It has been known for over a century that the ifight muscles of insects belonging to the two orders Diptera (the true ffies), and Hymenoptera (ants, bees and wasps) are specialized in certain aspects of their morphology to a degree not found in other groups of insects. These muscles are composed of bundles of very large fibres, which in turn are made up of numerous relatively gigantic fibrils together with a much greater number of small, spherical, homogeneous bodies termed sarcosomes. The sarcosomes are juxtaposed in longitudinal rows in between the fibrils and, together with the fibrils, are embedded in a transparent matrix, the sarcoplasm. During the past several decades, various authors have suggested on the basis