Tactile stimuli are hard for arthropods to detect through the armor of their rigid exoskeleton. To solve this problem, flies use bristles (Fig. 2.1). When a bristle is deflected, the pivoting of the shaft in its socket deforms the dendrite of a neuron attached to the shaft's base. The resulting depolarization sends an action potential to the central nervous system (CNS). Flies can pinpoint sensations because axons from different bristles get “wired” to different CNS target cells during metamorphosis, although much remains to be learned about the topology of these neurosensory maps (cf. Ch. 6).

Mechanosensory bristles are formed by 5 cells: 2 superficial cells that secrete cuticle (the shaft and socket cells) and 3 subepidermal cells that do not (the neuron, sheath, and glial cells). These 5 cells descend from a “sensory organ precursor” (SOP). The SOP divides to produce one daughter (IIa) that yields the outer cells, and another (IIb) that yields the inner cells. The sheath cell wraps the neuron's dendrite, while the glial cell wraps the axon. A sixth cell – the “bract cell” – is found in association with bristles on the distal leg and proximal wing. It secretes a thickened hair (“bract”) that is pigmented like the bristle shaft but much smaller. The bract cell is not part of the SOP clone. The way in which it is recruited from epidermal cells is discussed later.