In the fate map [stage 5], the blastoderm region that will form the larval epidermis is shaded blue. The major part of this region (i.e., the ventral neurogenic region) also gives rise to the precursors of the ventral nerve cord (see Martinez Arias, this volume). The Drosophila larva is acephalic; i.e., most of the head involutes to form the rostral part of the alimentary tract (pharynx [ph] and atrium) and the dorsal pouch (dp, shaded gray). Only small parts of the head segments, which bear the sensory antennomaxillary complex, are exposed to the outside at the anterior tip of the larva (pseudocephalon, psc). The rest of the bodywall is formed by the thoracic and abdominal segments.

The drawing of the stage 17 embryo shows the main cuticle specializations formed by the larval cuticle. Prominent among these are the ventral denticle belts (vdb), the dorsal trichomes
(tri), sensory structures (sh), and the cephalopharyngeal skeleton (cps; see drawing of L3) formed by the epidermis of the involuted head. The primordia of the adult epidermis, which are set apart from the larval epidermis during midembryogenesis (stages 13-15; Bate and Martinez Arias 1990; Cohen et al. 1991; see Cohen, this volume), are represented in the fate map as differently colored ovals.

By early stage 17, some of these primordia have invaginated as imaginal discs connected to the larval epidermis only by a thin peripodial stalk (leg discs, ld; wing disc, wd; haltere disc, hd). Some other primordia (genital disc, gd; labial disc, lbd; eye-antennal disc, ead) invaginate late in embryogenesis, shortly before hatching. The primordia of the abdomen (abdominal histoblasts, hib), prothorax (pd), and labrum (clypeolabral disc [bud], clb) remain permanently in the larval epidermis.

The third larval instar [L3] depicts the spatial arrangement of the imaginal discs and histoblasts at late larval stages. All discs have grown in size and cell number; after puparium formation, there are but one to two more rounds of division before the epidermal cells become postmitotic. In contrast, the abdominal histoblasts have not grown substantially; for them, proliferation takes place during the pupal phase. (Al, A8) Abdominal segments 1, 8; (T1) thoracic segment 1; (1-6) head segments 1-6.