Jumping spider's leg (family Salticidae). Coloured scanning electron micrograph (SEM) of the tip of a jumping spider's leg. The hairs are used for gripping and also as a sensory tool. Spiders do not have muscles in their legs. Instead they use muscular contractions in their body to force fluid into the legs, causing them to extend. Jumping spiders are so called because they can leap between 10 and 40 times their own body length when pouncing on prey.