The ultrastructural organization of femoral muscles in Musca domestica (Diptera).

The femoral muscles of Musca domestica are organized as typical synchronous muscles. The nuclei occupy the central region of the fibres. The myofibrils have a lamellar shape with the major diameter oriented radially with respect to the nuclei; they have an extremely regular organization. In cross section each thick filament is surrounded by 10-12 thin filaments. The mitochondria are relatively abundant and are organized in three concentric layers: perinuclear, intermyofibrillar and subsarcolemmal. They have an elongated shape and possess numerous cristae, which are oriented trasversally with respect to the long axis of mitochondria. The S.R. is organized as a complex system of tubules which completely surround the contractile material. The T-system has a characteristic shape, and takes contact with the sarcoplasmic reticulum forming peculiar triadic structures.