The primordia of the adult musculature derive from the embryonic mesoderm (see Bate, this volume). In the abdomen, these primordia (amp in L3) are represented by individual or small groups of cells that become associated with the peripheral nerves. In the thorax, head, and tail, the adult muscle precursors form larger clusters of cells attached to the imaginal discs as the so-called adepithelial cells (id, wd, hd, adepithelial cells of leg discs, wing disc, and haltere disc, respectively). Adepithelial cells and abdominal muscle precursors proliferate throughout the larval period.

During prepupal and early pupal stages, most larval muscles degenerate, and the adult muscle precursors establish a new muscle pattern. Larval muscles of the thorax degenerate early in the prepupa; the dorsal pharyngeal and abdominal muscles persist into the pupal phase and are actively involved in metamorphosis (Robertson 1936). Some larval dorsal abdominal muscles survive into the early adult stage.

In the abdomen of the prepupa, myoblasts are clustered along the remainder of the larval nerves. In the following 10-20 hr, these myoblasts fuse and form parallel arrays of anterior-posteriorly directed (longitudinal tergal muscle, lm) or dorsal-ventrally directed (abdominal tergosternal muscle, atsm) fibers. After completion of the abdominal epidermis, these fibers insert at newly formed apodemes. In the thorax, the three pairs of larval dorsal longitudinal muscle (lm) fibers are involved in the formation of the longitudinal indirect flight muscles (lfm) (Fernandes et al. 1991).

Adepithelial cells of the everted wing discs migrate toward the notum and concentrate around the larval dorsal longitudinal muscle fibers which thereby serve as a scaffold to direct the pattern of adult myoblasts. Other adult muscles (e.g., tergosternal indirect flight muscles, tsm; leg muscles, lem) develop without such scaffolds formed by larval fibers. The major groups of muscles of an adult fly are shown on the facing page. Individual muscles are numbered according to Miller (1950).