Peritrophic membranes and protease activity in the midgut of the malaria mosquito, Anopheles stephensi (Liston) (Insecta: Diptera) under normal and experimental conditions.

Formation and solubility of the peritrophic membranes (pm) in the midgut (stomach) of female Anopheles stephensi depended on salt concentration in the gut lumen. High calcium drastically reduced the solubility of the pm in vitro. Thin-layer chromatography revealed the presence of N-acetylgalactosamine and galactose in the pm. In contrast to Aedes aegypti no N-acetylglucosamine was present indicating significant differences between Anopheles and Aedes mosquitoes with respect to pm formation and composition. Experimental distension of the stomach epithelium did not trigger secretion of proteases, but membrane-bound granules were released from the stomach cells, and pm were formed. When females were fed blood 18 hr after an enema with salt solution, when practically no granules were present in the gut cells, protease activity was increased, and blood was digested normally, but no pm were formed. A functional correlation of the membrane-bound granules and pm formation is postulated. alpha-Amanitin prevented normal digestion, protease activity was only insignificantly increased, and no pm formed after blood intake.