How can fluid pressure straighten a spider’s leg?

A jumping spider’s legs do not work exactly like smaller versions of human legs. At two important joints, the spider has muscles for bending but lacks opposing extensor muscles that would directly pull the joint straight. Fluid pressure supplies an alternative way to extend them.

The fluid is hemolymph, which also carries substances around the body. Muscular action in the front body region can raise its pressure. The connected fluid spaces transmit pressure into the legs. At a specialized joint, a flexible folded membrane changes shape as the pressure acts on it, allowing the connected hard leg segments to move apart.

Pressure is force distributed over area. In a deliberately simplified flat-patch comparison, 40 kilopascals acting across half a square millimeter gives 20 millinewtons of force. This arithmetic explains how a small area can still experience a useful force. An actual joint has a changing curved membrane and lever geometry, so this value is not a measured extension force or a prediction of jump distance.

Flexor muscles work with the hydraulic system to coordinate movement. The entire jump also depends on other joints, body motion, contact with the ground and control signals. Hydraulics explains an important joint mechanism, not a claim that every movement or every spider’s propulsion comes from pressure alone.