Embryonic Development Of The Larval Appendages And Their Homology Between Mecoptera And Lepidoptera

The eruciform larvae of holometabolous insects are primarily characterized by bearing a varying number of abdominal prolegs in addition to three pairs of well-developed thoracic legs. However, whether these prolegs are serially homologous with the thoracic legs and whether the prolegs of these eruciform larvae(Mecoptera, Lepidoptera and Hymenoptera Tenthredinidae) are evolutionarily homologous are long-standing controversial problems, which have not been satisfactorily resolved to date. Based on comparative embryology and anatomy, the embryonic characters of the mecopteran and lepidopteran larvae have been studied. We compared and discussed the serial homology between the mandible and maxillia of the scorpionfly and proleg homology relationship between them. This can help explore the evolutionary evolution of the holometabolous insects on the basis of embryonic morphogensis and evolutionary aspects.Based on the comparative morphology, histological sections and anatomical analyses, we have studied and compared the embryonic development of appendages and muscle arrangement pattern of the scorpionfly Cerapanorpa byersi Hua and Huang, 2007(Mecoptera: Panorpidae) and the armyworm Mythimna separata(Walker, 1865)(Lepidoptera: Noctuidae), discussing serial homology of mandible with maxilla and proleg homology between Mecoptera and Lepidoptera. The result showed the molar and incisor lobes of the mandible are differentiated at the base, reprenting endites, and they are serially homologous with the lacinia and galea of the maxilla, respectively.We examined the embryonic features and histological structure of the prolegs of the scorpionfly Cerapanorpa byersi Hua and Huang(Mecoptera: Panorpidae) and the Oriental armyworm Mythimna separata(Lepidoptera: Noctuidae) to investigate whether the prolegs are homologous between these two holometabolous insect orders. In the scorpionfly, we found that the thoracic leg rudiments are differentiated into five segments–the coxa, femur, tibia, tarsus, and pretarsus. Paired lateral process primordia arise on abdominal segments A1–A8 in line with the thoracic legs in early embryonic stages, but degenerate into triangular protuberances in later stages, persisting into the larval stage, and paired medial processes appear along the midventral line before dorsal closure and eventually develop into unjointed, cone-shaped prolegs. Histological observation based on the serial sections showed that the lumina of the prolegs are not continuous with the hemocoel, differing distinctly from that of the basic appendicular plan of thoracic legs. These results suggest that the prolegs are not appendicular derivatives but secondary outgrowths in Mecoptera. In the armyworm, lateral process primordia appear on A1–A10 in line with the thoracic legs in the early stages, although only the rudiments on A3–A6 and A10 develop into segmented prolegs with the lumina continuous with the hemocoel and others degenerate eventually, suggesting that the prolegs are true segmental appendages serially homologous with the thoracic legs in Lepidoptera.Moreover,we compare the muscular system of thoracic legs and lateral appendages. The result that the muscles connected with lateral processes appear to be has a similar arrangement with the muscles associated with the thoracic legs was present as evidence not supporting the serial homology between the prolegs and thoracic legs in the scorpionfly. Similarly in Lepidoptera, a similar distribution pattern of muscles between prolegs and thoracic legs suggests a possibility that the prolegs are serially homologous with the thoracic legs.Hence we speculate that the prolegs of the scorpionfly are not true abdominal appendages. The abdominal prolegs and thoracic legs are not serially homologous. Compared with that of Mecoptera, Our embryological observations of M. separata show that the prolegs are similar to the thoracic legs in development and position, indicating the prolegs are derived from segmental appendages. Therefore, we conclude that the larval prolegs are not evolutionarily homologous between Mecoptera and Lepidoptera.