[♪ INTRO].

Ever gone to sweep up your floor and noticed a little dead spider  mixed in with the dust bunnies? If you paused for a closer look, maybe you noticed that its tiny legs  were curled up tight against its body.

This spidery leg-curl of death happens because spiders don’t use muscles to extend their legs. They harness the power of fluids instead! Spider legs are basically  a series of little tubes.

The joint where these tubes connect to the body, what you could think of as the  “hip”, works much how you’d expect. It’s equipped with both extensor muscles to extend the legs and flexor  muscles to curl them in. So, paired muscles that work against one another.

Like how your biceps and triceps allow  you to both flex and extend your forearm. But the other joints in spiders’  legs only have flexor muscles. To straighten out those  joints, spiders pressurize them by pumping them full of hemolymph,  the spider equivalent of blood.

Some scientists think the evolutionary  reason for relying on hydraulics is that getting rid of extensor muscles leaves room  for bigger, more powerful flexor muscles. And that curling-in motion of flexors is what spiders use to grip their  prey and climb your basement walls. So, not having to bother with extensors lets  them really maximize their flexing abilities.

Spiders can even use their unique  leg anatomy to power mighty leaps. First, they pressurize their leg joints  while contracting those flexor muscles. Then they suddenly release them, and the  near-instant extension launches them into the air.

But when a spider dies, its body experiences all the usual side  effects of death, including rigor mortis. That’s when a corpse becomes stiff  because its muscles contract. And it happens because a dead body stops  producing adenosine triphosphate or ATP, the energy source that powers our muscles.

ATP interacts with calcium ions and some specific  proteins to control the action of muscles. There’s a lot going on at  the molecular level here, but the key thing to know is that  you need ATP for the muscle to relax. Since a dead body stops producing ATP,  the muscles soon run out of the stuff, so they get stuck in clench mode until they  decompose enough that rigor mortis ends.

And, for a spider, being dead also  means no more pressurized hemolymph. So there’s nothing to combat  the flex of those legs, which is why they end up permanently curled. Now, if the thought of spiders stuck in the  tell-tale death pose makes you a little sad, maybe this will cheer you up:.

Spiders’ hydraulic legs are so cool that some  scientists are taking inspiration from them to design robots with joints that are  powerful but lightweight and flexible. So the next time you spot a curled-up spider, just imagine that its legacy  is an awesome giant spiderbot! Thanks for watching this episode of SciShow!

And specifically, thanks  to our patrons on Patreon. Without their support, we wouldn’t  be able to do what we do here, from covering the science news you need to  know to answering quick questions like this. So, thanks patrons!

And if you’re not a patron but  would like to learn more about joining our community of science lovers,  you can head to Patreon.com/SciShow. [♪ OUTRO].