The epidermis is shown here at the prepupal stage around 10 hr after puparium formation (apf), early pupal stage (24 hr apf), and in the adult (female). Colors indicate the origin of the corresponding epidermal region from a particular imaginal primordium (see color key).

In the prepupa, the thoracic discs evert and fuse with each other after displacing the larval epidermis (Robertson 1936; Bryant 1978; see Fristrom and Fristrom, this volume). The lateral region (pleura) and dorsal region (notum) of the thorax are formed by the prothoracic, wing, and haltere discs; the latter discs also form the dorsal appendages, wings, and halteres. The ventral thorax (sternum) and the legs are formed by the three pairs of leg discs. Inside the developing thorax, the discs of the head partially evert and fuse with each other. The labial disc forms the distal part of the proboscis, including the labellum; the clypeolabral disc (bud) gives rise to the clypeolabrum, and the eye-antennal disc gives rise to the upper proboscis, maxillary palps, head capsule, and the eyes. Eversion of the head takes place at 12 hr apf. The abdominal histoblasts have enlarged. The part of the genital disc giving rise to the posterior abdomen (A8-A10), including the external genitalia, has started to evert. In the early pupa shown schematically, head eversion has taken place. The abdominal histoblasts cover most of the abdominal segments; they will fuse with each other and with the epidermis formed by the genital disc at about 32 hr apf.

The epidermis of an adult fly (female) is shown on page 27. (acv) Anterior crossvein; (ant) antenna; (apl) anal plate; (cly) clypeus; (cx) coxa; (eye) compound eye; (fe) femur; (fro) frons; (gen) gena; (hib) abdominal histoblasts; (hl) haltere; (hu) humerus; (lab) labellum; (mxp) maxillary palp; (men) mentum; (not) notum; (pcv) posterior crossvein; (pl) pleura; (ps) posterior spiracle; (scu) scutellum; (stpl) sternopleura; (tar) tarsus; (ter) tergum; (ti) tibia; (tr) trochanter; (vII-vV) wing veins; (ver) vertex (dorsal head capsule); (vpl) vaginal plate; (wmg) anterior wing margin.