Stiffness of an arthropod leg joint.

The stiffness of the tibia-metatarsus joint of several spider-species was determined for different loading conditions. The relationship between force applied to the metatarsus tip and deflection is linear and free of hysteresis during axial loading (z'), but not under lateral (y') and dorsoventral (x') loading, where joint stiffness increases with amplitude and deflection rate. Up to lateral deflections of 2.5 degrees relaxation and nonlinearity of the joint-stiffness can be described by power functions. Considering strain induced in the spider's tibia, the viscoelastic properties of the joint result in a nonlinear amplitude transfer and a high-pass filtering of mechanical vibrations applied to the spider's leg. However, this contributes only slightly to the corresponding transfer characteristics measured for the electrical response of biological strain receptors.