A Staff Report from the Straight Dope Science Advisory Board

Do spiders have hydraulic legs?

September 28, 2004

Dear Straight Dope:

My boyfriend is a trivia freak, and he has recently gotten me hooked on the Straight Dope website. We frequently have arguments about his more unbelievable pieces of trivia. Recently he told me spiders run on a sort of hydraulic system and that's why their legs bunch up when they die. Please help me prove him wrong--it would give me so much joy to be right just once.

I'm afraid your boyfriend is partially correct on this one. All arthropods, be they spiders, scorpions, mites, centipedes, lobsters, barnacles, pillbugs, or insects, need some internal hydraulics in order to move their appendages properly. Your boyfriend wins on a technicality, though, since he said "a sort of hydraulic system"-- if he hadn't been so vague, he might've been wrong. Proving that the system isn't purely hydraulic is easy enough: poke a small hole in a spider or caterpillar or whatever and they don't collapse in a heap. The fluid leaks out slowly, the body retains most of its rigidity, and the critter can continue to move its limbs. None of this would be true if they operated purely on hydraulics, which would require a high and constant level of internal fluid pressure. It's the combination of hydraulics and internal muscles that make arthropods work. Arthropods need hydraulics to move because of the way their bodies are constructed. Humans can move their limbs to and fro because they have opposing (antagonistic) sets of muscles. To raise your arm, for example, you contract your biceps muscle; to lower it you relax your biceps and contract your triceps. 

Some arthropods, like spiders, can't do that. 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 (or if not in the legs, there are other places where the muscles aren't in antagonistic pairs--it depends on the type of arthropod). That's where hydraulics come in. Spiders, for example, 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. A human in that predicament . . . well, there's always Viagra. A spider is fated to curl up and die.  

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