21Proc. Arthropod. Embryol. Soc. Jpn. 50, 21 (2016)Ⓒ 2016 Arthropodan Embryological Society of Japan
ISSN 1341–1527

A Challenge to the Subcoxal Theories of Pleural and Sternal Origins inInsects*
Yuta MASHIMO and Ryuichiro MACHIDA
Sugadaira Montane Research Center, University of Tsukuba, Sugadaira Kogen, Ueda, Nagano 386–2204, Japan
E-mail: mashimo@sugadaira.tsukuba.ac.jp (YM)

The segment of insects is composed of the dorsal
tergum, lateral pleuron and ventral sternum. Based on hisembryological studies, Heymons (1899) suggested that thepleuron originates from the most proximal podomere or thesubcoxa. His idea has been elaborated as the “subcoxal theoryof pleural origin” by later morphologists such as Snodgrass,Weber and Matsuda. Ferris suggested the secondary nature ofsterna, and Weber accepted this idea and developed the“subcoxal theory of sternal origin”. The subcoxal theories of
pleural and sternal origin either have been widely acceptedbut variously discussed according to authors without reallylooking for reliable developmental facts in support (cf.
Matsuda, 1970). In the present study, we critically examined
the formations of pleura and sterna, using a cricket Gryllus
bimaculatus as material, and discuss the origins of pleuron
and sternum to review the subcoxal theories of pleuron andsternum.
A pair of ectodermal protuberances appears, and soon
each of them differentiates into the proximal part or thetergum and the distal part or the thoracic appendage, whichthen divides into the coxopodite and telopodite by a deep
demarcation. The terga develop and expand, and in the middle
stage of development a curving sha1low depression appears
in the middle part of each tergum. Whereas the coxopodite
divides into the proximal subcoxa and the dista1 coxa, thetelopodite elongates and is segmented. In the anteroventral
parts of the meso- and metathoracic terga, the spirac1e
invaginates. The spiracles have been usually regarded as
pleural in origin (cf. Uchifune & Machida, 2005), but it may be
more likely that the spiracles should be tergal derivatives inlight of the serial homology between the spirac1es andposterior tentorial pits formed in the gnathal tergal region.

Katatrepsis completes, and the terga rapidly extend
dorsally. On the completion of dorsal closure, the larval cuticle
is secreted. Simultaneously, the subcoxa1 region rapidly andsubstantially extends up to the definitive extension of pleuron.
The pleuro-coxal joint invaginates near the subcoxa-coxal
boundary, and from it the pleural suture differentiates andruns dorsally.
In the middle stage of development, most of cells havereceded from the ventral area of each segment in association
with the neurogenesis, and consequently the ventra1 surfaceof the segment is only covered with a thin cellular layer or acellular sheath of developing ganglia. In the final stage ofembryogenesis when the larval cuticle is secreted, thesubcoxa starts to rapidly extend medially, and form thedefinitive ventral epidermis or the eusternum.
The present embryological, critical observations oncrickets revealed that the pleura and sterna either aresubcoxal derivatives. As for the sclerites containing spiracles,
different from current understandings, their origin may beassigned to terga.
References
Heymons, R. (1899) Beiträge zur Morphologie und Entwicklungsgeschichte der Rhynchoten. Nova Acta. Abhandlungen derKaiserlichen Leopoldinisch Carolinischen Deutschen Akademie derNaturforscher, 74, 353–456.
Matsuda, R. (1970) Morphology and evolution of the insect thorax.
Memoirs of the Entomological Society of Canada, 76, 1–431.
Uchifune, T. and R. Machida (2005) Embryonic development of
Galloisiana yuasai with special reference to external morphology(Insecta: Grylloblattodea). Journal of Morphology, 266, 182–207.

* Abstract of paper read at the 51st Annual Meeting of the Arthropodan Embryological Society of Japan, June 12–13, 2015 (Urabandai, Fukushima).