Arthropods don't have an internal skeleton like we do; they have an exoskeleton--a rigid, jointed external structure protecting their innards, and the muscle connections are sometimes simplified. Flexor muscles attached to the inside of the exoskeleton enable arthropods to pull their legs inward, but in some arthropods there are no opposing extensor muscles to push the legs out again. Spiders extend their limbs by forcing blood into them, much as you pump hydraulic fluid into a backhoe's digging arm to extend the claw. This system is remarkably efficient. The mechanics of a jumping spider's legs enable it to leap 25 times its length. If a spider loses too much blood, however, it no longer has enough internal pressure to fully push its legs out.
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