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.