Hydraulic leg extension is not necessarily the main drive in large spiders

Abstract

SUMMARYUnlike most other arthropods, spiders have no extensor muscles in major leg joints. Therefore, hydraulic pressure generated in the prosoma provides leg extension. For decades, this mechanism was held responsible for the generation of the majority of the ground reaction forces, particularly in the hind legs. During propulsion, the front leg pairs must shorten whereas the hind legs have to be extended. Assuming that hind legs are essentially driven by hydraulics, their force vectors must pass the leg joints ventrally. However, at least in accelerated escape manoeuvres, we show here for the large cursorial spider species Ancylometes concolor that these force vectors, when projected into the leg plane, pass all leg joints dorsally. This indicates a reduced impact of the hydraulic mechanism on the generation of ground reaction forces. Although hydraulic leg extension still modulates their direction, the observed steep force vectors at the hind legs indicate a strong activity of flexors in the proximal joint complex that push the legs against the substrate. Consequently, the muscular mechanisms are dominant at least in the hind legs of large spiders.

More than a safety line: jump-stabilizing silk of salticids

“…After take-off, increasing body angles in spiders of either group imply initial rearward torque ( figure 2a,b), consistent with other salticids reported previously [21,22]. By contrast, wandering spiders pitch forward, probably because of the different relative position of CoM and leg extension mechanisms [40,41]. To counteract this initial pitch moment and prevent in-air rotation, previous studies suggest that spiders could adjust leg movement or use silk to reverse body orientation towards the ends of long jumps [21,22,40].…”

Biomimetic Spider Leg Joints: A Review from Biomechanical Research to Compliant Robotic Actuators

“…The spider leg consists of seven distinct, tubular segments that build an exoskeleton, which surrounds and protects all of the inner entities. From a functional point of view, the seven links of each leg can be reduced to three main acting joints used for walking [18,19]: the complex, muscle-actuated hip joint assembly with two degrees of freedom for stance and pan as well as the two main leg joints, the femur-patella and the tibia-metatarsus joints. Both of them show one rotational degree of freedom [20].…”

Crawling at High Speeds: Steady Level Locomotion in the Spider Cupiennius salei—Global Kinematics and Implications for Centre of Mass Dynamics

“…In unloaded legs these joints can be extended only by means of hydraulic pressure, which is generated in the prosoma and transmitted via lacunae to the respective joints. Although there are alternative extension mechanisms in legs with ground contact [7], hydraulic pressure also seems to be important for leg movement during stance (cp. [7] for a review).…”

“…Although there are alternative extension mechanisms in legs with ground contact [7], hydraulic pressure also seems to be important for leg movement during stance (cp. [7] for a review). Besides some related groups [8] spiders are the only animals that combine hydraulics and an exoskeleton consisting of stiff segments.…”