The precursor cells of the larval sensilla can be first identified in the ectoderm of a stage 11 embryo (Ghysen and O'Kane 1989; Bodmer et al. 1989; see Jan and Jan, this volume). These cells develop from specialized parts of the ectoderm ('proneural clusters'; Ghysen and Dambly-Chaudière 1989). Within the proneural clusters (prc), cell-cell interactions take place to select the sensillum precursors (sp) from the epidermal precursors. These interactions seem to be similar to those involved in determining the neuroblasts at an earlier stage. During stages 11 and 12, sensillum precursors perform two or three divisions to produce the different cells that make up the sensilla (Bodmer et al. 1989). Typically, a sensillum contains one bipolar sensory neuron (sne), three accessory cells (acc) that form sheaths around the dendrite, and a sheath cell associated with the axon (Hartenstein 1988). Some sensilla have more than one neuron (e.g., basiconical sensilla, which have three neurons). Two major classes of sensilla exist: the external sensilla (mechanoreceptors and chemoreceptor) and chordotonal organs (stretch receptors). In the external sensilla, the accessory cells remain in the epidermis and produce a specialized cuticular apparatus involved in receiving the stimulus. In the chordotonal organs, neurons and accessory cells come to lie subepidermally. A special type of sensillum is the larval photoreceptor (Bolwig's organ, (bo), which consists of 12 modified bipolar sensory neurons and no accessory cells (Steller et al. 1987; Green et al. 1993).

Beside the sensilla, which are innervated by bipolar sensory neurons, there exists another type of multidendritic sensory neuron that is not accompanied by accessory cells (Bodmer and Jan 1987; not shown). The first wave of sensilla (chordotonal organs; external sensilla with two or more sensory neurons) starts to differentiate during stage 13 (see left); the remaining sensilla (second wave) follow during stages 14 and 15. The sensory axons (pn) grow toward the CNS. Axons of the early differentiating sensilla form two tracts (anterior and posterior fascicles) in each segment, which are joined by later-appearing sensory axons and motor axons.

At stage 17, all sensilla have differentiated. Sensory axons have reached their target in the CNS, and motor axons have grown toward the musculature. Movement of the embryo starts during mid-stage 17.