Why Do Spiders Curl Up When They Die?

June 1, 2023

Have you ever swept up a spider in your home and wondered why its legs were curled up into a tight little ball? Well, a group of mechanical engineering students has figured out exactly why.

It turns out that dead spiders don't have the hemolymph pressure that normally pushes their legs out straight. Living spiders have muscles called flexors that work in pairs to curl and extend their legs, like your biceps and triceps. They also have a section of their bodies called the prosoma that ejects fluid into their legs to get them moving—like a heartbeat. When a spider dies, this fluid is no longer being pumped in so the legs remain curled up into a spidery ball.

That's a big problem for a jumping spider, because if its legs are curled up it won't be able to make that powerful leap of 25 times its body length. That's why it's important for them to stay flexed so they can quickly change from crawling to launching themselves.

A team led by Faye Yap, a graduate student at UC San Diego, decided to see if they could reanimate a dead spider and straighten its legs. They used a simple technique: they used puffs of air to put pressure on certain points on the wolf spider's legs in the same way that a heartbeat would pump blood through its system. It worked, and they were able to resurrect a dead spider with perfectly straight legs—and even a bit of a jump! It's not just spiders that can benefit from this sort of reanimation: scientists are starting to use these same principles to design robots with flexible joints.


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