The taste bristles of the adult labellum develop from the labial discs within the first 30 hr of pupation. The neuron-specific antibody Mab22C10 and the tissue-specific beta-galactosidase activity in the A37 strain were used as cell markers for the developing sensilla. These experiments revealed that the sensory progenitors of all the labellar bristles are specified in three waves occurring 0, 6, and 16 hr after pupation. The incorporation of the thymidine analog 5-bromo-2'-deoxyuridine (BrdU) into the cells of the developing proboscis further supports the temporal pattern of the sensillum specification. Mitotic activity in the bristles specified in the first wave is completed before the next wave is initiated. The eight cells that constitute a single bristle share a common lineage. The trichogen and tormogen cells are siblings from a single progenitor and the thecogen and the five neurons share a common ancestry. Our results provide a basis for the identification of molecules that specify the development of the polyinnervated bristles of the adult and the specification of neurons with different stimulus specificities.