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Have you ever noticed that dead spiders (ones not squashed by your shoe that is) die with their legs all curled up underneath them?  What is it that makes them curl up so tightly? Let’s take a quick look at spider’s legs and see what we can discover.

Spider Muscles

As early as 1944 (and maybe earlier) scientists have been earnestly studying the spiders legs. When we think of legs, arms and any appendage we think it is muscles that move them back and forth, to and fro. For humans this is correct but for arachnids it is only a partial answer. For our 8 legged friends, however, a combination of muscle and hydraulics of sorts is used and it hasn’t changed much in about 300 million years. Spiders do have muscles to contract called ‘flexor’ muscles but lack the ‘extensor’ muscle which extends the leg back out.

Enter the hydraulics

To accomplish the extending of a spiders legs and enable it to run and walk a sort of hydraulics is used. Simply put, fluids (spiders blood) are pumped into the

the leg as needed to extend it back out as the flexor muscle relaxes. Then the flexor muscle contracts again as pressure is relieved and this process happens over and over which enables the spider to move. Let’s not forget that spiders can jump as well and the jumping spider can jump 25 times its body length. This feat is done in the same fashion as running, however the blood pressure is elevated much higher. It isn’t long lived; just a quick burst will give the spider the kick it needs. It is repeatable as you can often see spiders jump or skip several times in a row.

At first thought you might think it is the heart that pumps the blood so it is the heart that creates the pressure for leg movement. Studies in the 1940?s  concluded that the spider’s heart could not create enough pressure for some of the incredible leaps and fast running, at least on its own. The heart is in the abdomen away from the legs and a large aorta runs through the pedicel connecting the thorax. If ‘pressure ‘ from the heart was the answer you’d notice swelling and deflating of the abdomen as pressure built and relaxed. It may be a combination of blood pumped and different pressures that combine by way of a sphincter or valve or perhaps other muscles in the prosoma that create the hydraulic effect on the fluid needed.

So why do they curl up?

The simple answer is there is no more hydraulic pressure to extend the legs and the flexor muscles revert back to their original length with no resistance, thus the legs curl in. It’s interesting to note too, that when spiders are alive and lose a leg or part of one that they don’t seem to ‘leak’ all their precious fluid. Spiders have mostly an open circulatory system so blood flows freely throughout the body so a wound or snapped off leg should be fatal. They have however a special mechanism to seal off the leg before this occurs and they lose pressurization. Once the leg grows back with molting or the wound is sealed fluid can be pumped in again.