Embryonic development of the alimentary canal of the scorpionfly Panorpa obtusa Cheng (Mecoptera: Panorpidae).

The embryonic origin of the alimentary canal, especially the midgut, is a controversial problem in insects, and it has not been satisfactorily resolved to date. The organogenesis of the digestive system in the embryonic development was observed in the scorpionfly Panorpa obtusa Cheng using light, transmission, and scanning electron microscopy. The embryonic development lasts about 150-160 h at 24°C. The stomodaeum is formed from an invagination in the medioposterior portion of the protocephalon mid-ventrally posterior to the labral segment at 76 h after oviposition. The proctodaeum arises as an invagination from the caudal end of the abdomen at 78 h. Four anal forks are formed from within the opening of proctodaeum. Three pairs of proctodaeal evaginations are formed from the anterior part of the proctodaeum, and eventually developing into Malpighian tubules, thus are of ectodermal origin. The cardiac and pyloric valves develop from stomodaeum and proctodaeum, respectively, and also of ectodermal origin. The midgut epithelium originates from anterior and posterior midgut rudiments in blind ends of the stomodaeum and proctodaeum, and it is of endodermal origin. The two cell-bands (rudiments) cover the yolk ventrally and then dorsally, elongate to each other, and eventually fuse to form the midgut. The midgut formation pattern is briefly discussed in different insects.