The ultrastructure of a novel muscle attachment in the Tephritid fruit fly Urophora affinis

Abstract

The tergosternal abdominal muscles in segments 1–6 of the tephritid fly Urophora affinis have lateral insertions into the pleura as well as terminal insertions into the terga and sterna. The lateral attachments occur at each Z band along the length of the muscle fiber. At the level of each Z band, there is a transverse row of large sockets on the lateral side of the muscle fiber which extend as far as the central nuclei. Fingerlike extensions of the epidermal cells penetrate these myoepidermal sockets and junctions connect them to the sarcolemmal membrane. The epidermal–cuticular junctions at these insertions involve no muscle attachment fibers; instead, cuticular "microapodemes" extend into the epidermal cell. Hemidesmosomes line the epidermal membrane attaching it to these microapodemes. Microtubules extend between the myoepidermal junctions and the epidermal–cuticular junctions though in lower densities than is typical of muscle insertions. These lateral insertions are believed to be a modification of the transverse tubule system. This unique muscle attachment appears to be an adaptation to prevent protrusion of the pleural walls of the abdomen when the tergosternal muscles contract.