(A) Schematic cross section of mature sensillum, showing cuticular apparatus (bristle shaft and socket) and underlying cells. (B) Schematic cross sections of developing sensillum. Sensillum progenitor (SOP; pI) divides into two daughters, pIIa and pIIb. pIIa divides into the support cells forming the sensillum shaft and socket (tr, to). pIIb gives rise to the neuron (ne), glial cell (gl), and inner sheath cell (th). (C) Drawing of the back of the Drosophila thorax (notum), showing pattern of mechanosensory bristles. Large bristles (machrochaetae) form an invariant pattern and are individually named (anp, anterior notopleural; apa, anterior postalar ; pdc, posterior dorsocentral; pnp, posterior notopleural; psc, posterior scutellar). (D) Larval imaginal wing disc giving rise to wing blade and notum. Proneural clusters are labeled by expression of proneural gene scute. Each one of the macrochaetae can be assigned to one proneural cluster.