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Proc. Arthropod. Embryol. Soc. Jpn. 48, 33–38 (2017)Ⓒ 2017 Arthropodan Embryological Society of Japan
ISSN 1341–1527[SHORT COMMUNICATION]

Formation of Subcoxae-1 and 2 in Insect Embryos: The Subcoxal
Theory Revisited*
Yukimasa KOBAYASHI
Sayamadai 2–21–18, Sayamashi, Saitama 350–1304, Japan
E-mail: dineutus@hotmail.co.jp

Abstract
In their embryological study of the carabid ground beetle Carabus insulicola, Kobayashi et al (2013) revealed that the basalmost
region of thoracic appendages, or subcoxa, can be subdivided into the subcoxae-1 and 2, and they form the larval pleuron. The study
also showed that the subcoxae-1 and 2 are the equivalents of the anapleural and katapleural rings, respectively, of the larval pleuron.
The study thus provided tangible evidence for the subcoxal theory whose validity has been much debatable. However, theapplicability of the subdivision of the subcoxa (subcoxae-1 and 2) to other hexapod embryos remains unverified. The present paper
reveals that the subdivision could be discernible in the embryos of several holometabolous orders, i.e., Coleoptera, Megaloptera,
Neuroptera, and Trichoptera, thus suggesting the applicability of the subcoxal theory at least to holometabolous insects. In nonholometabolous orders, however, we have not yet convincing evidence for the subdivision of the embryonic subcoxa.

History of the subcoxal theory
The subcoxal theory postulates that the basalmost
podomere of the thoracic appendage, or subcoxa, participatesin the formation of the larval pleuron. The theory assumes
that all the pleural areas between the tergum and sternum arederived only from the subcoxa. This idea was pioneered by
the embryological observations by Heymons (1899) in thehemipteran Naucoris cimicoides. He identified the subcoxa inthe region proximal to the coxa of the embryonic thoracic
appendage (Fig. 1A, scx), and interpreted that the region
forms the larval pleuron. Roonwal (1937) also described thatthe basalmost region of the thoracic appendage in the embryoof the locust Locusta migratoria forms the episternum andepimeron in the larval pleura (Roonwal, 1937, Figs. 70, 71, 72).
He also regarded the region as the subcoxa. The Roonwalʼs
subcoxa in each of the meso- and metathoracic segments
contains a spiracular opening near its anterodorsal corner, but
the Heymonsʼ subcoxa does not contain it and the opening islocated at the dorsal outside of his subcoxa, or on theunderside of the paratergal lobe (Fig. 1A, ptl). Thus, as will be
discussed later, the ʻsubcoxaʼ sensu Heymons and that sensu
Roonwal are different in the range included.
Heymonsʼ idea was subsequently elaborated as thesubcoxal theory by Snodgrass (1927, 1935) and Weber (1928,1952), based on the comparative morphology of the larval andadult thorax. They recognized two series of sclerites

concentrically arranged and proximal to the coxa in broad
hexapod taxa, and assumed that these sclerites are formed byfragmentation of the subcoxal rings, that is, the inner (distal)katapleural ring (katapleurite) and the outer (proximal)
anapleural ring (anapleurite) (Fig. 1B, apl, kpl). In pterygote
insects, the two rings are generally divided into the anterior
and posterior sclerites by the pleural suture runningdorsoventrally. Thus, the katapleurite is divided into theanterior katepisternum and posterior katepimeron, and theanapleurite into the anepisternum and anepimeron (Fig. 1C,
aepm, aeps, kepm, keps, pls). Weber (1928, 1952) alsopresumed that not only the pleuron but also the sternum(basisternum and furcasternum) are derived from these rings(Fig. 1C, bs, fs). However, Snodgrass (1958) subsequently
rejected his own theory by concluding that only the
katapleurite is subcoxal in origin, but the anapleurite does notexhibit any signs of an appendicular nature. Nonetheless,Matsuda (1956a, 1956b, 1960, 1963, 1970) reevaluated andrefined the theory by including many of his own observationsof thoracic sclerites in a wide range of insect taxa. In particular,
he stressed the importance of the paracoxal suture (pleuralcosta of Ferris, 1940) that separates the pleural walls into thekatapleural and anapleural rings, because the suture forms apleural apodeme that provides surface for the attachment ofmany thoracic muscles. He also pointed out that theanepisternum of many insect orders is further divided by the* This article, which was received in 2013 and should have been published in 2014, was printed in 2017 being much delayed due to various circumstances.

34

Y. KOBAYASHI

Fig. 1 History of the subcoxal theory. A. Heymonsʼ theory. Naucoris cimicoides (Hemiptera). Ventral view of the right mesothoracic leg ofan embryo, showing the subcoxal region (scx) proximal to the coxa (cx) (modified from Heymons, 1899). B, C. Weberʼs theory on theorigin of the pleuron and sternum (modified from Weber, 1952). Ventral views of a schematic thoracic segment in the prototype (B)and the orthopteroid type (C).
aepm: anepimeron, aeps: anepisternum, apl: anapleural ring: bs: basisternum, fm: femur, fs: furcasternum, fup: furcal pit, kepm:
katepimeron, keps: katepisternum, kpl: katapleural ring, pls: pleural suture, ptl: paratergal lobe, st: sternum, tar: tarsus: tb: tibia:
tg: tergum, tr: trochanter.

anapleural suture, which distinguishes a ventral
preepisternum anterior to the katepisternum (cf. Matsuda,
1970, Fig. 14). Matsuda (1970) naturally accepted thepossibility that these sclerites are derived from the subcoxa,
but also pointed out that the embryological data for supporting
the subcoxal theory is still very insufficient. Strangely
enough, however, according to both the original subcoxal
theory as well as the refinements of Matsuda (1970), no
consideration is given to whether the spiracle could be
included in the proximal anapleural ring or not.
Even assuming the katapleural and anapleural rings,
which are identified only from morphological observations,are derived from the subcoxa, there is no distinction between
these rings in the original embryonic ʻsubcoxaʼ of Heymons
and Roonwal. In recent years, Uchifune and Machida (2005)
clearly demonstrated that the embryonic subcoxa of
Galloisiana yuasai (Grylloblattodea) forms the sclerites ofboth pleuron and sternum, but the distinction of the
katapleural and anapleural rings is not shown in the embryonicsubcoxa.
Subcoxal theory revisited
Recently Kobayashi et al (2013) recognized theembryonic subcoxa at the basalmost region of each of thethoracic appendages in the young embryo of the carabid
ground beetle Carabus insulicola (Fig. 2A, B, scx1, scx2). As
development proceeds, the region is divided into the proximal
subcoxa-1 and distal subcoxa-2 by the paracoxal suture (Fig.
2C, pcxs). In the stage of katatrepsis, the two subcoxae arefurther subdivided anteroposteriorly into several regions,
namely the prospective preepisternum, anepisternum,
anepimeron, katepisternum, katepimeron, and pleuralapophyseal pit (Fig. 2D, peps, aeps, aepm, keps, kepm, pla),
and they differentiate into the larval thoracic pleurites (Fig.
2E). It was thus confirmed for the first time in the embryonicdevelopment that the subcoxae-1 and 2 are the equivalent of
the anapleural ring and katapleural ring, respectively. Also, asin the case of G. yuasai (Uchifune and Machida, 2005), thespiracle of C. insulicola is located in the preepisternum, andthus the spiracle belongs to the anapleural ring, or thesubcoxa-1 (Fig. 2D, E, spr).
According to the subcoxal theory sensu Weber (1928,1952) and sensu Matsuda (1970), the ventral part of thesubcoxa is involved in the formation of the sternum. In thefirst instar larva of C. insulicola, except for the large
semicircular presternum in the prothorax, most of the regionsof the thoracic sterna are membranous (Fig. 2E), which meansthat boundaries between the supposed sclerites areindiscernible. However, since the anterior part of thesubcoxa-1 of each thoracic segment extends ventro-medially,
it is highly probable that the definitive sterna are derived fromthe subcoxal element.
Despite the complete absence of abdominal appendages
in the larvae of C. insulicola, two pairs of appendage-likeswellings, the medial and lateral ones, temporarily arise ineach of the first eight abdominal segments (Fig. 2B, msw,lsw). In the ninth abdominal segment, only the medialswellings arise. The lateral swellings soon degenerate in thesubsequent stage (Fig. 2C). The medial swellings arepresumed to be serially homologous to thoracic appendages,
and thus regarded as abdominal appendages. The appendages(medial swellings) in the first segment develop into
pleuropodia which can be divided into the proximal subcoxa
and coxa and distal telopodite (Fig. 2B, pp). The appendages insegments II to IX are composed of only the subcoxa and coxa.
As development proceeds, the subcoxa in each abdominal
segment is divided into the subcoxae-1 and 2 like those ofthoracic segments (Fig. 2D). The abdominal subcoxae-1 and 2
correspond to the prospective tergopleurite and epipleurite of
the larval abdominal pleuron (Fig. 2D, E, tpl, epl). The coxal
part of each appendage flattens out and participates in theformation of the hypopleurite of the larval pleuron (Fig. 2D, E,
hpl). Therefore, the larval abdominal pleuron originates fromboth the subcoxa and coxa. Regarding the formation of theabdominal sternites, observations by Kobayashi et al (2013)strongly suggested that the three sternites, the mediosternite,

SUBCOXAL THEORY REVISITED

35

Fig. 2 A–F. SEM micrographs of developing embryos (ventral views) of the carabid ground beetle Carabus insulicola (modified from
Kobayashi et al., 2013). A. Embryo at the stage of 33% DT. B. High magnification of rectangle in A, showing thoracic segments. C.
Embryo at the stage of 40% DT, showing thoracic segments. D. Embryo at the stage of about 60% DT, showing thoracic segments.
E. Light micrograph of the first instar larva (ventral view) of C. insulicola fixed with alcoholic Bouinʼs fluid several hours after
hatching (modified from Kobayashi et al., 2013). The regions corresponding to the subcoxa (scx), subcoxa-1 (scx1), subcoxa-2 (scx2),
coxa (cx), and telopodite (tp) are highlighted in green, purple, red, blue, and brown, respectively.
ab1: first abdominal segment, ab1.tg: first abdominal tergum, aepm: anepimeron, aeps: anepisternum, at: antenna, cllr:
clypeolabrum, epl: epipleurite, fm: femur, hc: head capsule, hpl: hypopleurite, ist: inner sternite, kepm: katepimeron, keps:
katepisternum, lst: laterosternite, lsw: lateral swelling (highlighted in orange), md: mandible, mst: mediosternite, msw: medialswelling, mx: maxilla, pcxs: paracoxal suture, peps: preepisternum, pla: pleural apophyseal pit, pp: pleuropodium, pp(cx): coxal partof pleuropodium, pp(tp): telopodal part of pleuropodium, spr: spiracle, tar: tarsus, tb: tibia, tg: tergum (highlighted in yellow), th1:
prothoracic segment, th1.tg: prothoracic tergum, th2.l: mesothoracic leg, tpl: tergopleurite, tr: trochanter. Scales = A: 500 µm, B:
100 µm, C, D: 200 µm, F: 2 mm.

laterosternite, and inner sternite, are formed by medialextension of the subcoxal element (Fig. 2D, E, mst, lst, ist).
In summary, observations by Kobayashi et al (2013)convincingly demonstrated that the origin of the larval
anapleural and katapleural rings of C. insulicola can be traced
back to the embryonic subcoxa-1 and 2, respectively. In thisspecies, moreover, the coxa and subcoxae-1 and 2 are

identified in the proximal region of the abdominal appendage,
and these three regions participate in the formation of
abdominal pleuron. Thus, at least in this species, the subcoxal
theory is applicable to both thoracic and abdominal segments.
The applicability of the subcoxal theory to other hexapod
taxa largely depends on whether the subdivision of thesubcoxa into the subcoxae-1 and 2 could be discernible in their

36

Y. KOBAYASHI

Fig. 3 A–C. SEM micrographs of developing embryos (lateral views) of the whirligig beetle Dineutus mellyi (modified from Komatsu andKobayashi, 2012). A. Embryo at the stage of 52% DT. B. High magnification of rectangle in A, showing thoracic segments. C.
Embryo at the stage of 60% DT, showing thoracic segments. D–F. SEM micrographs of developing embryos of the dobsonfly
Protohermes grandis (Komatsu and Kobayashi, unpublished data). D. Ventral view of an embryo at the stage of about 40% DT. E.
High magnification of rectangle in D, showing thoracic segments. F. Lateral view of an embryo at the stage of about 40% DT. The
regions corresponding to the tergum (tg), subcoxa-1 (scx1), subcoxa-2 (scx2), and coxa (cx) are highlighted in yellow, purple, red,and blue, respectively.
ab1: first abdominal segment, lbp: labial palp, ltg: lateral tracheal gill, mxp: maxillary palp, pcxs: paracoxal suture, pp: pleuropodium,scx: subcoxa, spr: spiracle, th1: prothoracic segment, th.l: thoracic leg, th1.l: prothoracic leg, th2.l: mesothoracic leg, th3.l:
metathoracic leg, y: yolk. Scales = A–C: 200 µm, D–F: 100 µm.

embryonic stage or not. For the discrimination of the delicate
boundary between the two subcoxae, it is necessary to inspect
the data obtained by SEM observations. To date I have found
two regions corresponding to the subcoxae-1 and 2 in theproximal part of thoracic appendages in the following
published or unpublished data; Dineutus mellyi in Coleoptera(Komatsu and Kobayashi, 2012), Tribolium castaneum in
Coleoptera (Coulcher, 2011), Protohermes grandis in
Megaloptera (Komatsu and Kobayashi, unpublished data),
Chrysopa perla in Neuroptera (Konopová and Zrzavý, 2005),
and Nemotaulius admorsus in Trichoptera (Kobayashi andAndo, 1990).
For example, in the embryo of the whirligig beetle D.
mellyi at the stage of 52% DT (percent developmental time),
Komatsu and Kobayashi (2012) identified the subcoxa at thebasalmost region of the thoracic appendage (Komatsu andKobayashi, 2012, Fig. 5A, B, scx). They also regarded theregion just distal to the ʻsubcoxaʼ as the coxa. However, theirʻsubcoxaʼ actually corresponds to the subcoxa-1, because, in
meso- and metathoracic appendages, the ʻsubcoxaʼ (subcoxa-1)
houses a spiracular opening near the anterior corner (Fig. 3A,
B, scx1, spr), although the openings close in the subsequentSUBCOXAL THEORY REVISITED

stage (Fig. 3C). Their interpretation of the coxa (Komatsu andKobayashi, 2012, Fig. 5A, B, cx) is also incorrect; that is, theirʻcoxaʼ is actually the subcoxa-2 (Fig. 3B, C, scx2), because thesame region is identified as the ʻsubcoxaʼ in more advanced
stages (60% DT, 80% DT) (Komatsu and Kobayashi, 2012,
Figs. 7B, 9, scx).
In the embryo of the red flour beetle T. castaneum, thebasalmost region of the thoracic appendage is regarded as thesubcoxa which can be identified by both the SEM observationand expression patterns of marker genes (Distal-less andSerrata) (Coulcher, 2011, p. 97. Fig. 3.10). In my interpretation,
the Coulcherʼs ʻsubcoxaʼ probably corresponds to thesubcoxa-2, because the region does not include a spiracle. In
this species, the region which is more basal to his ʻsubcoxaʼ
and clearly delimited by a deep suture (paracoxal suture)
could be interpreted as the subcoxa-1 housing a spiracle.
In the embryo of the dobsonfly P. grandis, SEM
observations reveal the presence of subcoxae-1 and 2 which
are divided by the paracoxal suture at the basalmost region ofthoracic appendages (Fig. 3D, E, F, scx1, scx2, pcxs). In
another megalopteran species, Sialis lutaria, Bäcker et al.

Fig. 4 SEM micrograph of an embryo of the caddisfly
Nemotaulius admorsus at the stage of about 60% DT,
showing the proximal parts of thoracic appendages(ventral view) (Kobayashi and Ando, unpublisheddata). The specimen of this micrograph is the same
specimen used for Fig. 38 by Kobayashi and Ando
(1990). The regions corresponding to the tergum(tg), subcoxa-1 (scx1), subcoxa-2 (scx2), and coxa(cx) are highlighted in yellow, purple, red, and blue,
respectively.
ab1: first abdominal segment, ab10: tenth abdominal
segment, at: antenna, fm: femur, hl: head lobe, lb:
labium, lr: labrum, md: mandible, mx: maxilla, pcxs:
paracoxal suture, pp: pleuropodium, tar: tarsus, tb:
tibia, th1.l: prothoracic leg, tr: trochanter. Scale =100 µm.

37(2008, Fig. 9C) demonstrated the presence of katapleural andanapleural rings (their ʻtrochantinopleuriteʼ and ʻeupleuriteʼ,
respectively) in the pleural region of the larval thorax, andassumed those rings being subcoxal in origin. Although theauthors did not refer to the embryonic subcoxa of this species,
I believe that their ʻeupleuriteʼ and ʻtrochantinopleuriteʼ
correspond to the subcoxae-1 and 2, respectively.
In the closely related order Neuroptera, or in C. perla, an
SEM micrograph of a middle stage embryo by Konopová and
Zrzavý (2005, Fig. 8a) also strongly suggests the presence of
subcoxae-1 and 2 just ventral to the prospective tergum.
In the embryo of a caddisfly, N. admorsus, Kobayashi andAndo (1990) regarded the basalmost region of the appendage
as the subcoxa, and furthermore interpreted the region
between the subcoxa and the prospective tergum as theepicoxa sensu Kukalová-Peck (1983, 1987) (Kobayashi andAndo, 1990, Fig. 38, ecx). However, their ʻsubcoxaʼ andʻepicoxaʼ correspond to the subcoxa-2 and subcoxa-1,
respectively, which are clearly divided by the paracoxal suture(Fig. 4, scx1, scx2, pcxs). As pointed out by Kobayashi et al(2013), their subcoxa-1 almost corresponds to the KukalováPeckʼs epicoxa, which she postulated as the basalmost
podomere of the insect leg. However, they do not employ theterm epicoxa, because, unlike her epicoxa, which is used only
for the pleural region, the subcoxa-1 is assumed to participate
in the formation of the sternum.
These examples thus suggest that the subdivision of thesubcoxa into subcoxae-1 and 2 in the embryonic stage could
widely occur at least in such holometabolous orders as
Coleoptera, Megaloptera, Neuroptera, and Trichoptera. In
non-holometabolous orders, however, we have not yet
convincing data showing the subdivision of the subcoxa. The
ʻsubcoxaʼ in the embryonic thoracic legs of G. yuasai indicatedby Uchifune and Machida (2005, Fig, 15A, Scx) is most likely
to correspond only to the subcoxa-1, and the real subcoxa-2,
which may be very small area, may be concealed in theproximal part of the coxa (Uchifune and Machida, 2005, Fig.
15A, Cx). The same interpretation is probably applied to the
Roonwalʼs (1937) ʻsubcoxaʼ in L. migratoria: that is, his
ʻsubcoxaʼ is actually the subcoxa-1 housing a spiracle.
Inversely, as mentioned before, the Heymonsʼ (1899) ʻsubcoxaʼ
in N. cimicoides probably corresponds to the subcoxa-2,
because his subcoxa does not include a spiracle. Matsuda(1960) pointed out that the paracoxal suture is frequently
poorly defined in non-holometabolous insect orders, such as
Plecoptera, Dermaptera, and Isoptera. Thus it may be moredifficult to discriminate the two subcoxae in the embryonicperiod in the non-holometabolous taxa cited above than in
Holometabola.
In conclusion, in order to evaluate the applicability of thesubcoxal theory in a wide range of insect taxa, it is necessary
to accumulate the detailed embryological data on thedevelopment of the proximal part of appendages especially in
non-holometabolous orders.

38

Y. KOBAYASHI

Acknowledgments: I thank Messrs. Y. Oosawa, S.
Komatsu, and K. Niikura, and Dr. Y. Takami for their support
and cooperation to our embryological study of adephagan
beetles. I am grateful to Dr. F. Hayashi and the late Dr. H.
Ando for providing me the eggs of Protohermes grandis andNemotaulius admorsus, respectively. I also thank Dr. T.
Uchifune for his valuable comments on the manuscript.

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