FORMATION AND ULTRASTRUCTURE OF THE PERITROPHIC MEMBRANE IN LARVAL MIDGE CHIRONOMUS-TENTANS (DIPTERA, CHIRONOMIDAE)

The ultrastructural features of the cardia from fourth instar larvae o f the midge Chironomus tentans are consistent with it being the source of the peritrophic membrane. The group of anterior epithelial cells d isplay short basal membrane infoldings, abundant rough endoplasmic ret iculum, Golgi complexes, mitochondria, free ribosomes, vesicles and el ectron-dense secretory granules. The secretory material is secreted in to the narrow luminal cleft between the evenly spaced microvilli and t he apposing non-secretory esophageal valve. The valve and apices of se cretory cells act as a press to mold the secretory product into the de finitive peritrophic membrane. The single 255-488 nm thick uniform per itrophic membrane is devoid of holes and is composed of three layers i n the posterior cardia, two layers in the rectum but only one 110 nm t hick discernable layer in the midgut. Since the inner loosely arranged third layer is absent in the midgut and the rectum, it is postulated that it is incorporated into the electron-dense second layer. The seco nd or middle layer which represents the whole peritrophic membrane in the midgut displays unique vertical striations that enclose 6.5 nm dia meter channels. It is suggested that these newly described open-ended channels facilitate passage of water, salts, digestive enzymes and dig ested food material in either direction.