The legs of spiders move using a combination of the hydraulic pressure of body fluid and muscle flex.

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Smaller spiders (usually those weighing under 3g) use a hydraulic catapult method to move around and catch prey, whereas larger spiders (those weighing over 3g) rely on a combination of a hydraulic catapult and muscle-based contraction.

Spiders have four pairs of legs, and each pair has a specialized task for locomotion. The front two pairs are situated in front of the spider’s center of mass, and the rear two pairs are behind its center of mass. During forward motion, the front two pairs flex inward, creating a rearward pulling force.  The third leg pair acts as a pivot point, like a pole vaulter using the pole to swing their momentum over the bar. The fourth leg pairs extend from hydraulic pressure creating a rearward push force.

Each leg consists of seven tubular sections with three distinct regions. The hip joint located at the body allows movement left and right as well as up and down while the other two active elements, femur-patella, and tibia-metatarsus, allow movement up and down only. The hip joint has both extension muscles to push out the legs and flex muscles to pull in the legs.  The femur-patella and tibia-metatarsus only have flex muscles. To extend the legs, hemolymph fluid, similar to blood in vertebrates, pumped from the spider’s body fills the lower side of the femur-patella and tibia-metatarsus joints, pressurizing a bellow-like structure to extend the leg. With flex-only muscles attached to the inside circumference of the spider’s body, the spider can maximize its grip on prey. As a liquid, the hemolymph can fill in the gaps between muscle fibers for an efficient extension as well as leg structure stiffening.

When the spider prepares to jump on its prey or away from danger, it pressurizes the legs for an extension while simultaneously flexing its muscles in place. When the spider relaxes its flexed muscles, the pressurized legs extend initiating the jump sequence. Depending on the desired trajectory of the jump, the spider can manipulate the timing and force of each leg as well as using in-flight leg extension or contraction to manage angular momentum and aerodynamics.  Its silk dragline provides the spider with an emergency stop.

While spiders under a mass of 3g (like Cupiennius salei) primarily use this hydraulic catapult method, spiders with mass over 3g (like Ancylometes concolor) would start to bounce uncontrollably during full hydraulic pressurization, causing its legs to lose contact with the ground, unless it used excessive muscle energy to manage the bounce.  Thus, more massive spiders use a combination of hydraulic extension and muscle flex at launch but rely more heavily on muscle flex in their front legs.

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“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.”

Journal article
Energy and time optimal trajectories in exploratory jumps of the spider Phidippus regiusScientific ReportsNabawy M, Sivalingam G, Garwood R, Crowther W, Sellers W.

“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.”

Journal article
Biomimetic Spider Leg Joints: A Review from Biomechanical Research to Compliant Robotic ActuatorsRoboticsLandkammer S, Winter F, Schneider D, Hornfeck R.

“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.”

Journal article
Hydraulic leg extension is not necessarily the main drive in large spidersThe Journal of Experimental BiologyWeihmann T, Günther M, Blickhan R.

“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)

Comparative Biomechanics: Life's Physical World, Second EditionJune 17, 2013
Steven Vogel

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