Why Spider Legs Work Like Tiny Hydraulic Systems

Spiders lack extensor muscles in their major leg joints, so they use blood-like fluid pressure to extend their legs, which curl when that pressure stops.

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Spider legs work in a surprisingly different way from human arms and legs. At several major joints, spiders do not have the extensor muscles that many other animals use to straighten their limbs. Instead, they rely on pressure from hemolymph, the fluid that serves many of the roles of blood in their bodies. Muscles inside the spider help create pressure in the central body region. That pressure pushes hemolymph into spaces within the legs and helps extend the joints. Other muscles handle flexion, pulling the legs back inward. The result is a biological hydraulic system that can produce fast, coordinated movement without needing a conventional muscle pair at every joint. Research on spider locomotion has confirmed the important role of hemolymph pressure in extending major leg joints. The mechanism also helps explain a familiar sight: a dead spider often has its legs curled underneath its body. Once the hydraulic pressure that normally helps extend the legs disappears, the flexing system is no longer balanced by active hydraulic extension, allowing the legs to fold inward. The original article also notes that damage to the central body can interfere with this pressure system. This unusual form of movement has attracted interest beyond zoology. Engineers have studied spider joints as inspiration for soft robotic systems because hydraulic extension can create useful movement with a relatively simple mechanical arrangement.

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