Abstract
The ultrastructure of the anal organ of the full-grown larva of Drosophila melanogaster is described. The thin cuticle is characterized by epicuticular depressions which contain particulate material. In AgNO(3)-treated larvae, silver grains tend to penetrate the cuticle at the epicuticular depressions. At the basal surface, the epithelial cells exhibit narrow, parallel membrane infoldings which bear a particulate coat on the cytoplasmic surface. The infoldings are also attached around the cytoplasmic surface of endocuticular tubercles, thereby greatly increasing the absorptive surface area. At the apical surface, the membrane invaginations, which are closely associated with mitochondria, anastomose freely and extend deeply into the cytoplasm. The lateral membranes are linked by desmosomes and septate junctions. They are highly folded, are closely associated with mitochondria, and enclose intercellular channels and spaces. The epithelial cells are rich in mitochondria, glycogen particles and tracheoles. Numerous vesicles, multivesicular bodies, lysosome-like dense bodies and sparse endoplasmic reticulum are found in the cytoplasm. In concentrated medium, the epithelial cells show complete absence of the membrane infoldings and invaginations and reduction in the number of mitochondria. The ultrastructural features of the anal organ are consistent with its function in ion transport.
