Understanding the limits to the hydraulic mechanism of the spider leg: The effects of speed and size on limb kinematics in vagrant spiders

Among invertebrates, spiders (order Aranea) 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 how this may affect speed with changes in body mass, we measured maximal running speeds in 128 individual Lycosids and Sparassids, which varied in body mass between 0.0054 – 3.01 g, over 3 orders in magnitude. We show maximum speed scaled with M^0.353, while mean running speed scaled much lower as M^0.197. We further show no strong limitation of the hydraulic mechanism with leg extension speed being equal to or greater than leg flexion speed. We suggest the reduction in leg flexion speed, which was only apparent in the distal most joint on the limb, might be a result of the requirement for flexor muscles to act against the hydraulic system. We further explored the role of the limbs and found an alternating pattern of joint use among limbs and a reduced movement speed (increased leg dragging) in the rearward facing fourth limb with size. The pattern of joint use along each limb may represent a strategy to avoid interference with adjacent limbs during running. The reduced movement in the rearward facing hindlimbs with body size may be linked to the increased size of the abdomen in larger spiders, and may suggest a limitation to speed in larger bodied arachnids. <br>