GROUNDPLAN STRUCTURE AND HOMOLOGY OF THE PLEURON IN HYMENOPTERA BASED ON A COMPARISON OF THE SKELETOMUSCULATURE OF XYELIDAE (HYMENOPTERA) AND RAPHIDIIDAE (NEUROPTERA)

Abstract

The groundplan structure and homologies of the mesothoracic pleurosternum of Hymenoptera are postulated using the subcoxal theory of the origin of the pterothoracic pleura of insects, and the cryptosternite theory of an invaginated sternum in most holometabolous insects, based on a comparison of the skeletomusculature of Xyelidae (Hymenoptera) and Raphidiidae (Neuroptera). The following are hypothesized for Hymenoptera: (1) the mesosternum is invaginated except for the prepectus, which is a detached remnant of the presternum; (2) lines that delineate ventral regions on the mesepisternum of many Symphyta are secondarily evolved sulci and arc not pleurosternal sutures; (3) a basalar cleft and an anepisternum are present in the groundplan structure; (4) in Hymenoptera other than Xyelidae the basalar cleft and anapleural cleft intersect so the anepisternum is detached as a separate sclerite, the postspiracular sclerite; (5) the pre-episternum is enlarged secondarily at the expense of the katepisternum, resulting in the paracoxal suture and a narrow katepisternum closely paralleling the posteroventral margin of the episternum anterior to the mesocoxa; (6) the mesepimeron is not divided into an anepimeron and katepimeron by a paracoxal suture but in some taxa is divided secondarily into an upper and lower mesepimeron by marks that delineate the line of confluence between different sets of muscles; and (7) the trochantin is absent from the groundplan structure. Postulated homologies support the hypothesis that Xyelidae represent the basal lineage of Hymenoptera but do not support the hypothesis that the rest of Hymenopera had a biphyletic origin from Xyelidae. The study exemplifies the positive feedback relationship between morphology and systematics and demonstrates the necessity of systematics and phylogenctic hypotheses for testing hypotheses of homology derived through comparative morphology. Schematic figures illustrate the subcoxal theory of pleuron origin and the postulated groundplan of the hymenopteran mesopleuron; mesothoracic muscles that were studied to help homologize pleurosternal features between Xyelidae and Raphidiidae are tabulated and skeletomusculature is documented by scanning electron photomicrographs.