Abstract
The sensilla of Drosophila larval thoracic and abdominal segments appear in a constant temporal sequence during stage 13/14 (9·5–11·5 h) of embryonic development. Those sensilla innervated by more than one dendrite (basiconical sensilla, chordotonal organs, some of the trichoid sensilla and campaniform sensilla) appear earlier than sensilla innervated by a single dendrite (majority of trichoid sensilla and campaniform sensilla). Furthermore, a dorsoventrally directed gradient underlies the sequence in which sensilla of a given type appear. Sensory axons are emitted in the same sequence. Thus, axons of the polyinnervated sensilla appear first. Together with a distinct set of efferent axons they form ‘pioneer tracts’ of the two fascicles of the segmental nerves. Cytodifferentiation of the sensillum cells resembles the development of larval epidermal cells in many aspects. Thus, the sheath processes formed by sensillum accessory cells and the axons formed by sensory neurones develop from processes transiently formed by all cells. During the phase of cuticle secretion, apical portions of the presumptive accessory cells are modified to form the cuticular apparatus responsible for receiving the sensory stimuli. Finally, two sets of subepidermally located cells which differ with respect to their morphology and, probably, their origin envelop somata and axons of the sensory neurones.
