Z-line and supercontraction in the hydraulic muscular systems of insect larvae

An electron microscopic study has been made on the body wall muscles from Lepidoptera (Noctua pronuba, Philosamia cynthia) and Diptera larvae (Tipula tipula), whose locomotion is based on a hydraulic muscle antagonism. All the fibers examined had interrupted Z‐bands, formed of single dense bodies, irregularly placed around the sarcomere; myosin filaments could pass through the spaces between one Z‐body and the next, making it possible for the sarcomere to shorten its resting length by up to 60%. In supercontracted muscles, both the thick and the thin filaments pass through the Z‐line; since these actin filaments are very long, they can overlap and interact with the myosin filaments of the adjacent sarcomeres. The muscles examined can be placed in the category of supercontracting muscles and can be considered a type of striated muscle, with morphological and functional characteristics similar to those of helical or smooth muscle.