In vivo negative staining of the midgut continuous junction in the mosquito, Culex tarsalis (Diptera: Culicidae).

The midguts of female mosquitoes, Culex tarsalis, were examined electron microscopically during the digestion of a meal from either artificial sources (i.e., 100% serum or defibrinated rabbit blood) or vertebrate hosts. Intense intercellular staining was apparent when the meal was derived from the vertebrate host or defibrinated rabbit blood; less intense staining with 100% serum. The staining was attributed to the "leakage" of a component of whole blood, presumably hemoglobin, into the intercellular junctional spaces. The staining component demonstrated an affinity for the outer membrane leaflet of the plasma membrane. This study provides evidence in support of "leaky guts" as a means of infecting an arthropod host without a midgut amplification cycle. The ramifications of this concept are pointed out with reference to vector competence and midgut barriers to infection by arboviruses.