The occurrence and significance of the pertirophic membrane, with special reference to adult lepidoptera and diptera.

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Abstract
Adults of a number of lepidopterous families and most adult cyclorrhaphous Diptera possess a tubular chitinous peritrophic membrane arising at the level of the oesophageal invagination. Membranes containing chitin, and produced by the general midgut epithelium, are present in adults of some other families of Lepidoptera and in some nematocerous and orthorrhaphous Diptera. In some species of these latter groups (e.g. the blood-sucking females of mosquitoes and tabanids) membranes appear to be mainly formed following ingestion of food and are produced by a restricted region of the general midgut epithelium. Chitin-containing membranes enveloping the food are present also in Thysanura, Embioptera, Mallophaga, Coleoptera (Dermestidae and Carabidae), Mecoptera, and Hymenoptera (some Formicidae). It has been established that the midgut epithelium is able to secrete a chitinous membrane. It is probable that the ancestral insect midgut epithelium possessed the capacity to secrete chitinous membranes enveloping the food. This capacity has been lost in some insects and in others it has been restricted to a particular zone of the midgut. It is possible that the well-defined tubular membranes are the result of the restriction of this capacity to the extreme anterior end of the midgut.