The mushroom bodies are two characteristically shaped structures of the insect central brain. In Drosophila melanogaster they contain more fibers in females than in males. Within the first week of adult life the total number of fibers increases by about 15% and decreases again in flies older than 3-4 weeks. The number of mushroom body fibers is significantly reduced in flies kept under social isolation or deprived of their antennal input, but not in flies subjected to visual deprivation.