Embryonic Development Of The Panorpidae(Mecoptera)

Mecoptera is one of the most primitive orders in holometabolous insects because theybear a pair of prominent compound eyes in the larval stage, representing the intermediate linkbetween the Holometabola and Hemimetabola and occupying an important position inphylogenetic study of insect (especially in Holometabola). Study on the organogeneticfeatures and appendage homology in Mecoptera embryogenesis can help explore thephylogenetics and evolution of the holometabolous insects based on both morphogensis andevolutionary aspects.Based on the comparative embryology, histology and ultrastructural methods, we usedlight and electronic microscopy techniques to study the embryonic development ofPanorpidae in detail, in attempt to solve some long-standing controversial problems, such asthe homology between mandibles and maxillia, organogenesis (including embryonicmembrane, the alimentary canal, and nervous system), and the homology of thorax legs andprolegs. Combining with the abdominal appendages in representative species of othereruciform larvae Leucania separate (Lepidoptera) and Dolerus tritici (Hymenoptera), as wellas Bittacus planus (Mecoptera), we analyzed the developmental models of eruciform larvae,explored the origin and evolution of prolegs in different insect groups, and further revealedtheir homology. Our study may provide more evidences for the phylogenetic and evolutionarystudies of the holometabolous insects. The main results are as follows:The segmentation on the mandible of Panorpidae is unconspicuous in early stage anddistinct in middle stage of the embryonic development. As the development of mandibles anddifferentiation of the molar lobe and incisor lobe, the basal segment of the mandible isgradually disappeared. Based on the segmentation, we considered that the mandible consistsof two parts, the smaller basal mandibular subcoxa and longer distal mandibular coxa. Thereduced mandibular subcoxa is homologous with the cardo of maxilla. The molar and incisorlobes are endites, serially homologous with the lacinia and galea of maxilla, respectively.The results show that the invaginations of stomatodeum and proctodeum of Panorpobtusa are formed by the ectoderm, simultaneously occurring at the surface of embryonichead center and the end of abdominal segment. Six Malpighian tubules are derived from theectodermal processes at the end of lateral proctodaeum wall. The midgut originates from the endodermal cell bands, which are situated at the anterior and posterior embryo. Two pairedcell bands in both directions combine together and extend towards each other, finally enclosethe yolks and form the midgut rudiment. The ganglion cells formed by the neuroblast take animportant role in ganglion, nerve fiber, and lateral nerve cord formations.During the investigation of the thoracic and abdominal appendage development ofDicerapanorpa magna, we found that the thoracic leg rudiments are located at the lateralsides in each segment and finally develop as the five-segmented thoracic legs, including coxa,femur, tibia, tarsus, and pretarsus. Compared with the thoracic legs, the proleg primordia aresituated at the inner side near the midventral line of the abdomen. The primary true abdominalappendages along the same line of the thoracic legs are disappeared gradually duringembryogenesis, only leaving flat vestiges on the lateral sides of the abdomen.However, the prolegs in other eruciform larvae (Leucania separate and Dolerus tritici)are primary structures, which are developed from the proleg rudiment directly. The prolegprimordia of L. separate are located along the same line of the thoracic leg rudiments.However, the positions of the proleg primordia in both D. tritici and Panorpa obtusa aremuch closer to the inner midventral line of the abdomen. The proleg primordia of L. separateand D. tritici finally developed into stout prolegs as the important locomotion organs, whereasthe prolegs of Panorpidae and Bittacidae (Mecoptera) larvae are small and short,fundamentally no locomotory functions. The incompletely degenerated prolegs are very likelyto function as protecting the abdomen.Hence we speculate that the prolegs of panorpids are not the true abdominal appendages.The abdominal prolegs and thoracic legs are not serially homologous. Compared with that ofMecoptera, the prolegs of other eruciform larvae are distinct in origin, function or morphology.Therefore, the prolegs are not homologous, but very likely evolve independently. Accordingto the distinct origin of the prolegs, we modified the concept of eruciform larvae temporarilyas that they can be subdivided into primitive and secondary types.