AskNature

“Comparison of jump performance against other arthropods shows that Phidippus regius is firmly in the group of animals that use dynamic muscle contraction for actuation as opposed to a stored energy catapult system. We find that the jump power requirements can be met from the estimated mass of leg muscle; hydraulic augmentation may be present but appears not to be energetically essential.”

“Although a lot of research is focused on the hydraulic extension, the muscles also play an important role and need to be considered. The lack of extension muscles in the main joints leads to the important fact that almost the whole space inside the exoskeletal tubes can be filled with flexion muscles [36,37]. As mentioned, the contraction of the longitudinal muscles generates retraction forces and initiates a powerful flexion. This is necessary because the muscular flexion is the main actor for gripping prey as well as for climbing or rappelling.”

“Thus, in the hind legs of A. concolor, muscular (flexor) torque contributions exceed hydraulic (extensor) torque contributions in any joint throughout the entire period of force application. However, if the animals did not use hydraulics at all, force vectors would be even steeper. Thus, apparently, large spiders still use hydraulic torques. Although hydraulics are globally active during accelerated locomotion like in starts and jumps, large spiders apparently do not use it primarily for propulsion.”

“One particular hydraulic device is worth a little more attention here, partly because its existence comes as yet another surprise and partly because it achieves antagonism for contractile muscle in an unusual way. The eight legs of a spider differ little from the six of an insect, but a curious special feature of spider legs has been known for almost a century. While properly equipped with flexor muscles (ones that decrease the angle between one segment and another), they lack the antagonistic extensor muscles (ones that increase that angle toward 180 degrees). Biologists casually assumed that elasticity of the interarticular membranes provided the antagonistic force, not on the face of it an unreasonable idea. But Ellis (1944) remembered that spiders die with legs severely flexed. If elasticity did the extension, they would more likely die with legs extended or at least not so flexed–as do insects. He found that cutting off the tip of a leg prevented reextenson until the tip was resealed; and he found that mild exsanguination reduced a spider’s ability to extend any of its legs. He suggested that extension in spider legs was hydraulic, not muscular or elastic. The idea was confirmed by Parry and Brown (1959), who measured resting pressures of 6.6 kilopascals and transient pressures of up to 60 kilopascals (over half an atmosphere) in spider legs. An isolated leg could lift more weight as the pressure inside it was increased, and the spiders turned out to have a special mechanism to seal off a joint that prevented fatal depressurization when a leg was lost.” (Vogel 2003:421)