Light and electron microscopic study of the hindgut of the ant (Formica nigricans, Hymenoptera): I. Structure of the ileum

The study of the ileum of the ant Formica nigricans by light and electron m icroscopy revealed the existence of three differentiated regions: proximal, middle, and distal ileum. The middle region constitutes most of the length of the organ. Its wall is made up by a folded simple epithelium lined by a cuticle, which is surrounded by an inner circular muscle-layer and various external longitudinal muscle fibers adjacent to the hemolymph. A subepithe lial space is present between the epithelium and the circular muscle layer. Epithelial cells show extensive infoldings of the apical,:and to a lesser extent the basolateral plasma membrane. Apical infoldings are characterized by the presence of 10-nm particles (portasomes) covering the cytoplasmic s ide of the membrane. Mitochondria are abundant throughout the cytoplasm, al though they mainly are present underneath the apical infoldings. Lateral bo rders of epithelial cells display an apical junctional complex, mainly cons tituted by a long and convoluted pleated septate junction. These features s upport the view that epithelial cells in the middle ileum are specialized i n ion:solutes and water transport. The proximal ileum connects with the: am pulla into which the Malpighian tubules drain. As opposed to the middle-ile um, epithelial cells of the proximal ileum show less developed basolateral infoldings, and the apical plasma membrane is devoid of portasomes and only occasionally invaginates. These features suggest that the proximal ileum p lays no relevant role in ion and water transport. The distal ileum penetrat es into the rectal sac, forming a valve-like structure; this region presuma bly controls the amount of urine reaching the rectum. (C) 1999 Wiley-Liss, Inc.