An electron microscopical study of the innervation and sarcoplasmic reticulum of the fibrillar flight muscle of Drosophila melanogaster

The peripheral nerves of Drosophila are of the ‘tunicated’ type; their mesaxons are derived from several independent invaginations of the lemnoblast plasma membrane and possess septate desmosomes. Fine branches of the nerves and tracheoles penetrate deeply into the fibres of the flight muscle and draw the sarcolemma in with them. The myoneural junctions occur on the invaginated portions of the sarcolemma. The axons shed their lemnoblastic elements and become closely apposed to the muscle plasma membranes at the junctions. The sarcoplasmic reticulum of the flight muscle seems to consist mainly of the extensive infoldings of the sarcolemma and their processes, as in Tenebrio. ‘Dyads’ formed by the association of isolated vesicles with the membranous infoldings are present in Drosophila. Most of these ‘dyads’ seem to be in register with the myofibrils, two pairs being associated with each sarcomere.