The location of midgut bacteria relative to meconial peritrophic membranes (MPMs) and changes in bacterial numbers during midgut metamorphosis were studied in Anopheles punctipennis (Say), Culex pipiens (L.), and Aedes aegypti (L.) pupae and newly emerged adults. After adult emergence in Aedes, Anopheles, and most Culex, there were few to no bacteria in the midgut. In most newly emerged adult mosquitoes, few bacteria were found in either the lumen or within the MPMs/meconia. In a few Culex specimens, high numbers of bacteria were found in the MPMs/meconia and low numbers in the lumen. In all three species bacterial counts were high in fourth instars, decreased after final larval defecation, increased in young pupae, and increased further in old pupae. A very effective gut sterilization mechanism is operating during mosquito metamorphosis and adult emergence. This mechanism appears to involve the sequestration of remaining larval gut bacteria within the confines of the meconium and one or two MPMs and the possible bactericidal effect of the exuvial (molting) fluid, which is ingested during the process of adult emergence.

In mosquitoes, in addition to larval and adult peritrophic membranes (PMs), a PM (meconial peritrophic membrane or MPM1) forms in the pupa around the meconium, the sloughed, degenerating larval midgut epithelium. Often, a second membrane (MPM2) forms in temporal proximity to adult emergence. Differences in the occurrence, persistence, and timing of disappearance of the meconium/MPMs and gas were studied by dissecting the midgut contents from pupae of known ages postpupation and from adults of known ages postemergence. MPM1 was found in all Anopheles and Culex studied and nearly all Culiseta. The occurrence of MPM1 varied in the Aedes species. In one series of Aedes aegypti (L.) dissections, no fully formed MPM2 was found in any specimens. The occurrence of MPM2 appeared to be associated with adult emergence and varied among and within the seven species studied. It typically was seen in recently emerged adults but was observed occasionally in old pupae. Much of our data supports the idea that MPM2 formation is stimulated by midgut epithelium distention.