The larval sensory organs remain unchanged during the three larval molts. With few exceptions (e.g., Keilin's organ in ventral thoracic segments), all larval sensilla degenerate during metamorphosis. The imaginal discs and histoblasts, which form the adult epidermis, produce another complete set of adult sensilla. The first precursors of adult sensilla appear during the late third larval instar. Among them are the precursors of the macrochaetae (mac) of head and thorax, the chordotonal organs of legs and wings, the large campaniform sensilla of the wing blade (cam), some of the chemoreceptors of the proboscis, leg, and wing margin (wcs), and some of the mechanoreceptors of the legs (ls). The pattern of division and differentiation in the adult sensory primordia investigated so far is similar to the embryonic pattern (Hartenstein and Posakony 1989). The first sensory axons form 'pioneer tracts' for the later axons. In several cases, these axons also fasciculate with the larval nerves (e.g., the axons of the thoracic macrochaetae follow the remainder of the larval second thoracic nerve, T2n). In the prepupa, the first set of sensilla and their axons are in place. The hatched regions of the drawing schematically represent 'proneural regions' of later-born sensilla. Among these are the microchaetae of the head and notum, many sensilla of the proboscis and antenna, and most of the mechanoreceptive sensilla of the wing margin, legs, halteres, and genitalia. These sensilla start differentiating in the early pupa (16-24 hr after puparium formation). Their axons follow the tracts established by the early differentiating sensilla. The last sensilla to appear are those of the abdomen (~32 hr after puparium formation). The drawing on the facing page schematically shows the sensory organs and peripheral nerves of an adult fly. The main structural and functional subclasses of sensilla are represented in different colors. (abn) Abdominal nerves; (bas) basiconical sensilla; (cns) central nervous system; (iom) interommatidial bristles; (mic) microchaetae; (T1d, T2d, T3d) dorsal nerves of thoracic segments 1, 2, 3; (T1v) ventral nerve of thoracic segment 1; (wms) mechanoreceptors of wing margin.