Neurons within the precursor of the adult leg, the imaginal disc, innervate a larval sense organ, Keilin's organ. Electron microscopical investigations of first instar larvae show that five dendrites end at the organ: three insert at the bases of the three hairs of the organ and two end against the cuticle, without any apparent cuticular specialization. In third instar larvae, the imaginal leg discs invaginate into the body cavity, and only four of the dendrites (the outer segments of which become greatly elongated) remain in contact with Keilin's organ. The axons of the neurons that supply Keilin's organ project into a ventral neuropile region of the central nervous system, with a pattern that resembles the projections of other larval sensilla. Electrical activity can be recorded from neurons of the imaginal disc in response to mechanical stimulation.