Disposition of membranes and the entry of haemolymph-borne ferritin in flight muscle fibers of the fly Phormia regina.

The disposition of the surface plasma membrane and its inwardly directed derivatives, corresponding to the T-system of other fibers, and of the corresponding sarcoplasmic reticulum (SR) elements has been examined in a dipteran asynchronous flight muscle (Phormia regina). The presence of uni-axonal neuromuscular junctions within clefts approaching the center of the fiber is described. The most conspicuous SR component is present in the dyads adjoining plasma membrane derivatives, but it is also sparsely represented elsewhere. The accessibility of the plasma membrane-limited compartments within the fiber to the ambient haemolymph, in the living insect, has been investigated by tracing the distribution of ferritin by the circulatory system. The proportion of fiber volume occupied by the T-system and SR components in asynchronous and synchronous muscle is compared and the functional implications of these proportions is discussed.