Ultrastructure of the larval rectum of the scorpionfly Panorpa liui (Mecoptera: Panorpidae).

The rectum is an important part of the alimentary canal responsible for ion and water reabsorption of insects. However, it has rarely been studied in the larvae of Panorpidae, the largest family in Mecoptera. Here, we investigated the ultrastructure of larval rectum of the scorpionfly Panorpa liui Hua, 1997 using light and transmission electron microscopy. The rectum comprises tracheal muscular layers, connective tissue, non-cellular basal lamina, junctional cells, rectal epithelium, cuticle with irregular outlines, and a central lumen. The rectal epithelium is infolded to form six longitudinal rectal folds, which are distinct from rectal pads or papillae. In each rectal fold, the apical and basal plasma membranes of epithelial cells are infolded and the lateral plasma membranes form septate and scalariform junctions. The well-developed rectal folds are postulated to be closely associated with reabsorption of ions and water in the larvae. The associations of rectal folds with larval behaviors are briefly discussed in Mecoptera.

Ultrastructure of the dorsal ocellus of Bittacus planus larvae (Mecoptera: Bittacidae) with evolutionary significance.

The Bittacidae are unique in holometabolous insects in that their larvae bear a dorsal ocellus on the frons. The fine structure of the dorsal ocellus, however, has not been investigated to date. Here, the ultrastructure of the larval dorsal ocellus was studied in the hangingfly Bittacus planus Cheng, 1949 using light, scanning, and transmission electron microscopy. The dorsal ocellus of the larvae comprises a cornea, corneagenous cells, and retinula cells. The cornea is a laminated structure. A layer of corneagenous cells is located below the cornea. Numerous retinula cells are arranged tightly beneath the corneagenous cells. The retinula cells modify their adjacent membranes into numerous linear microvilli, which form an analogue of the rhabdom among adjacent retinula cells. The results show that the dorsal ocellus of larval Bittacidae is a highly vestigial organ and appears to be degenerating during the postembryonic development. The presence of the vestigial dorsal ocellus is likely to represent an ancestral plesiomorphy of holometabolous insects, providing new evidence for exploring the evolutionary origin of holometabolous larvae.