43Proc. Arthropod. Embryol. Soc. Jpn. 52, 43–44 (2019)Ⓒ 2019 Arthropodan Embryological Society of Japan
ISSN 1341–1527

The Paracoxal Suture in Insect Embryos: Its State and Importance for
Understanding the Basalmost Podomeres*
Yukimasa KOBAYASHI
Sayamadai 2–21–18, Sayama-shi, Saitama 350–1304, Japan
E-mail: dineutus@hotmail.co.jp

In the late embryo of the ground beetle Carabus
insulicola, the subcoxae, or the basalmost podomeres, ofthoracic appendages are divided into the proximal subcoxa-1
and distal subcoxa-2 by a semi-circular furrow (Kobayashi
et al., 2013). By comparing those subcoxae with the larval
pleural sclerites, Kobayashi et al. assumed that subcoxae-1
and 2 correspond to the larval anapleural ring (anapleuron)and kataplelural ring (katapleuron), respectively; thus, thesemi-circular furrow should be equivalent to the paracoxal
suture sensu Matsuda (1970). Kobayashi (2017) further
pointed out that the demarcation between subcoxae-1 and 2
could be detected in some holometabolan embryos, such as
Megaloptera, Neuroptera, and Trichoptera, but is obscure in
the embryos of non-holometabolous taxa.
On the other hand, in the embryo of the orthopteran
Gryllus bimaculatus, Mashimo and Machida (2017) revealed
that the thoracic pleura enlarge rapidly without dividing
into subcoxae-1 and 2. They defined the dorsal edge of thedeveloping subcoxa as the boundary between the tergum
and appendage (BTA). Based on this definition, they insisted
that the paracoxal suture of the Carabus embryo is the BTA,
and that the boundary between the ʻsubcoxaʼ and ʻcoxaʼ in theembryo of the grylloblatodean Galloisiana yuasai observed byUchifune and Machida (2005) is also the BTA. In other words,
the subcoxa-1 sensu Kobayashi et al. and the whole subcoxa
sensu Uchifune & Machida are the region which should beincluded in the tergum.
To explore the background of this disagreement ininterpretation, I have surveyed the literature (chiefly byMatsuda, 1970) describing the state and distribution of
the paracoxal suture in all hexapod orders. As a result, thefollowing are key considerations for discussing the embryonicparacoxal suture. (1) Based on the distribution of pleurocoxal muscles of Orthoptera, it is assumed that almost all thepterothoracic pleural areas belong to the katapleuron. The
anapleuron may potentially exist near the dorsal edge of thepleuron, and consequently the paracoxal suture may disappear
secondarily. This state of orthopteran pleura, as pointed out by
Matsuda (1970), is considerably specialized, and the result of
Mashimo and Machida (2017), in which the paracoxal suture

is not observed in embryogenesis, seems to reflect thisspecialization. Thus, one should be cautious when generalizingthese results. (2) In contrast, in several polyneopteran orders
such as Grylloblattodea and Blattodea, the anapleuron isrelatively large, and both the small katapleuron and theparacoxal suture are found only in the episternum. In theseorders, the paracoxal suture is absent in the epimeron. (3) In
most holometabolous orders, the paracoxal suture is present
in both the episternum and epimeron of adult pterothoracic
pleura. In the larvae, however, there are no data explicitly
indicating the state of the paracoxal suture.
I have also reexamined the correspondence between theembryonic subcoxae-1 and 2 and the larval pleural scleritesof C. insulicola. In this species, unlike Gryllus, it is difficult toassume the rapid enlargement of the subcoxa in late embryonicstages. I have therefore reached the conclusion that the division
into subcoxae-1 and 2 is valid, and the embryonic paracoxal
suture is not equivalent to the BTA. However, to confirm
whether the embryonic subcoxae-1 and 2 really develop into
the larval anapleuron and katapleuron, I plan to observe thethoracic muscles of Carabus larvae with a micro-CT.
Based on the second consideration described above, theʻsubcoxaʼ of the Galloisiana embryo (Uchifune and Machida,2005) is probably equivalent to subcoxa-1. Hence the boundary
between the ʻsubcoxaʼ and ʻcoxaʼ is not the BTA, probably
showing the paracoxal suture plus the boundary between theepimeron and coxal meron.
I cordially thank Heather Bruce for English edits andcorrections.
References
Kobayashi, Y. (2017) Formation of subcoxae-1 and 2 in insect embryos:
The subcoxal theory revisited. Proceedings of the Arthropodan
Embryological Society of Japan, 48, 33–38.
Kobayashi, Y., Niikura K., Oosawa, Y. and Y. Takami (2013) Embryonicdevelopment of Carabus insulicola (Insecta, Coleoptera,Carabidae) with special reference to external morphology andtangible evidence for the subcoxal theory. Journal of Morphology,274, 1323–1352.
Mashimo, Y. and R. Machida (2017) Embryological evidence* Abstract of paper read at the 54th Annual Meeting of the Arthropodan Embryological Society of Japan, May 18–19, 2018 (Sugadaira, Nagano).

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substantiates the subcoxal theory on the origin of pleuron ininsects. Scientific Reports, 7, 12597.
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 ofGalloisiana yuasai Asahina, with special reference to external
morphology (Insecta: Grylloblattodea). Journal of Morphology,266, 182–207.