Homozygotes die as prolonged stage 17 embryos without hatching. In late stage 17 embryos, sarcomere-like bundles of thick and thin filaments associated with incomplete Z-disc-like structures are seen. Sarcomere structure is highly disorganised in prolonged stage 17 embryos; thick and thin filaments appear randomly scattered and no regularly arranged Z-disc-like structures can be seen. The ends of muscles are abnormally flat, due to the complicated folding of the muscle cell membrane seen in wild-type direct and indirect muscle attachments having largely disappeared. The size of the tendon matrix at indirect muscle attachment sites is also reduced compared to wild type. Most heterozygotes are viable but they cannot fly. In adult flies, the thick filaments of the indirect flight muscles (IFMs) are loosely packed or untied, particularly along the periphery. Myofibril branching is occasionally seen, with the branching myofibrils seen connecting to and integrated with neighbouring myofibrils. Incomplete and apparently broken Z-discs are frequently seen. The IFMs of heterozygous late pupae are virtually normal in morphology, and there is no difference in sarcomere length and thin/thick filament ratio compared to wild type.