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Proc. Arthropod. Embryol. Soc. Jpn. 51, 1–15 (2017)Ⓒ 2017 Arthropodan Embryological Society of Japan
ISSN 1341–1527[REVIEW]

The Phylogeny of Hexapoda (Arthropoda) and the Evolution of
Megadiversity
Rolf G. BEUTEL1), Margarita I. YAVORSKAYA1), Yuta MASHIMO2), Makiko FUKUI3)
and Karen MEUSEMANN4,5)
Entomology Group, Institut für Spezielle Zoologie und Evolutionsbiologie mit Phyletischem Museum, FSU Jena, 07743 Jena,
Germany2)
Graduate School of Symbiotic Systems Science and Technology, Fukushima University, Kanayagawa 1, Fukushima, Fukushima
960–1296, Japan
3)
Graduate School of Science and Engineering, Ehime University, Bunkyocho 2–5, Matsuyama, Ehime 790–8577, Japan4)
Evolutionary Biology & Ecology, Department for Biology I (Zoology), University of Freiburg, 79104 Freiburg, Germany
5)Center for Molecular Biodiversity Research (ZMB), Zoological Research Museum A. Koenig, 53113 Bonn, Germany
E-mail: rolf.beutel@uni-jena.de (RGB)
1)

Abstract
Phylogenetic analyses of single genes and transcriptomes confirm that Hexapoda are a subgroup of Pancrustacea, arguably
most closely related with the specialized cave-dwelling Remipedia. The earliest evolutionary history in the marine environment
remains unknown. The monophyly of Hexapoda is clearly supported by molecular evidence, by the specific tagmosis, and bymorphological apomorphies implied by the Pancrustacea concept. The basal branching pattern, i.e., the interrelationships of theentognathous orders remain ambivalent. The monophyly of Insecta (= Ectognatha), Zygentoma (incl. the “living fossil”
Tricholepidion), Dicondylia (= Zygentoma + pterygote insects), and Pterygota is confirmed. The acquisition of wings was arguably
the most important single (and unique) innovation in hexapod evolution, providing an efficient escape mechanism, drastically
improving the dispersal ability, and making three-dimensional space easily accessible. The basal branching pattern in Pterygota
remains ambiguous (referred to as “Palaeoptera problem”). The long disputed monophyly of Polyneoptera is confirmed bymorphological and developmental features, and also by transcriptomic data. The controversial Zoraptera is placed in this lineage,
which is mostly characterized by plesiomorphic morphological features. The branching pattern within Polyneoptera remains
ambiguous even though a large clade comprising Xenonomia (= Grylloblattodea + Mantophasmatodea), Eukinolabia (= Embioptera+ Phasmatodea) and Dictyoptera (= Mantodea + Blattodea incl. termites) is suggested by analyses of transcriptomic data, with thespecies-rich Orthoptera as its sister taxon. Acercaria (= Paraneoptera excl. Zoraptera) and Holometabola form a clade Eumetabola,
even though this is weakly supported by morphological data if at all. The monophyly of Acercaria remains ambiguous with respect
to Psocodea (Psocoptera + Phthiraptera). Thysanoptera form a clade Condylognatha with Hemiptera. The great diversity of thelatter group is likely linked with mouthparts specialized on piercing and sucking fluids, and with a close connection to plants.
Holometabola, by far the most species rich group of insects, comprise three large clades Hymenoptera, Neuropteroidea [=Neuropterida + (Strepsiptera + Coleoptera)], and Mecopterida (Amphiesmenoptera + Antliophora). The successful evolutionary
interaction between angiosperm plants emerging in the Cretaceous and holometabolous groups has triggered unparalleleddiversifications.

About 1,000,000 extant insect species are named anddescribed while estimates with different approaches suggest
a total of at least 5 million (Stork et al., 2015). With theseenormous numbers, Hexapoda (insects in the widest sense,
i.e., including the entognathous orders) very clearly surpassthe diversity of all other known groups of organisms (e.g.,
Grimaldi & Engel, 2005: fig. 1.3). The extreme speciesrichness is only one of many factors that made insects a highly
attractive and interesting group. Aside from studies related tothe tremendous medical and economic importance of insects,
the morphology, embryology, phylogeny and evolutionary
biology have attracted numerous researchers since a longtime (e.g., Grimaldi & Engel, 2005; Friedrich et al., 2014; Kjer
et al., 2016). A milestone in insect systematics was laid by theGerman dipterist Willi Hennig with his “Stammesgeschichte
der Insekten” (Hennig, 1969) in the late 1960s (Fig. 1), by thattime exclusively based on morphological characters, but witha revolutionized method of phylogenetic evaluation

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R. G. BEUTEL, M. I. YAVORSKAYA, Y. MASHIMO, M. FUKUI AND K. MEUSEMANN(“Phylogenetische Systematik”; Hennig, 1950, 1966). In thelast two decades, molecular data became more and moreavailable and dominant in the exploration of phylogenetic
relationships of insects, with data sets of rapidly increasing
size (e.g., Trautwein et al., 2012; Yeates et al., 2016; Kjer et al.,2016). This tendency culminated in a recent study based on
1478 orthologous genes with taxon sampling of 144 terminals
covering all insect orders plus outgroups, and including rare
phylogenetic key taxa such as for instance Tricholepidion
gertschi (Zygentoma?) or Nannochorista (Mecoptera?) (Misof
et al., 2014).
The present study summarizes the present state ofinsect phylogenetics, covering recent studies based on
innovative anatomical techniques and large and welldocumented morphological character sets, investigations ofembryological features, and evaluations of extensive
molecular data, transcriptomes or genomes. Possible factors
leading to strong diversification are discussed as well as
future perspectives of insect phylogenetics and evolutionary
biology.
The position of Hexapoda
A critical issue is the phylogenetic origin of Hexapoda.
Traditionally the group is placed in a monophylum Tracheata(=Antennata or Atelocerata) together with Myriapoda (e.g.,
Kraus and Kraus, 1994). It was long accepted that a common
ancestor of this lineage acquired terrestrial habits in the
Silurian, and that specific adaptations such as the tracheal
system and Malpighian tubules have evolved only once, as
autapomorphies of this unit.
Today it is widely accepted that insects are not theclosest relatives of myriapods but of aquatic crustaceans (e.g.,
Richter, 2002). The monophyly of a clade Pancrustacea(= Tetraconata) was consistently supported by analyses ofmolecular data. This started with a study based only on few
hundred base pairs of ribosomal genes and a very limited
taxon sampling (Friedrich and Tautz, 1995), continued withevaluations of an increased number of genes plus
morphological characters (e.g., Giribet et al., 2001, 2005), andwas finally also confirmed by analyses of multi-gene data sets
(Regier et al., 2008, 2010) and transcriptomic data (v. Reumont
et al., 2009, 2012; Meusemann et al., 2010; Misof et al., 2014).
The precise position of Hexapoda in the clade Pancrustacea is
not fully clarified yet. However, it is evident that insects are“terrestrial crustaceans”, and probably closely related withthe highly specialized cave-dwelling Remipedia (Fanenbruck
et al., 2004; v. Reumont et al., 2012; Misof et al., 2014), a groupwith only ca. 20 known free-swimming, cave-dwelling andeyeless species.
The Pancrustacea hypothesis implies that hexapods
have acquired terrestrial habits independently, probably
already in the Ordovician (Misof et al., 2014). Consequently,
the tracheal system, Malpighian tubules, sperm transfer via aspermatophore, and other features considered as
autapomorphies of “Tracheata” have evolved independently inhexapods and myriapods. That these features evolved several
times (also in Arachnida) can be explained by a strong andsimilar selective pressure related to the change to theterrestrial environment.
The example of Tracheata shows, that seemingly
plausible evolutionary scenarios are only as good as theunderlying phylogenetic hypothesis. The picture can changevery distinctly when a long accepted systematic concept turns
out to be wrong.
The earliest evolution of Hexapoda and potential stemgroup fossils
Following the Pancrustacea concept it is evident that
Hexapoda must have had an origin in the marine environment.
However, no ancestral aquatic fossils are known so far. A
Devonian fossil described as Devonohexapodus bocksbergensis
was interpreted as an aquatic stem group of hexapods (Haas etal., 2003). This would have had a strong impact on theinterpretation of the earliest evolution of the group. However,it was shown by Kühl and Rust (2009) that the single (andsomewhat deformed) specimen belongs to a species alreadydescribed, Wingertshellicus backesi. The systematic affinities
remain rather unclear. However, Wingertshellicus is not
closely related to Hexapoda, but probably ancestral within
Euarthropoda (Arthropoda excl. Onychophora andTardigrada), close to the root of this large monophylum (Kühl
and Rust, 2009). Consequently, the earliest evolutionary
history of insects remains in the dark. The example of“Devonohexapodus” shows that fossils, especially poorly
preserved ones and single specimens, should be treated very
carefully and critically. Wrong identifications can lead toserious evolutionary misinterpretations.
The monophyly of Hexapoda
The monophyly of Hexapoda was never seriously
questioned in the morphological era of insect phylogenetics(e.g., Hennig, 1969), even though Kristensen (e.g., 1975) notedthat the morphological support is weak, with basically only
one apomorphic character complex, the tagmosis with arelatively short 3-segmented thorax and a distinctly longer
abdomen, with 11 segments in most groups (groundplan of
Insecta and probably also Hexapoda). A crucial evolutionary
innovation linked with this feature is the far-reaching divisionof labor within the postcephalic body. The locomotor organs
with their strongly developed musculature are concentrated
in the thorax, three pairs of legs in all hexapods and two pairsof wings in most groups of pterygote insects. The abdomencontains the largest section of the digestive tract, theexcretory organs, the main part of the fat body, and the genitalorgans.
That this unique and complex apomorphy has greatly
contributed to the diversification in the early evolution ofhexapods is unlikely. Four of five apterygote orders have a low
diversity, with only several hundred species each. However, it
is likely that a compact tagma equipped with strong

PHYLOGENY OF HEXAPODA

Fig. 1 Phylogeny of Hexapoda, compiled from Die Stammesgeschichte der Insekten (Hennig, 1969), from Kjer et al. (2016) (Royal SocietyInterface).

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R. G. BEUTEL, M. I. YAVORSKAYA, Y. MASHIMO, M. FUKUI AND K. MEUSEMANN

musculature was a precondition for the formation of the flightapparatus, which evolved early as a key feature of pterygote
insects. The large and rather compact fat body, mainly locatedin the abdomen, enables insects to mobilize energy-richstored substances very efficiently. This was probably also a
precondition for the evolution of flight.
The monophyletic origin of insects in the widest sense
was challenged by a relatively early molecular study based on
mitochondrial genomes (Nardi et al., 2003), with Collembola(but also the honeybee and a louse) placed among somecrustacean taxa. The analysis in this study had already beenquestioned methodologically by Delsuc et al. (2003).
Moreover, Hexapoda was fully confirmed as a clade by studies
based on single genes (e.g., Sasaki et al., 2013), multi-gene
analyses (Regier et al., 2010) and transcriptomic studies (e.g.,
Meusemann et al., 2010; v. Reumont et al., 2012; Misof et al.,2014). Moreover, the Pancrustacea concept adds new strong
morphological evidence for a monophyletic origin. It implies
that an entire series of characters are additional
autapomorphies of Hexapoda. This includes Malpighiantubules, the tracheal system, sperm transfer via
spermatophores, the formation of a massive fat body, and also
the loss of several organs or structural complexes. The
second antennae, the ventral food rim, nephridial organs, andthe mid gut glands are completely reduced (Beutel et al.,2014). A conspicuous developmental feature obviously linked
with terrestrial habits is the loss of the planctonic nauplius
larva (Fig. 2: metanauplius of Triops cancriformis[Notostraca]), which is probably a groundplan apomorphy ofPancrustacea. An anamorphic development with a freeswimming larval form with a highly specialized feeding
apparatus with three appendages is replaced with an
epimorphic development (full number of segments after
hatching from the egg) and larvae adapted to a moist but
terrestrial environment (Fig. 3).
Basal branching events: the entognathous orders
Collembola, Protura and Diplura
In contrast to the monophyletic origin, the basal
branching events in Hexapoda are still a controversial issue.
Hennig (1969) tentatively suggested a monophylum
Entognatha comprising Collembola, Protura and Diplura,
characterized by mouthparts more or less completely
enclosed by extensions of the genae, and also by partly orcompletely reduced compound eyes and short or missing
Malpighian tubules. Kukalová-Peck (1991) suggested a clade
Cercophora for the first time (Fig. 4), with Diplura as thesister group of Insecta (= Ectognatha), implying the paraphylyof Entognatha. This hypothesis was based on the presence of
cerci, a sperm axoneme pattern 9 + 9x2 + 2, and paired claws,
and is also strongly supported by the formation of an amnion(Ikeda and Machida, 1998, 2001; Machida, 2006; Sekiya andMachida, 2009). As a third alternative, some molecular studies
suggested a clade Nonoculata, combining the eyeless Protura
and Diplura (e.g., Luan et al., 2005). However, recently

published embryological evidence is incompatible with thisoption (Tomizuka and Machida, 2015). The presently available
morphological evidence is clearly ambivalent, supporting
either Entognatha and Ellipura (= Collembola + Protura)(Hennig, 1969), or alternatively Cercophora. Interestingly,
analyses of transcriptomic data do not yield a well-supported
solution (compare Meusemann et al., 2010; v. Reumont et al.,2012; DellʼAmpio et al., 2014; Misof et al., 2014; current 1KITE
transcriptomic analyses). The crucial position of Dipluravaries depending on the taxa sampled for this apparent key
taxon. Moreover, heterogeneity along the tree considering
early hexapod lineages might cause confounding signal within
datasets.
Collembola - the springtails - are by far the most speciesrich group of the apterygote hexapods, with more than 7,000

Fig. 2 Metanauplius larva of Triops cancriformis (Branchiopoda,
Notostraca), ventral view. Courtesy Dr. Martin
Fritsch (Museum für Naturkunde Berlin) and Prof.
Dr. Stefan Richter (Universität Rostock).
an1: antennule, an2: antenna, md: mandible

Fig. 3 Baculentulus densus. Prelarva shortly after hatching(with its chorion).

PHYLOGENY OF HEXAPODA

species. One conspicuous autapomorphy of the small andweakly sclerotized insects is the abdominal jumping device(furcula). Considering the high degree of morphological
specialization, the ecological versatility of the group is5

surprising. Whereas the other primarily wingless insects are
ground oriented and usually live in leaf litter or similar
environments, Collembola occupied very different niches,such as for instance tree canopies, marine tide pools, glaciers,

Fig. 4 Phylogeny of Hexapoda from the 1KITE project (www.1kite.org/; from Misof et al. 2014, modified, courtesy of Hans Pohl).
Presumptive morphological and developmental apomorphies mapped on transcriptomic tree. 1. Hexapoda: tagmosis with head,
3-segmented thorax and abdomen; Malpighian tubules (?, missing in Collembola); tracheal system; loss of 2nd antenna; loss ofventral food rim; loss of nephridial organs; loss of mid gut glands; sperm transfer via spermatophore. 2. Cercophora (= Diplura +Insecta): cerci (?); double claws; amnion; sperm axoneme with 9 + 9x2 + 2 microtubuli pattern. 3. Insecta (= Ectognatha): flagellar
antenna; Johnstonʼs organ; tentorial bridge (?); tarsus subdivided; pretarsus distinctly reduced; ovipositor; terminal filament. 4.
Dicondylia: secondary mandibular joint (?, possibly groundplan of Insecta); tracheal system with anastomoses; closed amniotic
cavity. 5. Pterygota: wings and associated structures; ligamentous endoskeleton reduced; legs with ligamentous diapghragm;
abdominal coxal vesicles absent; epicuticular wax layer; internal fertilization (?, probably not groundplan Pterygota). 6. Palaeoptera:
shortened, bristle-like antenna; loss of antennal circulatory organs; dentisetae of lacinia; larval galea and lacinia fused; aquatic
immatures. 7. Neoptera: ability to fold back wings and associated characters of the wing base (?, possibly groundplan of Pterygota),arolium (?), 3rd valvulae form sheath for 1st and 2nd valvulae (groundplan?). 8. Polyneoptera: embryo formation by the fusion ofpaired regions with higher cellular density; blastokinesis accompanied by full elongation of embryo on egg surface; forewings
modified as tegmina (groundplan? absent in Zoraptera, Embioptera, termites and wingless groups); enlarged anal field of hind wing(groundplan? absent in Zoraptera, Embioptera, termites and wingless groups), pad-like tarsal euplantulae (groundplan?, absent in
Zoraptera and some other groups). 9. Eumetabola: ocelli absent in immature stages. 10. Condylognatha: mandible(s) modified asstylet; labrum narrowed; maxillary palps absent; dorsal shift of anterior tentorial pits. 11. Hemiptera: four-segmented labial
rostrum; labial endite lobes and palps absent; buccal pump; brain and suboesophageal complex form compact unit. 12. Holometabola(= Endopterygota): holomatobolous development with complete metamorphosis; pupa as non-feeding immature stage(groundplan?); endopterygotism (wing buds internalized); larva with reduced number of antennomeres, simplified mouthparts andundivided tarsi. 13. Aparaglossata (= Holometabola excl. Hymenoptera): ovipositor distinctly modified or reduced; reduced numberof Malpighian tubules (maximum 8); paraglossae and their muscles reduced; stemmata as simplified larval eyes. 14. Coleopterida(= Strepsiptera + Coleoptera): median labral retractor absent; salivarium absent; number of antennomeres reduced (13 or less);
posteromotorism; cerci of adults absent. 15. Mecopterida: orthopteroid ovipositor reduced; telescoping female postabdomen;
ventral metasternal process elongated; larval stipes divided into basistipes and dististipes; larval M. craniodististipitalis present.
See text and also Beutel et al. (2014).

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R. G. BEUTEL, M. I. YAVORSKAYA, Y. MASHIMO, M. FUKUI AND K. MEUSEMANN

and even caves.
What may have contributed to the evolutionary successis the jumping capacity, an efficient escape mechanism. Why
Collembola were able to occupy a much greater variety ofhabitats than related groups remains unclear, but this has
likely also contributed to the diversification. Another factor isthe exceptionally high reproduction rate, often linked withparthenogenesis and a short generation cycle.
The rise of Insecta (= Ectognatha) and early splitting
events
A strongly supported lineage is Insecta (= Ectognatha),comprising the small apterygote orders Archaeognatha andZygentoma, and the extremely species-rich winged insects(Fig. 4). A convincing synapomorphy is the flagellate antenna,
with muscles only in the basal scapus and a chordotonal organ(Johnstonʼs organ) in the following pedicellus. An important
evolutionary novelty is the ovipositor with specificappendages (gonocoxae and gonapophyses) of the female
genital segments VIII and IX. The obvious advantage is thateggs can be deposited in narrow crevices or plant tissue.
Another important apomorphy is the subdivision of the tarsus.
This is a precondition for the development of tarsal attachment
devices, which later evolved in pterygote insects and enabled
them to walk efficiently on plant surfaces (e.g., Beutel andGorb, 2001, 2006).
Archaeognatha and Zygentoma are very similar in theirhabitus and lifestyle and share numerous plesiomorphic
features. Nevertheless, there is no doubt that Zygentoma
form a clade Dicondylia together with Pterygota (Fig. 4), a
concept unambiguously supported by morphological (e.g.,
Hennig, 1969; Kristensen, 1975, 1995), developmental(Machida, 2006, 2009; Masumoto and Machida, 2006) andmolecular evidence (e.g., Misof et al., 2014). The main
traditional argument is the dicondylic condition, the presenceof a second mandibular joint, but this has possibly evolved
earlier and may belong to the groundplan of Insecta (Blanke etal., 2015). Besides that, an additional joint reduces the degrees
of freedom at the mandibular base and allows more powerful
biting movements. The obvious advantage is that a broader
spectrum of food can be processed, including solid planttissues. The apterygotes largely rely on fungal hyphae andspores, algae, and soft decaying materials. However, it isimportant in this context that the secondary mandibular joint
was still a rather flexible gliding device in Zygentoma andmayfly immatures (Staniczek, 2000).
Pterygota: insects get airborne
There is little doubt that the most important single
innovation in insect evolution was the development of wings.
Pterygote insects comprise ca. 99% of all species. That wings
evolved more than once can be ruled out with certainty
considering the complexity of the pterothoracic flightapparatus, even though many different variations of thischaracter system evolved (e.g., Brodsky, 1994).

One obvious advantage of active flight is the increased
ability to escape predators. What is probably more importantis the enormously increased dispersal capacity compared toapterygote insects. This applies also to other successful nonrelated group–spiders and spider mites (e.g., Bell et al., 2005).
Juvenile spiders can fly passively using silk rafts. Using thismechanism, they can disperse over distances of 100 km andmore. That flight and an improved dispersal capacity haveplayed a very important role is also underlined by the fact thatall secondary wingless groups of insects have a very low
diversity, such as for instance Zoraptera or Grylloblattodea(e.g., Beutel et al., 2014).
In the Carboniferous–apparently linked with theevolution of wings–the first great wave of diversification took
place. Within a geologically very short time span almost allorders of hemimetabolous groups appeared, and towards theend of the period also Holometabola (e.g., Kukalová-Peck,1991).
Another important evolutionary novelty of pterygote
insects is internal sperm transfer. Apterygote hexapods, withthe possible exception of Protura (M. Fukui; unpublishedobservations) and some collembolan species (e.g., Schaller,
1971; Proctor, 1998), externally deposit a spermatophore,
which quite often does not lead to successful fertilization, forinstance when males of other species or females eat or
destroy the sperm package (e.g., Dallai et al., 2009). Moreover,indirect sperm transfer is usually restricted to very moist
habitats, especially soil, and it often requires the production of
stalk or silk thread material (e.g., Schaller, 1971; Proctor,
1998). The reproductive efficiency and economy is increasedin pterygotes with direct sperm transfer with a postabdominal
intromittent organ, the aedeagus. This has arguably also
contributed to the evolutionary success. Again, this also
applies to the successful Araneae. Male spiders transmit
spermatophores into the female genital tract with theirmodified pedipalps (Alberti and Michalik, 2004). Different
modes of internal sperm transfer have also evolved in
Opiliones (with a penis), Ricinulei, and within the orders
Acaria (in some cases with a penis) and Solifugae (Alberti andMichalik, 2004).
Even though the switch to internal fertilization, usuallywith a spermatophore, was a major evolutionary innovation in
insects, this feature is likely not an autapomorphy of
Pterygota. The pre-abdominal secondary copulatory organ of
Odonata has obviously evolved independently and it wassuggested that stem-group dragonflies had an external spermtransfer (Bechly et al., 2001: Namurotypus).
In different groups of pterygote insects, male and female
genitalia fit together like lock and key, a mechanism thatprevents interspecific copulation (see e.g., Shapiro and Porter,1989). It is conceivable that this has contributed to theextreme diversity of the genitalia and also to the diversificationof insects. However, more recent investigations suggest that
this is only a marginal phenomenon in insects (e.g., Shapiro
and Porter, 1989). What apparently plays the major role in the

PHYLOGENY OF HEXAPODA

evolution of highly diverse genitalia is sexual selection,
female cryptic choice (Eberhardt, 1985, 1996). The females
develop variations of the genitalia to prevent fertilization or toallow it only selectively, whereas the male genitalia vary to
overcome the obstacles. This is one example of anevolutionary arms race in insects.
The “Palaeoptera problem”
A deep-rooted and strongly disputed problem insystematic entomology is the basal branching pattern in
Pterygota (e.g., Kristensen, 1991). Hennig (1969) placed
Ephemeroptera and Odonata in a clade Palaeoptera, supported
for instance by aquatic immatures and strongly shortened,
bristle-like antennae. Boudreaux (1979) suggested a clade
Chiastomyaria including Ephemeroptera and Neoptera.
Potential synapomorphies are the indirect flight muscles andthe direct sperm transfer with a postabdominal aedeagus.
Staniczek (2000) suggested Metapterygota comprising
Odonata and Neoptera, based on an entire series of
apomorphies linked with the mandibles. Additional arguments
for this option are abdominal spiracles with a closing
mechanism, additional connections in the tracheal system(e.g., Kristensen, 1991), and the loss of the subimago, a winged
immature stage of mayflies that moults before the adult stage
is reached. More detailed morphological investigations
showed that Palaeoptera may indeed form a clade (Blanke et7al., 2012a, b). Mandibular features studied by Staniczek (2000)
are closely correlated and not independent characters, leading
to artefacts caused by concerted convergence (see Blanke etal., 2012b). Recent analyses of transcriptomes also tentatively
support the Palaeoptera concept (Fig. 4; Misof et al., 2014).
However, the results are still ambiguous. Like the issue of theentognathous orders, the “Palaeoptera problem” remains
unresolved at present.
Neoptera
Another key feature which evolved early in pterygote
insects characterizes Neoptera (= Pterygota excl.
Ephemeroptera and Odonata), the ability to fold back thewings over the abdomen. An obvious advantage is that winged
adults can now hide in relatively narrow spaces. This means
better protection and also reduced water loss in moist
microhabitats. The neopterous conditions is linked withspecific features of the wing base, a subdivided median plate,flexion lines in the wing base, longitudinal veins detachedfrom the axillary sclerites, and a pleural muscle attached tothe 3rd axillary (e.g., Wootton, 1979; Beutel et al., 2014). The
ability to fold back the wings was generally accepted as an
autapomorphy of Neoptera (e.g., Kristensen, 1975, 1995;
Beutel and Gorb, 2001). However, it was pointed out by
Willkommen and Hörnschemeyer (2007) that this condition
may be part of the pterygote groundplan, and that theFig. 5 Zorotypus caudelli. A. Apterous adult. B. Alate adult. C. Embryonic development of Zorotypus caudelli. DAPI staining. Black andwhite arrowheads show cephalic and caudal ends, respectively.

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R. G. BEUTEL, M. I. YAVORSKAYA, Y. MASHIMO, M. FUKUI AND K. MEUSEMANN

seemingly ancestral conditions found in Ephemeroptera andOdonata differ very distinctly.
Aside from the modified wing base there is not much insupport of monophyletic Neoptera. An ovipositor with 3rd
valvulae (gonoplacs) forming a sheath for the 1st and 2nd
valvulae was suggested as a groundplan apomorphy (see e.g.,
Kristensen, 1991).
Another possible autapomorphy of Neoptera is thearolium (Beutel and Gorb, 2001, 2006), a median pretarsal
attachment lobe. It is the first of several types of attachment
devices, which evolved in pterygote insects. An important
issue in this context is that tarsal or pretarsal adhesive
devices do not just serve as attachment structures. They
enable insects to walk efficiently on plant surfaces (Beutel
and Gorb, 2001). Plants modify their surfaces to prevent
insects from walking and feeding on them, and insects modify
their attachment devices to overcome these obstacles. This is
another example for an evolutionary arms race, in this casebetween two very different groups of organisms.
Polyneoptera and the first major diversification
Polyneoptera, one of the three large subdivisions ofneopteran insects (e.g., Beutel et al., 2013), emerged in thefossil record in the Carboniferous (e.g., Kukalová-Peck, 1991;
Grimaldi and Engel, 2005). The monophyly of this group wasstrongly debated, especially with respect to Plecoptera (e.g.,
Kristensen, 1991), and a neutral term “lower Neoptera” wasfrequently used. The placement of the small and enigmaticorder Zoraptera (Mashimo et al., 2014a; Fig. 5A, B) was also
very controversial, and a sister group relationship with
Acercaria was tentatively suggested by Hennig (1969).
Numerous conflicting hypotheses for the internal
relationships of Polyneoptera were suggested (e.g., Trautwein
et al., 2012; Beutel et al., 2013) and an almost completely
unresolved “lower neopteran” phylogenetic pattern(Kristensen, 1991: fig. 5.5) became known as “Kristensenʼs
comb”.
The monophyletic origin of Polyneoptera including
Zoraptera is now confirmed by different sources of evidence(Fig. 4), especially by embryological characters (Mashimo etal., 2014b; Fig. 5C). The embryonic development inpolyneopteran groups differs distinctly from what is found thein acercarian orders, but also in Ephemeroptera and Odonata(Palaeoptera), indicating that the polyneopteran pattern isapomorphic (Fig. 4; Mashimo et al., 2014b). Recent
phylogenetic evaluations of morphological data also confirmed
the monophyly of Polyneoptera including Zoraptera(Yoshizawa, 2007, 2011; Matsumura et al., 2015; Wipfler et al.,2015), like analyses of single genes (Yoshizawa and Johnson,
2005; Ishiwata et al., 2011; Wang et al., 2013), and finally alsotranscriptomic data analysed in the 1KITE project (Misof etal., 2014).
Polyneoptera comprise very small orders with distinctly
less than 100 described species (Zoraptera: ca. 40 spp.;
Grylloblattodea: ca. 35 spp.; Mantophasmatodea: ca. 20 spp.),some medium sized groups (e.g., Plecoptera: ca. 3,500 spp.;
Dermaptera: ca. 2,000 spp.), and as the only group very
successful in terms of species diversity Orthoptera (ca. 22,500
spp.). Orthoptera are characterized by jumping hindlegs anddifferent acoustic communication systems have evolved in
the group. Caelifera are strictly phytophagous whereas many
predacious species occur in Ensifera (e.g., Beutel et al., 2014).
The interordinal relationships are not fully clarified yet
but considerable progress has been made. It is generally
accepted that termites are a subordinate group of a clade
Blattodea (as sister group of Cryptocercidae), which forms
the sister taxon of Mantodea (Lo et al., 2000; Deitz et al., 2003;
Misof et al., 2014). It is also well established that
Grylloblattodea + Mantophasmatodea form a clade(Xenonomia), and also Phasmatodea + Embioptera(Eukinolabia) (Terry, 2004; Terry and Whiting, 2005; Wipfler etal., 2011). Features of the egg and blastokinesis suggesting
the monophyly of Xenonomia (= Chimaeraptera) were
described by Uchifune and Machida (2005). Fujita and Machida(in press) suggested micropyles clustered on the ventral sideof the egg as an embryological autapomorphy of Dictyoptera,and proposed a phylogenetic pattern Mantodea + Blattodea[= Blaberoidea + (Blattoidea + Isoptera)] based on theblastokinesis type. Jintsu et al. (2010) supported themonophyly of Eukinolabia based on structural affinities of theeggs of Embioptera and Phasmatodea.
The major branching events in Polyneoptera(“phylogenetic backbone”) are not fully clarified yet.
Transcriptome analyses (Misof et al., 2014) suggest a patternwith Zoraptera + Dermaptera as the sister group of theremaining orders, Plecoptera as the next branch, and thenOrthoptera as sister group of a large lineage comprising
Dictyoptera [Blattodea (incl. termites) + Mantodea],
Xenonomia and Eukinolabia (Fig. 4). However, this hypothesis
requires further confirmation. The recently described extinct
order Alienoptera is probably the sister taxon of Mantodea(Bai et al., 2016), thus representing a “connecting link”
between extinct predacious roaches and the praying mantises.
The two completely wing-less orders Grylloblattodea
and Mantophasmatodea comprise together only about 50
species. In Grylloblattodea, which also occur in Japan, loss ofwings is apparently an adaptation to the preferred cold
mountain areas (e.g., Wipfler et al., 2014). The advantage is the
reduced risk to be drifted into unsuitable habitats by strong
winds. The obvious disadvantage in both taxa is the drasticallyreduced dispersal ability, which apparently resulted in a very
low diversity. Additionally, both groups have only a marginal
relationship to plants if at all. It is likely that insect-plantinterrelationships have played an important role in thediversification of different insect groups, as for instance in
Orthoptera.
Acercaria (= Paraneoptera excl. Zoraptera, hemipteroid
assemblage)
Acercaria, the second major neopteran subgroup,

PHYLOGENY OF HEXAPODA

comprises approximately 120,000 described species. The
monophyly was generally accepted (e.g., Hennig, 1969;
Kristensen, 1975, 1991; Beutel and Gorb, 2001; Wheeler et al.,
2001; Friedemann et al., 2014). Potential morphologicalapomorphies are slender and elongated laciniae detachedfrom the stipes, an enlarged postclypeus and enlarged cibarial
dilators, tarsi with three segments or less, completely
reduced cerci, at most six Malpighian tubules, and a single
abdominal ganglionic mass (e.g., Kristensen, 1991; Yoshizawa
and Saigusa, 2001; Friedemann et al., 2014). Even though mostof these features are reductions, the morphological support
for a monophyletic origin appears strong. Nevertheless, as asurprising outcome, Acercaria were not supported as a cladein Misof et al. (2014), with Psocodea placed as sister taxon ofHolometabola (Fig. 4). This result, which appears implausible
from the morphological perspective, is presently reinvestigated with a distinctly extended taxon sampling.
The monophyly of Psocodea was confirmed in Misof etal. (2014), and also in earlier studies based either on single
gene analyses (Yoshizawa and Johnson, 2003, 2010) or onmorphology (Friedemann et al., 2014) Psocoptera wasrendered paraphyletic by a sister group relationship between
Liposcelididae and the ectoparasitic Phthiraptera. The
analyses based on transcriptomic data supported
Condylognatha, combining Thysanoptera with themegadiverse monophyletic Hemiptera, as earlier suggested
by Yoshizawa and Saigusa (2001), Friedemann et al. (2014) andothers, in contrast to Micracercaria (= Thysanoptera +Psocodea) suggested by other authors (see Kristensen, 1991).
A key feature of Acercaria is the presence of piercingsucking mouthparts. However, this condition does not belong
to the groundplan (e.g., Spangenberg, 2015). Psocoptera andbasal groups of the ectoparasitic lice (“Mallophaga”) havemaintained biting mouthparts, even though with somespecializations (e.g., v. Kéler, 1966; Spangenberg, 2015).
Obviously the character complex has evolved two or threetimes independently, in a very distinctive way inRhynchophthirina and Anoplura (Tröster, 1990), with a single
functional stylet-like mandible in Thysanoptera, and withstylets formed by the mandibles and laciniae in all hemipteranorders (e.g., Spangenberg et al., 2013; Spangenberg, 2015).
It is conceivable that advanced piercing sucking
mouthparts and predominantly plant-feeding habits havecontributed to a remarkable diversification of Hemiptera,
which comprise about 70% of all known acercarian species.
The majority of true bugs and thrips, and all species ofSternorrhyncha and Auchenorrhyncha are specialized onsucking fluids of plants (e.g., Beutel et al., 2014).
Auchenorrhyncha (cicada, plant hoppers, tree hoppers),
which comprise ca. 42,000 spp., show evolutionary parallels to
Orthoptera, the polyneopteran group with the highest
diversity. They also evolved a very good jumping capacity(absent in Cicadoidea) (see Friedemann and Beutel, 2014 andstudies cited therein), and acoustic communication systems.
This makes it plausible to assume that both character9complexes have contributed to the diversification in both nonrelated lineages.
The major hemipteran diversification started in the
Cretaceous, likely correlated with the early radiation ofangiosperm plants (Grimaldi and Engel, 2005: fig. 8.31). The
strong affinity to plants is also underlined by the fact that
hemipteran subgroups have evolved a very broad spectrum of
attachment devices (Friedemann et al., 2014). These
structures enable them to move very efficiently on different
plant surfaces (Beutel and Gorb, 2001).
Eumetabola
A clade comprising Acercaria and Holometabola issupported by molecular data (Fig. 4; Letsch et al., 2012; Misof
et al., 2014), even though there is hardly any morphological ordevelopmental evidence. Presently the loss of ocelli in
immature stages is the only plausible synapomorphy of thetwo large lineages (e.g., Beutel et al., 2014).
Holometabola
By far the largest subgroup of Neoptera is Holometabola(= Endopterygota), with an unparalleled diversity of about
described 800,000 species, roughly half of all known
organisms. In contrast to basal branching events in Hexapoda
and the internal phylogeny of Polyneoptera, the relationshipsof the holometabolan orders seem to be settled in the “age ofphylogenomics” (Fig. 4). A large and complex morphological
data set (Beutel et al., 2011), single copy nuclear genes(Wiegmann et al., 2009), transcriptomes (Peters et al., 2014;
Misof et al., 2014), and genomes (Niehuis et al., 2012)
converged upon the same interordinal pattern. Hymenoptera(ca. 132,000 described spp.) are placed as the sister group of allremaining orders, in contrast to Hennig (1969), Kristensen(1975, 1995) and Beutel and Gorb (2001), but in agreementwith Rasnitsyn and Quicke (2002). Morphologicalapomorphies of Aparaglossata (= Holometabola excl.
Hymenoptera; see Peters et al., 2014) are the loss of theparaglossae and their muscles, the partial reduction of theorthopteroid ovipositor, and a reduced number of Malpighiantubules (Beutel et al., 2011). A large clade Neuropteroidea
comprises the three neuropterid orders (Neuropterida) as
sister group of a monophylum Coleopterida, combining
Coleoptera with the long disputed Strepsiptera. Komatsu andKobayashi (2012) suggested a knob-like micropylar projection
as an apomorphic groundplan feature of Neuropteroidea, eventhough this structure is not present in most beetles. The
sister group relationship between the megadiverse beetles(ca. 360,000 described species) and the extremely specialized
endoparasitic Strepsiptera ends one of the longest
controversies in systematic entomology (e.g., Niehuis et al.,2012; Pohl and Beutel, 2013). The sister group of
Neuropteroidea is Mecopterida, a very large monophylum
already suggested by Hinton (1958) as “panorpoid orders”.
This group comprises Amphiesmenoptera, with Trichoptera
and the megadiverse Lepidoptera as sister taxa, and10

R. G. BEUTEL, M. I. YAVORSKAYA, Y. MASHIMO, M. FUKUI AND K. MEUSEMANN

Antliophora containing the small order Mecoptera, theectoparasitic Siphonaptera, and the extremely species-rich
Diptera. Kobayashi and coworkers analyzed embryologicalcharacters cladistically to reconstruct the relationships ofamphiesmenopteran subgroups (Kobayashi and Ando, 1988;
see also Kobayashi et al., 2003). The interrelationships of thethree antliophoran orders are presently not completely
resolved (Misof et al., 2014), especially with respect to theenigmatic mecopteran subgroup Nannochoristidae, treated asa separate order Nannomecoptera by Hinton (1981).
The most conspicuous feature of Holometabola andarguably a key innovation is the holometabolous developmentwith a complete metamorphosis and a non-feeding and largely
or completely immobilized pupal stage. The immobilized andunprotected pupa is apparently a risk-factor in the life cycle,
which means evolutionary costs. In contrast to this, the abilityof larvae and adults to use different resources and habitats isvery likely an advantage, resulting in a decreased intraspecificcompetition.
Another important feature is the endopterygote
condition (e.g., Kristensen, 1991). The wing buds lie below thelarval cuticle. This enables the larvae to penetrate very
narrow crevices, for instance under bark, or to burrow in planttissue, even including wood. This means a larval development
in an environment providing moisture and inaccessible formost predators.
These factors have likely played a role but do by not
sufficiently explain the extreme species richness. It is evident
that the major diversification did not take place in the early
evolution of the group, but independently in different lineages.
The diversity of several groups is low with only few hundred
species for instance in Megaloptera (ca. 320 spp.) or Mecoptera(ca. 550 spp.) (e.g., Grimaldi and Engel, 2005; Beutel et al.,2014). Extremely diverse – each with distinctly more than
100,000 spp. – are Hymenoptera (ca. 132,000 described spp.),
Coleoptera (ca. 360,000 spp.), Lepidoptera (ca. 175,000 spp.)
and Diptera (ca. 154,000 spp.). These four orders – referred toas “big4” (see http://big4-project.eu/) – are not closely relatedwith each other, and each of them includes basal groups with alow diversity, for instance Archostemata with only 40 species,
about 0.01% of the total diversity of beetles. Interestingly
Archostemata, like basal lepidopteran groups, are associatedwith gymnosperms, whereas the vast majority ofphytophagous beetles is linked with the highly diverse
angiosperm plants (e.g., Crowson, 1981).
A distinctly improved flight capacity may havecontributed to the diversification in Hymenoptera and
Diptera, with functional or anatomical dipterism, respectively.
Parasitism has likely played a role in Hymenoptera, probably
with an immense hidden diversity still to discover, especiallyof very small forms. Strong mechanical protection isapparently a key feature of Coleoptera, with ancestral forms
adapted to narrow crevices, especially under bark.
A major factor in the “megadiversification” of the “big 4”of Holometabola was likely a successful evolutionary

interaction with angiosperm plants, beginning in the early
Cretaceous or slightly earlier (e.g., McKenna et al., 2015). New
food sources became available for phytophagous insects, andreciprocally different groups of insects have immensely
contributed to the dispersal and evolutionary success ofangiosperms as pollinators. This connection is wellestablished in Lepidoptera and subgroups of Hymenoptera(bees etc.), beetles, and Diptera (e.g., Beutel et al., 2011).
These lineages underwent an explosive radiation in the late
Mesozoic. The positive evolutionary interaction resulted inabout 200,000 species of angiosperms and the enormous
number of ca. 800,000 spp. in Holometabola. This is probably
the most important example of successful co-evolution.
Perspectives
The investigation of hexapod diversity, morphology,
systematics and evolution has a long tradition, going back to
the 18th century and even earlier (e.g., Engel and Kristensen,
2013; Friedrich et al., 2014). The work of the German dipterist
Willi Hennig in the last century was unquestionably a
breakthrough, with a revolutionized phylogenetic
methodology (Hennig, 1950, 1966) and a comprehensive workon insect systematics (Fig. 1; Hennig, 1969). In the last two
decades, an impressive development of insect phylogenetics
took place, with a greatly accelerated acquisition of highquality anatomical data, but also with a breathtaking“evolution” of molecular systematics and analytical methods(Kjer et al., 2016).
Obviously, the anatomy and development of hexapods
are not in the mainstream of present day research.
Nevertheless, embryology as an essential branch ofevolutionary entomology is still carried out on a very highlevel in some research institutions, for instance thelaboratories of Prof. Dr. Ryuichiro Machida (Sugadaira
Research Station, Mountain Science Center, University ofTsukuba) (e.g., Mashimo et al., 2014b) and Emer. Prof. Dr.
Yukimasa Kobayashi (Tokyo Metropolitan University).
Morphological work has gained great momentum since thelast turn of the century, mainly due to new anatomical
techniques (e.g., Friedrich et al., 2014). They distinctly
accelerated the acquisition of high quality morphological data
and also greatly improved the documentation. In particular,
micro-computed tomography (µ-CT) combined with computer
based reconstruction has turned out as highly successful.
New techniques like for instance serial block-face scanningelectron microscopy (SBFSEM) or nuclear magnetic
resonance imaging (NMRI) also allow us to examine
extremely small objects (e.g., Knauthe et al., 2016).
Ultrastructural features play a minor role in insect
systematics. However, insect sperm with characters on thecellular level turned out as phylogenetically informative (e.g.,
Dallai et al., 2016). Like other limited character sets, it isinsufficient to resolve the phylogenetic relationships in ahighly diverse group like Hexapoda (Gottardo et al., 2016).
However, it provides crucial phylogenetic support forPHYLOGENY OF HEXAPODA

important branches such as for instance Cercophora (=Diplura + Insecta). Besides this, the evolution of thisunicellular character system is amazingly complex andapparently shaped by other mechanisms than other body parts(e.g., Gottardo et al., 2016).
In insect systematics based on morphology in a broad
sense, there is a clear tendency to use broader character sets,some of them comprising several hundred well-documented
characters (e.g., Beutel et al., 2011). This was made possible
by a remarkable renaissance of insect anatomy (e.g., Beutel
and Kristensen, 2012; Friedrich et al., 2014). However, eventhough the size of morphological matrices increased
remarkably, they are still dwarfed by recent molecular data
sets, as for instance 1,478 orthologous genes sampled fornearly 150 terminal taxa representing all insect orders andoutgroup taxa (Misof et al., 2014). Peters et al. (2014) presented
a two-stage procedure to combine molecular andmorphological evidence for Holometabola: the phylogenetic
branching pattern is reconstructed using extensive sequence
data (in this case transcriptomes) in the first step. In thesecond step character transformations on the phenotypiclevel are traced, using the obtained phylogeny (e.g., Mesquite;
Maddison and Maddison, 2011).
It is apparent that the future perspective of insect
phylogenetics lies in a complex, multifaceted approach, aspresently practiced in the 1KITE project (www.1kite.org/): aclose and efficient collaboration between specialists indifferent fields, molecular systematics, bioinformatics,
morphology, palaeontology, and last but not least
developmental biology. We are confident that this will lead to a
new level of insight in the evolution of the extremely
successful and fascinating Hexapoda.

11Bai, M., R. G. Beutel, K.-D. Klass, W. Zhang, X.-K. Yang, and B.
Wipfler (2016) †Alienoptera – a new insect order in the roach–
Mantodean twilight zone. Gondwana Research, 39, 317–326.
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Bell, J. R., D.A. Bohan, E. M. Shaw and G.S. Weyman (2005)
Ballooning dispersal using silk: world fauna, phylogenies,
genetics and models. Bulletin of Entomological Research, 95, 69–
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Beutel, R. G. and S. Gorb (2001) Ultrastructure of attachment
specializations of hexapods (Arthropoda): evolutionary patterns
inferred from a revised ordinal phylogeny. Journal of Zoological
Systematics and Evolutionary Research, 39, 177–207.
Beutel, R. G. and S. Gorb (2006) A revised interpretation of theevolution of attachment structures in Hexapoda, with special
emphasis on Mantophasmatodea. Arthropod Systematics &Phylogeny, 64, 3–25.
Beutel, R. G. and N. P. Kristensen (2012) Morphology and insect
systematics in the era of phylogenomics. Arthropod Structure &Development, 41, 303–305.
Beutel, R. G., F. Friedrich, T. Hörnschemeyer, H. Pohl, F. Hünefeld, F.
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Beutel, R. G., F. Friedrich, S.-Q. Ge and X.-K. Yang (2014). Insect

Acknowledgments: A generous invitation to give a
presentation at the 51st Annual Meeting of the Arthropodan
Embryological Society of Japan at Fukushima (2015) is
gratefully acknowledged by RGB. We also thank Prof. Dr. R.
Machida and Emer. Prof. Dr. Y. Kobayashi (Tokyo Metropolitan
University) for inviting us to submit this contribution andanonymous reviewers for their helpful comments. We arevery grateful for valuable images of crustacean larvae kindly
provided by Dr. Martin Fritsch (Museum für Naturkunde,
Berlin) and Prof. Dr. Stefan Richter (Universität Rostock).
Financial support from the Partnership Program University ofTsukuba-DAAD granted to RGB, Prof. Dr. Bernhard Misof,
and Prof. Dr. Ryuichiro Machida (Project-ID 57060275,
2014/2015) is also gratefully acknowledged.

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