Meconial peritrophic membranes and the fate of midgut bacteria during mosquito (Diptera : culicidae) metamorphosis

The location of midgut bacteria relative to meconial peritrophic membranes (MPMs) and changes in bacterial numbers during midgut metamorphosis were st udied in Anopheles punctipennis (Say), Culex pipiens (L.), and Aedes aegypt i (L.) pupae and newly emerged adults. After adult emergence in Aedes, Anop heles, 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 lume n or within the MPMs/meconia. In a few Culex specimens, high numbers of bac teria were found in the MPMs/meconia and low numbers in the lumen. In all t hree 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 durin g mosquito metamorphosis and adult emergence. This mechanism appears to inv olve the sequestration of remaining larval gut bacteria within the confines of the meconium and one or two MPMs and the possible bactericidal effect o f the exuvial (molting) fluid, which is ingested during the process of adul t emergence.