Article: Spiders’ Legs Are Hydraulic Masterpieces

Very interesting article from the WSJ.  Don’t believe I’ve re-posted this one yet:

Spiders’ Legs Are Hydraulic Masterpieces:  The compressed-fluid system that arachnids use to move their limbs has inspired scientists and engineers.

By Helen Czerski

I’ve recently started to hear tales of unwelcome houseguests. But instead of unwashed dishes and loud music, the complaints focus on the visitors’ habit of creeping out from tiny hiding places, scuttling across floorboards and making people jump. In early autumn, the house spiders come out to play.

I’m no arachnophobe, but thinking about spider anatomy for too long does make me wince. I find them fascinating in a slightly icky way, and this is a rare case where I’m happier being fascinated from a distance. But the way a spider moves is a beautiful lesson in mechanics. Within their spindly frame, spiders are hiding a sophisticated system of hydraulics that is essential to their movement.

A spider going for a walk is deeply impressive. Each leg has seven segments, so with eight legs, that’s 56 individual body parts to coordinate. Spiders have an exoskeleton, an external frame made of chitin and protein with no internal bones. There are muscles inside the exoskeleton that pull on it to flex the legs and bend them inward. But for two of the major joints in each leg, there are no extensor muscles, which means there’s no muscular way to pull the legs outward again. This is where the spider’s internal plumbing comes in handy.

All eight legs are attached to the front part of the spider, the prosoma or cephalothorax, which also carries the eyes and mouthparts. Inside the prosoma there’s a fluid called hemolymph that takes the place of blood. Like our blood, it’s a transport system, carrying oxygen and nutrients. But instead of flowing through pipes like our veins and arteries, it just fills the spaces between other organs.

Hemolymph is mostly water, and that gives it a useful characteristic: It can’t be compressed. You can’t squeeze it and make it smaller. Just like a water-filled balloon, if you try to squeeze it in one place, it will push into the surroundings somewhere else so that its overall volume stays the same.

This is the key point about any hydraulic system: As the fluid is pushed into another space, it transmits a force. So instead of using muscles to extend its major leg joints, the spider squeezes on the fluid in its prosoma; the hemolymph is forced out and down the legs, pushing the leg joints outward. When the spider relaxes the internal pressure, the extra fluid flows back into the prosoma and the spider is ready for the next step. Using fluid pressure like this is so effective that it has been called a “hydraulic skeleton.”

This is why dead spiders are found with their legs curled up: The tension in the flexor muscles pulls the legs inward, and there is no longer any internal pressure in the hemolymph to push them back out.

Like any system, hydraulics has its advantages and disadvantages, the most obvious being the possibility of a puncture. Spiders do have mechanisms for sealing the hole if they lose a leg and can run around quite happily afterward, but they’re very vulnerable to punctures at sites that can’t be easily sealed.

We tend to associate hydraulics with heavy machinery, but the spider is a living demonstration that it has far more delicate and intricate possibilities. Scientists and engineers have already been inspired by spiders to design tools for use in space that operate using a similar hydraulic system. So next time you see a spider scuttle across the floor, it’s worth taking a moment to appreciate all of its hidden subtlety. Even your least favorite house guest can be fascinating company.

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