Paranotal Lobes Are Appendicular in Origin: Elucidation by Micro-CT Analysis of the Thoracic Muscular System in the Larvae of Carabus insulicola (Insecta, Coleoptera)

Abstract

In the embryo of Carabus insulicola, Kobayashi et al. (2013: J. Morphol., 274, 1323–1352) revealed that the subcoxa can be divided into subcoxae-1 and 2 by the paracoxal suture (PCXS), and these subcoxae form the larval pleuron. Mashimo and Machida (2017: Sci. Rep., 7, 12597), however, rejected the appendicular nature of subcoxa-1 and interpreted the PCXS as the boundary between the tergum and appendage (BTA), and thus they regarded subcoxa-1 as a tergal structure. To resolve the disagreement over the PCXS, we examined the larval thoracic muscular system of C. insulicola using an X-ray micro-CT. We conclude that subcoxae-1 and 2 are independent podomeres having intrinsic muscles in each region and that the paranotal lobe of the C. insulicola larvae is derived from the dorsal half of subcoxa-1. us, the paranotal lobe is appendicular in origin, contradictory to traditional interpretations of tergal origin. By comparing the muscle arrangement with the expression of leg patterning genes in Tribolium castaneum embryos, we suggest that subcoxa-1 corresponds to the crustacean precoxa, and the molecular BTA, or the boundary between the tergum and precoxa, is located more dorsally than the morphological BTA identied in C. insulicola. Combining these morphological and molecular data, we propose the “precoxal theory” for the origin of insect wings.