Whether you love them or hate them, spiders are amazing creatures that have evolved to survive in many different environments. Their long legs dramatically increase their size, making them appear larger than their body actually is, but when a spider dies, their legs almost always curl up underneath them.

The reason that this happens is that spiders move their legs using a fluid called hemolymph. This fluid, which they have instead of blood, acts very much like a hydraulic system.

When the spider wants to move its legs, it simply adjusts the pressure of the fluid by bringing some of it back into the main portion of the body, or adding more to the leg. Naturally, they can do all of this very quickly, which allows them to walk around, jump, attack prey, and perform any other spider activities they enjoy.

In addition, spiders don’t have extensor muscles in their legs, and only have flexor muscles. So, their flexor muscles are very strong, allowing them to have a great grip, which is important for capturing prey.

Throughout the life of the spider, it operates the hemolymph system and its flexor muscles properly, allowing it to function like normal. When a spider dies, however, it loses control of that system, resulting in its legs getting pulled in toward the body in the well known curled up position.

Interestingly, this same system explains why spiders often roll over onto their backs when they die. Having all of the hemolymph fluid pulled into the body throws off its normal center of gravity, which results in the spider flipping over in many cases.

Just a warning for those who are afraid of spiders, all of this can happen when a spider is injured or shocked as well, so just because you see a spider on its back with its legs pulled in toward its body does not mean it is dead.

On a morbid note, researchers found that the spiders are able to have a strong grip and precise control over their bodies while alive. So, some researchers thought that they could use those same systems after the spider died to control the motion of its corpse. They found some success in making what they call spider necrobots by injecting air into the bodies and releasing it in a way that controls their legs.

If you’re into that type of thing, you can see a video of a dead spider being controlled this way below.

What practical uses this could have is not clear. If the researchers could design an extremely small air pump that could be controlled at a distance, they could send the spider along on spy or attack missions, but that seems highly unlikely given today’s technology. Especially since the spider’s body would break down relatively quickly, leaving it immobile.

Whatever the case, this air powered spider does show how their body works, and why it curls up like that after death.

Thought that was fascinating? Here’s another story you might like: Why You’ll Never See A Great White Shark In An Aquarium