Understanding the Limits to the Hydraulic Leg Mechanism: the Effects of Speed and Size on Limb Kinematics in Vagrant Arachnids

Among invertebrates, spiders (order Araneae) may be unique in their relationship between speed and mass as they use a combination of direct muscular contractions to flex their appendages, and internally controlled hydraulic pressure to extend them. To explore this, we measured maximal running speeds in 128 individual lycosids and sparassids, which varied in mass between 0.0054 and 3.01 g. We show maximum speed scaled with M, while mean running speed scaled much lower as M. We show no strong limitation of the hydraulic mechanism, with leg extension speed being equal to or greater than leg flexion speed. The reduction in leg flexion speed, only apparent in the distal most joint of the limb, might be a result of the requirement for flexor muscles to act against the hydraulic system. We explored the role of the limbs and found an alternating pattern of joint use among limbs, which may represent a strategy to avoid interference with adjacent limbs during running. Furthermore, we observed a reduced movement speed (increased leg dragging) in the rearward facing fourth limb with size. This may be linked to the increased size of the abdomen in larger spiders and may suggest a speed limitation in larger individuals.