Fig. 1. Schematic representation of the development of the indirect flight muscles (IFMs) and the abdominal muscles. The IFMs are anatomically and functionally subdivided into two distinct groups: the dorsal longitudinal muscles (DLMs) and the dorsoventral muscles (DVMs). (A) The precursor myoblasts (yellow) for the IFMs are attached to the wing disc during the larval life. At the onset of metamorphosis larval muscles begin to histolyse, with the exception of three oblique muscles. (B) By 12 hours APF, histolysis is complete and the three larval templates are clearly visible. By this stage, myoblasts migrate into the muscle-forming regions. (C) The larval templates start splitting by 13 hours APF and the process is complete by 16-17 hours APF. At the same time, the DVMs form by the de novo fusion of myoblasts. (D) By 24 hours APF, the DLMs and the DVMs are complete. Muscle formation in the abdomen occurs later than in the thorax. The abdominal muscles develop from an adult myoblast pool (yellow) associated with the segmental and intersegmental nerves (A). (A-D) During early pupal stages (from 0-24 hours APF), myoblasts proliferate and migrate out along the nerves. (E) Fusion of these myoblasts begins by 28-30 hours APF. (F) The arrangement of the DLMs, the DVMs (I, II and III) and the dorsal muscles of the first abdominal segment in an adult fly: by 50 hours APF, the pattern of the adult muscles is largely complete.