Abstract
The Drosophila brain is highly variable in size. Female flies grown in densely populated larval cultures have up to 20% more Kenyon cell fibers in their mushroom bodies than flies from low-density cultures. These differences in the number of Kenyon cell fibers are accompanied by differences in the volume of the calyx. During imaginal life, volume changes are observed in the calyces, all parts of the optic lobes, the central brain, and central complex. They occur not only in the first week of adulthood but also between days 8 and 16. Factors causing these changes are little understood. In flies kept in pairs for 1 week, the size of the calyx but not of the lobula is influenced by the sex of the partner. Females have larger calyces if the partner is female than if it is male. Males seem to be affected in the same way. Females living solitarily in little food vials for the first week of their imaginal life have smaller lobulae than females having spent the week in a populated flight cage. Males and females of heterosexual groups of 40 animals have still smaller lobulae, medullae, and calyces than their siblings kept in isolation. In addition, brain size is influenced by yet unidentified factors causing long and short trends. The data suggest that in Drosophila, most neuropil regions of the brain are continuously reorganized throughout life in response to specific living conditions.