageal complex (sog, Figs. 11D,E,
12A), with only a narrow central passage for the pharynx;
it appears small in relation to the size of the head, occupying ca. 5% of the lumen; due to the bulging mushroom
bodies and large olfactory lobes it appears peanut-shapedin dorsal view. The optic lobes are well-developed, withthe closely adjacent lamina, medulla and lobula forming a thick cylindrical tract connecting the brain withthe compound eyes, surrounded by fibers of 0md1; themedulla is the largest of the optic neuropils; three thin

ARTHROPOD SYSTEMATICS & PHYLOGENY — 78 (1) 2020

ocellar nerves originate on the dorsal side of the brain between the mushroom bodies (ocn, Fig. 11C – D). The antennal nerves extend from their origin on the anterior tipsof the deutocerebral subunits (nan, Fig. 11C,D) throughthe scapus into the pedicellus and flagellum. The frontal commissures originate mesad the antennal nerves (fc,Fig. 11D); anterodorsally they connect with the frontal
ganglion (fg, Fig. 11C,E), which is asymmetric: most ofits mass is concentrated laterad 0bu2 rather than centrally
dorsad the pharynx, where it is very thin between 0hy9
and 0bu2. Medially closely adjacent connectives link the
suboesophageal complex with the prothoracic ganglion.

Brachyponera luteipes
The brain (br, Figs. 11F – G, 12B, 14B) fills out much
more space of the cephalic lumen than in Formica, ca.
23% of the entire volume; its shape is elongate-rectangular and it reaches the level of the compound eyes anteriorly. The olfactory lobes and mushroom bodies are verylarge and similar in their general shape to the corresponding structures of Formica (no detailed analyses of neuroanatomy were conducted in this study). The optic lobes
are less well-developed, present only as indistinct swellings of a thin optic nerve; lobula and medulla are very
close to the lateral protocerebrum; the lamina is weakly
developed close to the compound eye; due to the anterior
placement of the eye the thin optic nerve is rather long; it
curves laterad to fit through the bundles of 0md1. Ocelli
and ocellar nerves are missing. Due to the location of thebrain in the anterior cephalic region the antennal nerves
are very short; they arise as single thick strands but divide into two branches shortly after their origin on the
deutocerebrum (nan, Fig. 11F,G). The frontal commissures at the level of the anatomical mouth opening have anearly vertical orientation. The frontal ganglion is placed
medially above the anatomical mouth opening (fg, Fig.
11F,H). The suboesophageal complex appears elongatedlike the brain (sog, Figs. 11G,H, 12B).

3.13. Fat body
Both species
Fat body cells are concentrated in the anterolateral andanterior regions of the head; they also fill out most of thelumen of the mandibles and they are also present in partsof the maxillae and the labium; they are loosely arranged
between other internal structures of the head.

3.14. Tracheae
Both species
The tracheae were not a special focus of this work, butfrom observations on the sections and µCT-scanning
data the main branching pattern appears similar to that
described by Richter et al. (2019). Two main pairs of tracheal branches (HA_0002415) enter the head. One pairruns ventrolaterad very close to the bundles of 0md1, en-ters the anterior head region and releases smaller antennal tracheae. The branches of the other pair split directly
anterad the occipital foramen; the split branches run dorsad the brain; the main branches run below the brain.

4.

Discussion

4.1. Homology based terminology forcomparative anatomical studies
Richter et al. (2019) clarified the homology of the cephalic digestive tract in ants for the first time. This is justone example for persisting questions of the homology ofmorphological structures in ants. Establishing homology
is an important prerequisite for any comparative anatomical work, and also for the formation of meaningful evolutionary hypotheses. Furthermore, using a consistent terminology based on a solid homology framework can help
to make morphological data better suited for analyses bycomputer algorithms (Silva & Feitosa 2019). In the present work, we try to use the terminology suggested by the
HAO wherever possible to support the goal of achieving
a consistent terminology for all Hymenoptera and ultimately all insects. However, it should be noted that theterms in the HAO are not based on homology in the senseof structures being traced back to a common ancestor, andare thus not hypotheses of homology as applied in thepresent work (Seltmann et al. 2012). Several homology
issues are addressed throughout the discussion of the present study (mainly the identity of the hypo­stoma and theidentity of several muscles associated with the cephalicdigestive tract). However, a complete understanding of
formicid cephalic structures linked with general insect
morphology (e.g. Snodgrass 1935; Beutel et al. 2014)
will be a continuous process over the next years and requires more intensive anatomical investigations.

4.2. Volume measurements basedon µCT scansµCT-scans have a great potential for qualitative analysesof the anatomy of insects and other organisms (e.g. Fried­rich et al. 2014; Wipfler et al. 2016). Moreover, they area valuable source of quantitative data such as volumes ofindividual structures. This was used to great effect for instance by Lilico-Ouachour et al. (2018) to demonstratespecialized morphological traits in different castes of thehyperdiverse ant genus Pheidole Westwood, 1839. However, there are several potential technical problems withthis technique that we will discuss briefly. In our case,the contrast of the used µCT scans was not sufficient fora distinct separation of all tissues based on grey scales.
Perfect manual segmentation is possible on principle,
but would be extremely time consuming, and would nothave been justified by the aims of the present study. The
calculated volumes as such (see supplementary material
159

Richter et al.: Head anatomy of Formicidae

File 2) do not reflect the actual volumes of the scanned
structures perfectly. Furthermore, the preparation method
has a strong impact on the volume of internal structures.
Different preparation methods such as fixation, staining
and drying (or scanning in ethanol as was done in thepresent study) can influence the contrast as well as potential shrinkage, significantly influencing the resulting
volumes (see e.g. Sombke et al. 2015; Gutiérrez et al.
2018). An additional problem in the present study is that
only one specimen of each F. rufa and B. luteipes wereentirely segmented, thus disregarding potential intraspecific differences. An example for potential problems isthe apparently non-functional “Muscle X” found in one
F. rufa specimen. While in our case this aberrant feature
could be clarified in comparison with a series of otherspecimens, other irregularities might be more difficult tointerpret and could affect qualitative as well as quantitative morphological results. Taking all these problems
into consideration, we used only very clear tendencies in
the calculated volumes, also supported by visible relative
size differences between the two species to draw tentative
conclusions. In the previously mentioned study of Lilico-Ouachour et al. (2018), the authors do not give any
details on how structures were segmented before volume
calculation. The images (figs. 2 – 5) clearly show that not
all muscles were reconstructed. Especially the musclesof the maxilla and labium are missing completely (intrinsic muscles) or at least partly (extrinsic muscles). While
this is not necessarily a problem for the purpose of thatparticular study, we advocate that in the future, all aspects that can potentially influence the results of volume
calculations based on µCT-scans should be reported, toincrease the comparability of the results.

4.3. The head capsule
One of the most prominent features of the ant head is its
prognathism, with the mouthparts rather than the antennae placed at the forefront of the head. This was alreadyrecognized as apomorphic feature of Formicidae by
Bolton (2003), Boudinot (2015), Richter et al. (2019)and others. Among many plausible factors, prognathism
is often associated with ground-dwelling and/or predacious habits (e.g. in carabid beetles, Evans 1994) or witha preference for subcortical habitats (Beutel 1997). It isa feature that has evolved multiple times independently
across insects with different functional and morphological backgrounds (e.g. Beutel et al. 2014). In contrast toother groups of Holometabola (e.g. beetles and Ra­phi­dio­ptera), the prognathous orientation in ants is achievedby a distinct elongation of the postgenal bridge, resulting
in anteriorly directed mandibles opposite to the posteriorly directed postocciput. The exact position and angleof the occipital foramen on the back of the head is variable across Formicidae and possibly related to their specific lifestyle. The prognathous configuration also leads
to far-reaching changes in the orientation of internal cephalic structures such as the tentorium, muscles, brain,160

and digestive tract (see below). As in other prognathous
insects, ants show a high diversity of mandibular shapes,
particularly with regard to pronounced mandibularelongation (Formicidae: Gotwald 1969; Megaloptera:Contreras-Ramos 2011; Carabidae: Ball et al. 2011).
Prognathism of ants is linked to profound changes suchas the shift of the origin of M. craniomandibularis ex­
ternus 0md3 to the postgenal ridge, instead of the lateralhead capsule as in other Aculeata (Zimmermann & Vilhelmsen 2016). This very unusual condition potentiallyfacilitated the diversification of mandibular shapes in
ants. The prognathous head orientation is arguably a key
feature of ants and of high relevance to our understanding of their evolution. However, as pointed out above,the moderately inclined head of ants, differs distinctlyfrom a horizontally oriented prognathous and posteriorly
retracted head as found in other groups such as Coleo­
ptera and Raphidioptera.
Several other features of the head capsule are characteristic for ants and shared among all subgroups. A
concave occipital region, the lack of epicranial sutures
as zones of weakness associated with ecdysis, and the inflected clypeus are ground plan features of Hymeno­ptera(e.g. Beutel & Vilhelmsen 2007; Vilhelmsen 1996). The
general configuration of the occipital foramen is sharedwith many other apocritan groups (Zimmermann & Vilhelmsen 2016): it is narrow, more or less dumbbell or
hourglass-shaped and enclosed by the collar-like postocciput. However, the exceptionally strong constrictionof the postocciput is a potential autapomorphy of ants,
further narrowing the foramen and increasing the movability of the head. In worker ants, the combination of avery narrow postocciput and large prothorax with strongly developed cervical musculature (Keller et al. 2014;
Liu et al. 2019) leads to a very flexible head-to-thorax
articulation, allowing ants to lift and carry loads with
their anteriorly directed mandibles (e.g. Nguyen 2014).
An aspect of ant cephalic morphology that has rarely
been discussed previously is the raised middle portion of
the clypeus and anterior frontal region. Prentice (1998)
associates a strongly convex clypeus with the presenceof a proboscis. This condition creates space for strongly
developed muscles of the sucking pump, required for operation. As the clypeus is at least “moderately convex”
in most apoid taxa (Prentice 1998), which also applies
to Formica, this condition is probably generally related
to the uptake of liquid food (Paul & Roces 2019). Thevery strongly raised condition in ants such as species ofBrachyponera and other ponerines is possibly rather aside effect of the antennal approximation than an adaptation to a specific mode of food uptake.
Another insufficiently studied feature is the hypo­stoma. Its externally visible part, the hypostomal carina,
is sometimes used as a taxonomic character (usuallyreferred to as hypostoma, e.g. Eguchi 2006; Lucky &Ward 2010). The hypostoma forms the ventral marginof the oral foramen. It is distinctly concave in ants, forming a distinct emargination (hyc, Figs. 5F,H, 11A,B) together with the usually broad hypostomal carina (hysc,

ARTHROPOD SYSTEMATICS & PHYLOGENY — 78 (1) 2020

Figs. 1C,F, 2C,F, 11A,B). This hypostomal cavity (hyc,Figs. 5F,H, 11A,B) receives and protects the base of themaxillolabial complex, mainly the cardines and the postmentum, especially when the complex is retracted (Fig.
9B,D,F,H). Another feature of the hypostoma, alreadydescribed in detail for Wasmannia affinis (Richter et al.
2019), are the triangular processes (previously interpreted as part of the postgena) reaching into the oral foramen,
which partially separate the mandibular cavities from themain oral foramen (hysp, Fig. 11A,B). In all examined
species (including other aculeatans with this condition,
Osten 1982), the lateral margins of these processes interact with the mandibular bases, stabilizing the mandibles
as they operate. Their medial margins interact with thestipes when the maxillolabial complex retracts. However,among the three species examined, a distinct concavity ofthe process fitting with the stipes is only present in For­mica. This interaction is probably part of a tight closing
mechanism of the oral foramen (see below). The shape of
the processes is highly variable across ants, with a steep
lateral margin in both Wasmannia and Formica, but amuch more gradually ascending margin in Brachypon­era. Brachyponera is also lacking an additional smaller
process bordering the pleurostomal fossa mesally, which
is present in species of both other genera (white arrowhead, Fig. 11A). The differences in the shape and size ofthe processes may have an effect on mandibular movements likely relevant in a functional context.
The frontal carinae are are almost always present in
ants. They probably increase the mechanical stability of
the cuticle of the frontal region, especially around the antennal foramina which sustain the forces of the usuallylong antennae. The antennal insertions are obviously another key feature in the evolution of ants. Keller (2011)
analyzed this character system in great detail, revealing the phylogenetic relevance of the torular or frontal
lobes and the antennal acetabulum. Further analyses ofthis character system appear highly promising. The present study also revealed some differences in the antennal musculature. While the extrinsic antennal musclesof Formica are similar in size, one muscle is distinctly
larger in Brachyponera. Moreover, all muscles are more
steeply oriented in the head, related also to varying tentorial shapes. These differences are likely linked with thepattern of antennal movements. This was previously investigated by Ehmer & Gronenberg (1997) in the context
of the trap-jaw mechanism of Odontomachus. Fast retraction of the antenna is important in this case to avoid damage of the appendage by the powerful mandibular strike.
Although the authors did not homologize the muscles,
they describe an enlarged “retractor” muscle which appears to correspond to the enlarged 0an3 in Brachypon­era. A more detailed analysis of the antennal articulation
and associated musculature has certainly the potential to
improve our understanding of ant evolution. The antennae of ants are not only a very important sensory organ(e.g. Hashimoto 1990), but also used in communication(e.g. Lenoir 1982), underscoring their importance for theevolution of social behavior.

A highly variable feature is the shape of the head.
While the external contours often reflect underlying features such as specific muscular arrangements (see present
work; Paul 2001; Richter et al. 2019), its phylogenetic
information content is questionable due to the strong
adaptive aspect. Nevertheless, the highly variable andspecific shapes of the head capsule often provide useful taxonomic information (e.g. Brown 1948; Longino
2006). Aside from this they can be relevant in a functional and evolutionary context, as recently demonstrated
by analyses of the integration of different body parts inthe ant genus Pheidole Westwood, 1839 (Friedman et al.
2019).
One of the least studied character systems of ants isthe cephalic endoskeleton. Its main functions are stabilizing the head capsule and providing space for muscle attachment (such as the antennal muscles discussed above),as it is almost always the case in insects (e.g. Beutel etal. 2014). The present study and the previous work on
Wasmannia reveal some noteworthy differences. The tentorium varies considerably in shape. The general configuration with long anterior arms (ata, Figs. 5E – H, 11A,B,
13B, 14B, S2) with lamellar extensions (ml, ll, Figs.
5E – H, 11A,B, 13B, 14B), a short bridge (tb, Figs. 5E – H,
11A,B), and short posterior arms (pta, Fig. 5E – H) is very
similar across all species and also shared by other groupsof Aculeata (Zimmermann & Vilhelmsen 2016). What differs from other aculeates, however, is the frequent reduction of the dorsal arm (dta, Figs. 5E,F, 11A, S2F). Where­
as it is distinctly developed in Formica, Lasius (Kubota
et al. 2019), and also Opamyrma Yamane, Bui & Eguchi,
2008 (Yamada et al. 2020), it is entirely missing in the
myrmicines Wasmannia (Richter et al. 2019) and Melis­sotarsus Emery, 1877 (Khalife et al. 2018). In Brachy­
ponera, a triangular base of the dorsal arm is visible at theposterior end of the lateral lamella, followed by a very thin
and relatively short anteriorly directed tube. This particular configuration of the dorsal arm may be linked with thelarge size of the brain in Brachyponera, reducing the space
for the dorsal arm. Other differences concern the shapeand extension of the lamellae of the anterior arm. They
are very strongly developed in Brachyponera, smaller in
Formica and Lasius (Kubota et al. 2019), and thin but
rather elongated in Opamyrma (Yamada et al. 2020). The
lateral lamella is completely reduced in Wasmannia andapparently also in Melissotarsus (Khalife et al. 2018). Interestingly, the secondary tentorial bridge (Zimmerm­ ann& Vilhelmsen 2016: figs. 1, 2, stb) is reduced in all investigated species except for Melissotarsus (Khalife etal. 2018), and the posterior processes in all species but
Formica. Both structures are almost always parts of theaculeatan tentorium (Zimmermann & Vilhelmsen 2016).
At present, the available information is not sufficient for areliable evolutionary interpretation of endoskeletal character transformations across Formicidae. The structuralaffinities between Wasmannia and Melissotarsus on one
hand, and between Formica and Lasius on the other, suggest that the tentorial shape may be generally conservedon the subfamily level.
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Richter et al.: Head anatomy of Formicidae

Another structure of the cranial endoskeleton that hasreceived little attention is the torular apodeme. This internal expansion of the antennal acetabulum was foundin all species we have investigated to date, but is almost
entirely neglected in the literature. It is noteworthy that it
was already described for formicines by Lubbock (1877),
but obviously forgotten afterwards. While the process
is just an elongation of the acetabulum in Wasmannia(Richter et al. 2019: phragma of the acetabulum), it is adistinct, flattened process in Brachyponera and a distinct
tubular process in Formica. The single obvious functionof the process is that it serves as area of origin for M.
tentoriooralis (0hy2). According to Lubbock (1877) its
free end bends when the antenna is moved and elasticallyreverts to its original position, moving the entire appendage back to its resting position. However, the process isneither connected to the antennal muscles, nor directly tothe antenna. It thus appears unlikely that it plays a role inantennal movements. An alternative interpretation would
be that this structure stabilizes the torulus/ antennal insertion. Further investigations will be necessary to clarify
the diversity and function of this remarkable structure
which is apparently unique to ants. Especially the functional consequences of the shift in origin of 0hy2 shouldbe evaluated.

4.4. The mouthparts
The mandibles apparently played a key role in ant evolution. The highly modified secondary (dorsal) mandibular joint, described in detail for Wasmannia, is certainly
an apomorphy of Formicidae. It distinctly increases thedegrees of freedom at the mandibular base and thus theflexibility of mandibular movements (B. Wipfler pers.
comm.), probably contributing to the potential of thesemouthparts as an all-purpose tool (Gronenberg et al.
1997). A more detailed analysis of this specific character is currently in preparation (R. Keller and coworkers).
The importance of the mandible in the evolution of Formicidae is underscored by its sheer structural diversity.
The impressive spectrum of shape variations (Gotwald1969) includes forms with a strong degree of elongation (e.g. Lattke et al. 2018), different trap-jaw species(Larabee & Suarez 2014), and some cretaceous fossils
with bizarre and highly derived mandibles (e.g. Barden& Grimaldi 2016; Cao et al. 2020). Understanding theevolution of this diversity will likely lead to important
insights in ant evolution in general. The mandibles of
the generalist species investigated in the current work
are both of the characteristic triangular, torqued form as
defined by Keller (2011). An interesting question in thiscontext is whether this condition evolved once in the lastcommon ancestor (with multiple secondary variations),
or independently in the poneroid and formicoid clades,
respectively. The latter case would imply a remarkablescenario of parallel evolution. The current study did notreveal any structural features supporting convergence.
Consequently, the triangular condition is a potential
162

synapomorphy of the poneroid-formicoid lineage, i.e.
Formicidae excl. Martialinae and Leptanillidae (B. Boudinot, pers. comm.). As a triangular mandibular shape is
an isolated phenomen in related groups (e.g. some Mutillidae and Vespidae) and narrow curved mandibles occur in different stem group ants and also in Leptanillinae
and Martialinae (B. Boudinot pers. comm.; Rabeling etal. 2008; Cao et al. 2020), the triangular shape is possibly not part of the formicid groundplan. Observed differences such as the angle and length of the masticatory
margin plus dentition are most likely related to dietary
preferences. Further minor differences can be found in
the articulatory surface, such as a deeper mandibular acetabulum. Interestingly, a specific depression anterior tothe mandalus (the trulleum in Wasmannia and a muchsmaller crescent-shaped depression in Formica) is present in formicoid species, whereas this structural modification is completely missing in Brachyponera. The trulleum is likely linked with an additional stabilization of
the mandible via a canthellus in some myrmicine ants
(Richter et al. 2019). However, it cannot be excluded
that a depression anterior to the mandalus is functionallyassociated with mandibular gland secretion. An interesting character of the mandible of Brachyponera is thepresence of a lateral mandibular pit. This concavity is
characteristic for several ponerine lineages (Cryptopone
Emery, 1893, Euponera Forel, 1891, Pseudoponera Emery, 1900; Schmidt & Shattuck 2014). It is much deeperin B. sennaarensis than in the investigated B. luteipes
and B. chinensis (Billen & Al-Khalifa 2016: fig. 1c). In
B. sennaarensis and luteipes, the pit is associated with an
intramandibular gland of unknown function. Examined
serial sections of B. chinensis did not show an intramandibular gland (possibly an artefact of fixation), suggesting considerable intrageneric variation in this character.
This underlines the necessity to investigate even closely
related taxa for a reliable reconstruction of the evolutionary history of specific features.
The high diversity of mandibular shapes is also reflected by linked internal structures, especially the large
apodeme of 0md1 and the three mandibular muscles. The
presence of different fiber types has been investigated indetail by Gronenberg et al. (1997), and some differencesin the apodeme configuration were documented by Paul& Gronenberg (1999). Although our data are not sufficient to perform detailed analyses of fiber type configurations, they fit in the picture outlined in these previous
works. A general feature of ants is the presence of one
lateral and one mesal strand of fibers with fast contraction properties, and attached directly to the main body ofthe 0md1 apodeme. These are surrounded by insertion
sites of fibers with different degrees of slow contraction
properties, attached either directly or via thread-like extensions of the apodeme. Like Gronenberg et al. (1997)
we mainly found directly attached fibers in the ponerine
Brachyponera, and mostly fibers attached via threads
in Formica. The suggestion of different trends in fiber
type distribution across different phylogenetic lineages
thus gets new support. Concerning the apodeme confi­

ARTHROPOD SYSTEMATICS & PHYLOGENY — 78 (1) 2020

gu­ration, a tri-partition of the adductor tendon is likely
a general feature in ants. However, at least one of thethree branches is very small in most groups, and a clear
phylogenetic pattern with respect to the length or shapeof these subdivisions is presently not recognizable. Aninteresting feature is the presence of an accessory branchoriginating dorsally on the central branch, attachment
site of an isolated occipital fiber bundle of 0md1. Thishas been found in Wasmannia (Richter et al. 2019), For­mica and Brachyponera. Paul & Gronenberg (1999) described it only for Diacamma sp. and Ectatomma ruidum(Roger, 1860). It is conceivable that it was overlookedin other species and is in fact a very conserved feature.
Further study of this character system will likely yieldinteresting insights into the specific functional propertiesof the ant mandible and potentially also phylogenetically
relevant features.
Compared to the mandibles, the maxillolabial complex of ants is clearly conserved. While different modesof proboscis formation occur in many other aculeategroups (e.g. Osten 1982, 1988; Prentice 1998; Krenn
2002), this condition is unknown in ants. While adultants take up only liquid food (e.g. Paul et al. 2002; Paul& Roces 2019) like their aculeate relatives, ant species
feeding on plant sap mainly consume sugary exudates ofplant sucking insects or plant sap from open “wounds”,
rather than taking up nectar directly from flowers (Lachet al. 2010). Furthermore, their small body size often
enables them to crawl into flowers to reach nectar. The
selective pressure to reach deep into flowers with themouthparts and to evolve a proboscis as a consequenceis thus limited in ants. Another possible explanation is anancestral soil dwelling lifestyle (Lucky et al. 2013), making plants a much less accessible food source compared
to flying insects. What can be found instead in many ant
species is a tight closure of the preoral space. Keller(2011) documented different interlocking mechanisms
linking the labrum with the maxilla. This can be achievedby interaction of the labral processes with an extended
distal stipital margin (as in Brachyponera), the flat lateralpart of the stipes (as in Wasmannia, Richter et al. 2019),
or with the base of the maxillary palp (as in Formica).
Additional locking mechanisms can include stipital andpremental grooves interacting with the distal labral margin (Keller 2011; Richter et al. 2019). A tightly closed
preoral space is probably advantageous during activities like digging in the ground (Osten 1982) or hunting,protecting unsclerotized elements of the mouthparts andbuccal cavity.
The presence of small denticles on the surface of the
cardo of ants was described by Richter et al. (2019) for
the first time. Here we confirm that this surface sculpture
also occurs in Formica, and that additional small setae
occur in Formica and Brachyponera. Such an armatureon the proximal maxillary element is apparently common
in ants. While small setae may play a role as proprioceptors, the denticles possibly reduce energetic output forholding these maxillary parts in a particular conformation (B. Boudinot, pers. comm.).

Some variation in the armature of the maxillolabial
complex was already documented by Gotwald (1969).
Different specific structural elements such as a galealcrown, galeal comb, and lacinial comb are also present
in other apocritan groups (e.g. Popovici et al. 2014). An
interesting aspect is the structure of the lacinial comb.
While it is usually formed by setae (e.g. in Formica), we
observed a comb formed by thick cuticular spines without
articulation in Brachyponera. Gotwald (1969) did not
mention unarticulated spines for any other ant species.
However, illustrations of maxillae of many species now
assigned to Ponerinae (e.g. Hypoponera opacior [Forel,1893], Platythyrea schultzei Forel, 1910, Simopelta oc­
ulate Gotwald & Brown, 1967, Neoponera commutata[Roger, 1860]) show rather thick spines with a broadbase. It is thus conceivable that Gotwald misinterpreted
fixed spines as thickened setae, and that all these species
share the condition found in Brachyponera. This suggests an interpretation as an apomorphy characterizing
at least Ponerinae and possibly additional groups. A linkwith carnivorous habits is likely, as the concerned species
are mostly predators (e.g. Schmidt & Shattuck 2014). A
character with a certain degree of variation is the patch of
microtrichia on the internal/ventral surface of the lacinia.
While it is largely reduced in Brachyponera, there are
two patches (one larger, one of moderate size) in Formi­
ca and one moderately sized patch in Wasmannia. While
the microtrichial patch as such is obviously a conserved
groundplan condition in ants, its specific configuration
is likely correlated with functions of the lacinia, such as
food uptake or cleaning the body surface.
The musculature of the maxilla is highly conserved in
ants. Our findings conform with previous results such as
Paul et al. (2002) and Richter et al. (2019). The only distinct variation concerns 0mx4, which can be undivided
or composed of two separate bundles, with two tendons
even in the case of a single subunit (see Brachyponera).
However, in all cases the tendons insert on the stipes very
close to each other, and they even fuse in Formica. Variations in size, specific shape or also sarcomere length (seePaul et al. 2002) of the individual muscles may be interesting in a functional context. Omx1 for instance is larger
in Formica than in Brachyponera (relative to head size),
and an oblique 0mx3 is a characteristic of Wasmannia(Richter et al. 2019).
The sensilla of the maxillary and labial palps show
some variation, especially in the density of setae on the
different palpomeres. Our observations suggest a trend
towards reduced numbers in species with smaller bodysize (Brachyponera, Wasmannia, Richter et al. 2019),
whereas setae are densely arranged on all palpomeres inlarger species, as for instance in Formica. The sensilla ofthe labial palps were used for phylogenetic reconstruction by Hashimoto (1991). His coding of the density ofsetae for different subfamilies agrees with Richter etal. (2019) and the present study. However, our (unpublished) observations on Acromyrmex aspersus (F. Smith,1858) show that this larger myrmicine has a rather dense
pattern of microtrichia on its palpomere, in conflict with
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Richter et al.: Head anatomy of Formicidae

Hashimoto’s coding for the subfamily. This indicates thatthe character shows more variation than previously suggested.
The labial muscles are largely conserved like those of
the maxilla. While 0la5 originates on the posterior tentorium in A. aspersus, its usual area of origin is apparently
the postgenal area close to the occipital foramen. Other
differences concern mostly the size and sarcomere length(Paul et al. 2002), potentially interesting in a functional
context, but likely phylogenetically uninformative.
The infrabuccal pouch of ants has been studied withrespect to its function (e.g. Eisner & Happ 1962; Quinlan & Cherret 1978; Febvay 1981). The morphology
and ultrastructure were recently investigated (Wang etal. 2019). Though some of the morphological interpretations of Wang et al. (2019) are questionable (e.g. labelling the galeolacinial complex as “hypopharyngeal
plate”, fig. 3B), they provide a detailed documentationof different kinds of microtrichia and the cuticular surface structure of different areas of the pouch. The results
generally conform with our observations. Groups of microtrichia on cuticular scales are visible on the surfaceof the distal part of the pouch, whereas honeycomb-likeplates with more or less distinct folds are present proximally, with a transition area between the two patterns.
While the infrabuccal pouch is usually referred to as a“filtering device”, actual filtering is achieved by microtrichia and setae of the mouthparts and on the epi- andhypopharynx (including the buccal tube), with the pouch
functioning as a storage device for the processed material. Wang et al. (2019) demonstrated that the size of thepouch varies depending on the amount of material it contains. This makes comparisons between species difficult,even though distinct differences among taxa do occur.
In Formica, for instance, the pouch is unusually large,
thus displacing parts of muscle 0bu5/ 6. Another interesting variation is the presence of three folds on the dorsalside of the pouch in Brachyponera, which were observedin all investigated species and specimens of the genus.
These structures probably facilitate the enlargement of
the pouch when it contains a large mass of substrate. The
position of such “reserve folds” is possibly conserved, atleast at species level.
A structure rarely described in detail is the salivary
duct. Even though this structure is apparently rather
uniform, certain differences can be found within ants. A
similar condition is found in Formica and Brachypon­
era, where the duct runs through the ventral region of thehead capsule, forming a loop anterad the brain and finally connecting to the sclerotized salivarium. The wall isformed by a thin cuticle and a layer of tissue cells with adecreasing thickness at the level of the labium. The loop
is partly vertically oriented in Formica, which is not thecase in Brachyponera, probably due to the limited space
between the brain and infrabuccal pouch. A slightly different condition is found in Wasmannia affinis: the duct
appears thin throughout its entire length (although different protocols for histological sections may play a role)
and a loop is missing completely (Richter et al. 2019).
164

The loop probably facilitates the extension of the ductwhen the maxillolabial complex (including the salivarium) is protracted. However, this raises the question why
it is absent in some species. Further investigations of thesalivary duct may clarify functional issues and possibly
provide phylogenetic information.
Another interesting structure linked with the salivarium is muscle 0hy7. It is generally reduced in Aculeataaccording to Zimmermann & Vilhelmsen (2011), but amuscle originating on the prementum and inserting on
the salivarium was identified in Wasmannia (Richter et
al. 2019) and now also observed in Formica. It is missingin Brachyponera, but at least a very small muscle in thisposition is present in some other ponerines we examined.
The current data suggest that this muscle may be absent
in the groundplan of Aculeata, with reversal in ants. It
is probably used to open the salivarium, indicating thatan active opening of this cavity is important in many ant
species, but apparently not in all of them (see Brachypon­era). More work is required to assess the precise functional background of this muscle and its potential phylogenetic significance.
The labrum of ants is an interesting character complex for different reasons. In most ants it forms different
interlocking mechanisms with the maxillolabial complex, and various modifications have evolved with different functional backgrounds. The rectangular, bilobed
shape found in generalist species investigated here isconserved in most groups (see also Gotwald 1969). As
this shape also occurs in outgroup taxa (e.g. the crabronid Pison spinolae Shuckard, 1837, Cowley 1959), it ismost likely a groundplan condition. The labrum of someof the trap-jaw ants of the myrmicine Attini underwent
profound transformations to act as latch mechanism fortheir power amplified mandible strike (e.g. Gronenberg
1996; Bolton 1999). Interestingly, the modifications inStrumigenys F. Smith, 1860 and Daceton Perty, 1833
were considered as homologous when these genera werestill assigned to the same tribe (e.g. Bolton 2003). However, it is now known that Strumigenys is not closely related to the “dacetines” (Ward et al. 2015), and closer
scrutiny revealed differences in the labral modifications(J. Katzke unpubl. observation), even within the genus
Strumigenys (E. Economo unpubl. observation). Very diverse labral shapes have evolved in the Basiceros genus
group (e.g. Longino & Boudinot 2013), closely related toStrumigenys (Ward et al. 2015). This has recently been
investigated in detail for Basiceros (Probst et al. 2019),
revealing adaptions of the labrum for prey capture and a
distinct phylogenetic signal in the character system within
the genus. The authors discuss the possibility of a second
pair of labral muscles (“anterior frontolabral muscle”)
based on the presence of a second pair of tendons on thelabrum (Probst et al. 2019: fig. 2G), without presenting
direct evidence. The presence of such a muscle appears
highly unlikely considering its absence in all other ants
and in fact Aculeata (Zimmermann & Vilhelmsen 2016;
Richter et al. 2019). This underlines the importance andpotential of detailed anatomical documentation.

ARTHROPOD SYSTEMATICS & PHYLOGENY — 78 (1) 2020

Some noteworthy differences of the epipharynx wereobserved in the present study. In most ants the membranous main part of this structure, i.e. the part not fusedto the hypopharynx to form the prepharyngeal tube, iscovered with rows of minute microtrichia (see Formi­
ca, Wasmannia, Richter et al. 2019). Additionally, theepipharynx of Brachyponera bears a band of much longer microtrichia that extends along the anterior margin of
the functional mouth opening and expands into a more orless triangular patch around the epipharyngeal midline.
The microtrichia of the preoral cavity are usually interpreted as part of a filter apparatus preventing solid particles from entering the digestive tract (e.g. Eisner & Happ
1962; Wang et al. 2019). A varying vestiture might thus
be associated with the preferred food, which is probably
mainly arthropod prey in the case of species of Brachy­
ponera (Schmidt & Shattuck 2014). It is noteworthy inthis context that B. luteipes (Zhou et al. 2007) and also
B. senaarensis (Dejean & Lachaud 1994) belong to thefew ponerine species also feeding on seeds. However,whether the specific armature of microtrichia is related tothe preferred diet, cannot be clarified with the currently
available data. It is also important in this context that recent studies show that ponerine ants rely more on plantmaterials as food than was previously assumed (Hanischet al. 2019). Therefore, possible correlations between
structures involved in feeding and carnivorous habits
should be carefully considered in each case.

4.5. Cephalic digestive tract, glands andnervous system
The prepharynx of ants (pph, Figs. 11C – H, 12A – D,
13A,B, 14A,D,E, S1G,H, S2A,B), previously interpreted
as the pharynx (ph, Figs. 11C – H, 12A, 14B, S1G,H,
S2C), was only recently correctly homologized (Richter et al. 2019). Many features of this part of the digestive tract have not been investigated in detail so far. The
buccal tube (bt, Figs. 11D,E,G,H, 12A – D, S1G,H) has
been considered as a distinctive prepharyngeal subunit
based on a sharp bend separating it from the remaining
prepharyngeal tube. This is more strongly pronounced in
ants than in other groups of Aculeata, where the anteriormost part of the cephalic digestive tract is also moreor less distinctly curved (Zimmermann & Vilhelmsen
2016). It is evident, however, that the sharp bend in ants
is associated with the deep retraction of the maxillolabial complex, as it almost completely disappears when
this structure is in a protracted position (see Fig. 12B,D).
Comparisons with other aculeates that strongly retract
the maxillolabial complex (e.g. Mutillidae, Osten 1982)
could help to clarify whether this condition is unique toants, or whether a sharp bend in the prepharynx is generally associated with strongly retracted ventral mouthparts.
Another prepharyngeal character complex is the sclerotization pattern. The sitophore plate, the sclerotizedventral (hypopharyngeal) ventral wall of the prepharynx(Fig. 12A,B), is a groundplan apomorphy of Hymenoptera(Vilhelmsen 1996; Beutel & Vilhelmsen 2007). However, the epipharyngeal dorsal wall of the prepharynx also
displays distinct sclerotized elements in ants, just posterior to the buccal tube. In Wasmannia this is a simple
flat sclerite. In Brachyponera this structure additionallybears a sclerotized keel along its midline (epk, Figs. 11H,
12B, 14A,D). In Formica this area is rather weakly sclerotized and hump-shaped. In all three species this is theinsertion area of a part of M. clypeopalatalis (0ci1) andalso of the longitudinal pharyngo-epipharyngeal muscle,
which is generally very large in Hymenoptera (Beutel& Vilhelmsen 2007). Even though the presently available information is insufficient for a reliable evolutionary
interpretation, this character system should be carefully
considered in future studies. A possible explanation forthe presence of a sclerotized keel in Brachyponera is thestrong median constriction of the clypeus, which results
in a very narrow M. clypeopalatalis (0ci1). The keel
provides additional surface for the insertion of its fibers.
The most diverse sclerotized prepharyngeal structures
are the oral arms (oa, Figs. 1C,D,F, S1G,H). These hypopharyngeal sclerites stabilize the prepharynx and theirposterior parts are insertion sites of the oral muscles 0hy1
and 0hy2. Additionally, these posterior parts of the arms
enclose the opening of the pharyngeal gland, indicatingthat the oral muscles are likely involved in opening andclosing the aperture of this gland (Verhaegen unpubl. observations). The distinct differences in the shape of thesearms may contain phylogenetic signal and are potentiallyrelevant in the context of dietary specializations. The
only detailed comparative study on these structures by
Porto et al. (2019) was focused on Apoidea. The authors
could demonstrate that some of the changes in the shapeof the oral arms (pharyngeal rods in their terminology)
and the general configuration of the hypopharynx areuseful characters on higher and/or lower taxonomic levels (Porto et al. 2019).
A character system not investigated by Porto et al.
(2019) but certainly of interest in ants is the prepharyngeal and pharyngeal musculature (Figs. 11 C – H, 13, 14).
In addition to minor variations in shape and size of theindividual muscles, some characters may have distinct
functional implications and also contain phylogenetic
signal. A seemingly unusual feature of ants is the subdivision of M. clypeopalatalis (0ci1) into an unpaired
subcomponent inserting at the functional mouth opening(part a, retractor of buccal tube of Paul et al. 2002) and
one part inserting on the epipharyngeal sclerite discussed
above (part b). In other groups of Aculeata, the anterior
component was interpreted as 0ci1 and no posterior partwas described (Zimmermann & Vilhelmsen 2016). The
only muscle inserting on the dorsal prepharyngeal wall
described by these authors is M. clypeobuccalis (0bu1),
which is sometimes subdivided into two individual bundles (Zimmermann & Vilhelmsen 2016). It is conceivablethat these seemingly different configurations are rather
due to homologization problems than to evolutionarychanges in the character system. Beutel & Vilhelm165

Richter et al.: Head anatomy of Formicidaesen (2007) described M. clypeopalatalis (M. 43/ 0ci1)
as consisting of two subunits in basal hymenopterans,compatible with the presence of two separate bundles
in most insects (v. Kéler 1963). This suggests that ourinterpretation of the anterior unpaired muscle and of theanterior paired bundle as two subunits of 0ci1, and theposterior paired bundle as 0bu1 is consistent with thegeneral interpretation in the literature (e.g. Beutel etal. 2014). Another interesting case is M. tentoriooralis(0hy2). It was previously interpreted as a subunit of M.
frontooralis (0hy1) by Richter et al. (2019), even thougha possible interpretation as 0hy2 (M. 47) was discussed
in that study. Our investigations show that the frontocly­
peal ridge, which is the area of origin of this muscle insome other aculeate goups (Zimmermann & Vilhelmsen
2016), is very closely adjacent (Formica) or even fused(Brachyponera) to the antennal acetabulum, which also
carries the torular apodeme serving as area of origin of
the muscle in question in ants. Considering this configuration, a shift from the frontoclypeal ridge to the torular
apodeme, consistent with a homologization as 0hy2, ismuch more likely, than a shift from the middle frontal region, which would be implied by a homologization with
0hy1. It is an interesting question whether the muscle
shifted its origin first and the torular apodeme evolved
after this to optimize the attachment site, or whether themuscle shifted to a preformed torular apodeme, which
evolved for a different reason in the first place? Comparisons with other poneroid (e.g. Amblyoponinae) or morebasal formicid lineages (e.g. Leptanillinae) plus closely
related aculeate groups (e.g. Scoliidae and spheciform
Apoidea) will likely help to clarify the functional andevolutionary background of the involved structuraltransformations. A muscle affected by size reduction oreven complete loss is 0bu3. It is completely reduced in
Brachyponera and very small in Wasmannia and otherexamined ponerines. In clear contrast, it is large and subdivided into two bundles in Formica and also in Lasius(Janet 1906), suggesting that this may be an apomorphyof Formicinae. The absence of M. verticopharyngalis(0ph1) is very likely an apomorphy of the entire Formicidae, as this muscle is usually present in Aculeata (Zimmermann & Vilhelmsen 2016), but has never been foundin ants so far. One possible reason for this reduction is the
prognathous condition, which results in a shift of different cephalic structures. The brain usually largely fills out
the posterior lumen of the head, resulting in very limited
space for dorsal pharyngeal muscles in this region.
Two large glands are associated with the prepharyngeal/pharyngeal sucking pump, the prepharyngeal andthe pharyngeal gland. The shape of both may be phylogenetically relevant. The prepharyngeal gland almost entirely encloses the wall of the infrabuccal cavity, whereas
most cells are located behind it in the myrmicine Mono­morium pharaonis (Linnaeus, 1758) (Boonen & Billen2016), a condition also found in the myrmicine Was­
mannia affinis (Richter et al. 2019). A placement of two
gland cell clusters laterad the prepharynx and infrabuccal
pouch was observed in Formica and also in Camponotus

166

pennsylvanicus (De Geer, 1773) (Forbes et al. 1961). For
ponerine species only few descriptions of this gland areavailable, but it is apparently concentrated in the space
between the buccal tube and the remaining prepharynx
in different Brachyponera species (Billen & Al-Khalifa 2015). Different shapes and positions of the gland
are likely related to the spatial configuration of internal
structures, for instance the extension of the infrabuccal
pouch.
The pharyngeal gland also varies considerably inshape in different groups of ants. In most described species it appears “glove-like”, with several tubes connecting to a reservoir that opens near the anatomical mouth(e.g. Forbes 1938; Peregrine et al. 1973; Billen & AlKhalifa 2015; Billen et al. 2015; Richter et al. 2019).
This also applies to Formica, where the gland is formedby more than 40 individual tubes. In clear contrast to this,the gland of Brachyponera (including B. sennaarensis
Billen & Al-Khalifa 2015) is sack-shaped, with about
five more or less separated lobes instead of individualtubes. Additionally, the epithelial cells are more globular.
As this general gland shape was also observed in otherponerine species (Gama & Cruz Landim 1981; Schoeters & Billen 1997) (also in Pseudomyrmecinae, Gama
& Cruz Landim 1981), this is arguably a plesiomorphic
condition for the group. The groundplan condition of ants
is most likely the glove-shaped configuration, which was
also found in leptanillines (Billen et al. 2013) and othergroups of Aculeata (e.g. Herzner et al. 2013). Further
study may reveal additional phylogenetically informative
differences.
Some features of the brain are very easy to correlatewith the behavior and lifestyle of the two investigatedspecies. The large optic neuropils are certainly linked
with the importance of visual orientation for F. rufa (e.g.
Nicholson et al. 1999). Brachyponera on the other hand
has about ten times less ommatidia and seems less reliant
on visual cues, showing distinctly reduced optic neuropils and lacking ocelli and ocellar nerves. Interestingly,
the distinct anterior shift of the eye causes the optic nerve
to run at a distinct angle through the fibers of 0md1. Dueto the anterior placement of the eyes, fibers of this muscle can occupy regions that are occupied by the optic
lobes and the compound eye in Formica. This suggests
a possible tradeoff between the visual sense and additional space for attachment of mandibular muscle bundles. Another interesting aspect is the general size of thebrain relative to the head volume. It is well known that
relative brain size increases in smaller species in ants andother groups of organisms (e.g. Wehner et al. 2007; Seidet al. 2011; Lilico-Ouachour et al. 2018). The brain ofBrachyponera looks proportionally very large, occupying most of the posterior lumen of the head. This is evenmore pronounced than in Wasmannia affinis (Richter et
al. 2019), which is even smaller, and thus obviously not
only a result of miniaturization (Polilov 2015). Even
though specific neuroanatomical features of ants were already treated in several studies (e.g. Kelber et al. 2009;
McKenzie et al. 2016), there is probably still potential inARTHROPOD SYSTEMATICS & PHYLOGENY — 78 (1) 2020

comparing size and shape of the brain in different ant lineages. As the brain limits the space for other head structures, its shape is also influenced by the presence andextension of other organs (see optic nerve and 0md1).
Comparative studies could reveal tradeoffs between thecomplexity of the brain and the mechanical efficiency ofdifferent functional elements of the head.
The tracheal configuration is apparently a highly
conserved character system. The general branching pattern described here was also found in Wasmannia affinis(Richter et al. 2019), Camponotus pennsylvanicus (Keister 1963) and even in representatives of the basal hymenopteran family Xyelidae (Beutel & Vilhelmsen 2007).
In summary, our morphological investigations revealed a broad spectrum of characters relevant in thecontext of the evolution of ant head structures. Some ofthem have received very little attention before, like thetentorium and other endoskeletal elements, or also thesalivarium and associated structures. Another neglectedcharacter system is the cephalic digestive tract, with abroad spectrum of variation likely relevant in a functional
and phylogenetic context. The mandibles and the labrum
have already been investigated in some detail. Nevertheless, more structural information on these key elementsof the head of ants will likely lead to a better understanding of important transformations in the evolution of thegroup.

5.

Acknowledgements

Our very great thanks are due to Brendon Boudinot (UC Davis,
Department of Entomology and Nematology) for providing a very
thorough and detailed review within very short time. This helped
greatly to improve our study. Many helpful comments made by
the editor in chief Dr. Klaus-Dieter Klass (Senckenberg Museum
für Tierkunde Dresden) are also gratefully acknowledged. We also
thank the OIST Imaging Section for providing access to the SEM
and µCT scanner, especially Sasaki Toshiaki who helped withsample preparation and explained the SEM facilities, and Komoto
Shinya for general support with the µCT scanner. We are gratefulto An Vandoren for making the serial histological sections. We also
thank Chung-Chi Lin for collecting the Taiwanese Brachyponera
workers for histological examination and Thomas Parmentier forproviding specimens of Formica rufa, and the Agentschap voor
Natuur en Bos (Agency of Nature and Forest) for granting him
permission to collect these insects. We are also thankful to James
Trager for his terminological advice. This work was funded by subsidy funding to OIST, and Adrian Richter is grateful for a scholarship of the Evangelisches Studienwerk Villigst e.V.

6.

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Electronic Supplement Files

Authors’ contributions

at http://www.senckenberg.de/arthropod-systematics

A. Richter, F. Hita Garcia, R.A. Keller, E.P. Economo and R. G.
Beutel conceptualized and designed the study. A. Richter and
F. Hita Garcia generated the µCT scans; A. Richter did the 3D
reconstructions, SEM images, photographs, assembled the figure
plates and wrote the first draft of the manuscript. J. Billen provided
the histological series and contributed important details on histo­
logical features. All authors commented on the manuscript and re­viewed it together to arrive at the final version.

ASP_78-1_Richter_Electronic_Supplements.zip
DOI: 10.26049/ASP78-1-2020-06/1
File1: richter&al-formicidaehead-asp2020-electronicsupple
ment-1.pdf. — Fig. S1: Volume renderings of heads of F. rufa
(CASENT0790267: A,C – E,G) and B. luteipes (CASENT0709409:
B,F,H). A,B: ventral view of the head capsule, cut open to reveal
the dorsal endoskeleton and the insertions of the dorsal head mus­cles. C: posterior view, showing the functionless muscle X. D:
lateral view of the anterior head, showing the connection of thestipito-premental conjunctivum with the maxilla and hypopharynx/
infrabuccal pouch. E,F: lateral view of the anterior head, show­
ing the interaction of the labrum with the maxillolabial complex.
G,H: lateral view of anterior cephalic digestive tract, showing theoral arms. — Abbreviations: 0bu1 – M. clypeobuccalis; 0bu2 – M.
frontobuccalis anterior; 0bu3 – M. frontobuccalis posterior; 0ci1a –M. clypeopalatalis, unpaired portion; 0ci1b – M. clypeopalatalis,
paired portion; 0hy1 – M. frontooralis; 0hy2 – M. tentoriooralis;
0lb2 – M. frontoepipharyngalis; 0md1 – M. craniomandibularis
externus; 0mx1 – M. craniocardinalis externus; 0mx3 – M. tentoriocardinalis; 0mx4 – M. tentoriostipitalis anterior; ant – anten­
nifer; bt – buccal tube; cd – cardo; epk – epipharyngeal keel; esr –epistomal ridge; ga – galea; lc – lacinia; Ibp – infrabuccal pouch;lbr – labrum; lbrp – labral process; MX – functionless muscle X;
mxg – maxillary gland; oa – oral arm; ph – pharynx; pmx – palpus
maxillaris; pph – prepharynx; spc – stipito-premental conjuncti­
vum; st – stipes — Colors: beige/ brown – mouthparts; green –cephalic digestive tract (prepharynx and pharynx); grey – cuticle;
orange/ red – muscles; purple – glands. — Fig. S2: Histologicalsections of heads of F. rufa (A – E) B. luteipes (F,H) and B. chinensis (G). All sections transverse. A: Section at the level of themaxillolabial complex showing especially the interaction of thestipes with the hypostomal triangular process. B: Section on thelevel of the pharyngeal gland opening. C: Section on the level ofthe eyes, showing the Y-shaped pharynx. D,E: Sections showing
the stipito-premental-conjunctivum, D further posterior than E. F:
Section at the level of the posterior brain, showing the tiny dorsal
tentorial arm. G,H: Section at the level of the anterior mouthparts,
showing the mandibular pit gland. — Abbreviations: ata – anteriortentorial arm; dhy – distal hypopharynx; dta – dorsal tentorial arm;
hyr – hypopharyngeal rod; ibp – infrabuccal pouch; lcs – lacinial
sclerite; md – mandible; mdp – mandibular pit; mdpg – mandibu­
lar pit gland; oa – oral arm; ph – pharynx; phg – pharyngeal gland;phgo – pharyngeal gland opening; pph – prepharynx; pma – pre­mental arm; spc – stipito-premental conjunctivum; stis – stipes
internal sclerite; svd – salivary duct; — Symbols: white arrowhead – interaction of lateral stipes with anterior concavity of hy­popharyngeal triangular process.
File 2: richter&al-formicidaehead-asp2020-electronicsupple
ment-2.xlsx. — Volume measurements.
File 3: richter&al-formicidaehead-asp2020-electronicsupple
ment-3.docx. — Table S1: Comparison of morphological features
between B. luteipes, F. rufa and W. affinis, data on the latter are
based on Richter et al. (2019).

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