Function of a Muscle Whose Apodeme Travels through a Joint Moved by Other Muscles: Why the Retractor Unguis Muscle in Stick Insects Is Tripartite and Has No Antagonist

The course of the common apodeme of the tripartite retractor unguis muscle is described for the stick insects Carausius morosus and Acrophylla wulfingii. This apodeme travels through the femoro-tibial joint well outside the axis of rotation of the joint, but movements of the femoro-tibial joint do not affect the position of the tarsal claws, which are moved by the retractor unguis muscle. The independence of tarsal position upon tibial position is not produced by a neural compensation mechanism but by a sophisticated morphological arrangement combined with specialized physiological properties. These mechanisms consist of two parts. (1) Moderate claw flexions are mainly produced by the smaller tibial parts of the muscle and their influence on claw position is not affected by tibial position, because they lie distal to the femoro-tibial joint. (2) The retractor unguis muscle works against strong elastic structures and the claw assumes the position where the elastic force is counterbalanced by the muscle force. The maximum muscle force of the strong femoral part of the muscle is nearly independent of muscle length. Therefore, the force it transfers to the elastic structures is also nearly independent of tibial position.