The Nervous System of Certain Abdominal Segments of the Cecropia Larva (Lepidoptera: Saturniidae)1

Abstract

The muscles of the second and third abdominal segments and their innervations are described. The musculature of the third segment conforms, in general, to the pattern found in other lepidopterous larvae. In the second segment, which has no proleg, there are no muscles homologous with the retractors of the planta nor is there a branch of the ventral nerve root such as innervates these muscles in the third segment. This supports the theory that the proleg is a true metamorphic character, without phylogenetic significance. Also, the dissimilar musculature of the proleg in Cecropia as compared with other larvae supports Hinton's theory of the polyphyletic origin of prolegs in the Lepidoptera. Dorsal, ventral, and transverse pairs of nerve roots arising from each segmental ganglion seem homologous with those described for the pregenital segments of certain Orthoptera. Here, as in those Orthoptera, the ventral nerve fuses with the transverse nerve of its own ganglion, the dorsal nerve with the transverse nerve of the preceding segmental ganglion, and the spiracular muscles are innervated from the connective between the dorsal and the transverse nerves. These criteria of homology in such widely separated orders as Orthoptera and Lepidoptera support the concept of a basic segmental nerve pattern within the Hexapoda. The presence of these criteria of homology implies that holometabolous larvae, such as that of Cecropia, probably evolved from the hemimetabolous immature form, such as that of the orthopterans.