Peritrophic membrane formation in three temperate simuliids,Simulium ornatum, S. equinumandS. lineatum, with respect to the migration of onchocercal microfilariae

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Abstract
Engorged females of three temperate simuliid species, Simulium ornatum, S. lineatum and S. equinum, were collected immediately after their bloodfeeds from a cow host. Midguts were dissected out at selected time intervals thereafter, from two minutes to 24 hours post engorgement (p.e.) and peritrophic membrane formation observed. There was an initial rapid secretion of peritrophic membrane (PM) material from the midgut epithelium. The peritrophic membrane produced is Type 1, being formed by delamination from the entire midgut epithelium. This secretory phase continued for approximately two hours p.c. in all three species and was followed by a period of organization within the membrane. At between 12 and 24 hours p.e., the PM appeared more organized and was associated with the formation of several distinct laminae in both S. ornatum and S. lineatum, although in S. equinum it consisted of only a single homogenous layer. Both the rate of secretion and the resulting morphological appearance of the PM appeared species-specific and there was great variation in its thickness in all three species at all time intervals examined. It is suggested that microfilariae penetrate the PM during the initial period of its secretion before it has condensed to form a distinct structure. Furthermore, the migration of microfilariae may be restricted to, or enhanced by, the presence of very thin areas within the membrane where erythrocytes lie almost adjacent to the midgut epithelium.