Ultrastructural midgut events in Culicidae larvae fed with Bacillus sphaericus 2297 spore/crystal complex

Ingestion of Bacillus sphaericus 2297 spore/crystal complex by Culicidae larvae Anopheles stephensi, Culex pipiens subsp. pipiens and Aedes aegypti was rapidly followed by a dissolution of the protein crystalline inclusions inside the anterior stomach of the three species. During the first day of intoxication, B. sphaericus spores germinated within the midgut lumen, and were in a vegetative stage between 36-48 h after ingestion when the larvae began to die. Ultrastructural observations focused on larval midgut showed alterations which differed according to the mosquito species, being localized mainly in the gastric caeca and posterior stomach. With the bacterial concentration used, neither general cell swelling nor complete breakdown of the midgut epithelium was recorded before larval death. In A. stephensi larval midgut epithelium large low-electron-density areas appeared, rough endoplasmic reticula formed numerous concentrical structures and mitochondria swelled. Large vacuoles (of unknown origin) appeared early on in the C. pipiens midgut cells, and rough endoplasmic reticula broke into small vesicles. Midgut epithelial cells of A. aegypti showed mitochondria swelling except in the anterior stomach, and a vacuolisation of smooth reticula: these aspects remained unchanged until the larvae died.