Unlike most arthropods, spiders have no extensor muscles in their limbs and instead extend them by hydraulic pressure. Although all arthropods use muscles attached to the inside of the exoskeleton to flex their limbs, spiders and a few other groups still use hydraulic pressure to extend them, a system inherited from their pre-arthropod ancestors. The only extensor muscles in spider legs are located in the three hip joints. As a result a spider with a punctured cephalothorax cannot extend its legs, and the legs of dead spiders curl up. Spiders can generate pressures up to eight times their resting level to extend their legs, and jumping spiders can jump up to 50 times their own length by suddenly increasing the blood pressure in the third or fourth pair of legs. Unlike smaller jumping spiders, though larger spiders use hydraulics to straighten their legs, they depend on their flexor muscles to generate the propulsive force for their jumps.
Spiders
Muscular system of spiders.