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Early Mesoderm Development pages 36-37 | 38-39 | 40 | 41
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.
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).