The Hydraulic Mechanism of the Spider Leg

Hinge joints occur in the legs of various arthropods, notably Limuhis (Snodgrass, 1952); arachnids; diplopods, chilopods and pauropods (Manton, 19586). At a hinge joint flexion is muscular, but the anatomical form of the joint rarely permits the presence of antagonistic extensors (see Manton, 19580,%. 151 for an exception). In the recent account of the leg muscles of the house spider Tegenaria sp., Parry (1957) suggested that extension might be due to hydraulic forces in the leg. Manton (19584) attributed to the same cause the full extension of the arachnid leg when the limb tip is off the ground but considered that extension during a propulsive stroke was passive and due to the ground reaction on the limb and so, indirectly, to depressor muscles. In the present paper support is given to the idea that active extension occurs at the hinge joints of the Tegenaria leg and that the mechanism is a hydraulic one. First, we measure the internal pressure in the leg of intact spiders and find it to be surprisingly high. Secondly, we establish an empirical relation between the internal pressure of the leg and the torque at the hinge joints. Thirdly, we measure the torques actually developed at the hinge joints when a spider accelerates a mass attached to the leg, and use the relation between torque and pressure to show that the measured torques can be accounted for by the pressures which occur in the leg.