Midgut ultrastructure of Culex tarsalis (Diptera: Culcidae) before and after a bloodmeal.

The midgut ultrastructure of rasin-and blood-fed female mosquitoes, Culex tarsalis Coquillentt, was examined. The raisin-fed midgut is characterized by: (1) large nuclei, (2) small mitochondria, (3) short segments of rough endoplasmic reticulum, (4) rough endoplasmic reticular vesicular vesicles in the posterior midgut only, and (5) increased autophagic acitivity with age. Blood feeding elicits drastic changes in midgut epithelial structures: (1) nuclei are smaller, (2) mitochondria are much enlarged, (3) rough endoplasmic reticular vesicles disappear, (4) rough endoplasmic reticular whorls appear, (5) residual lyosomal figures are abundant, and (6) an intercellular accumulation of an electron-opaque material is noted. The significance of rough endoplasmic reticular whorls and vesicles in bloodmeal digestion is discussed. In addition, the concept of a functional host 'gut barrier' to infection by pathogens is examined as related to a possible by-pass mechanism.