Why a spider is like a waterbed

Scuttling out from its hidden lair, it jitters haltingly across the floor, jabbing the tiles in a skittering blur of hair, eyes, and long spindly legs. For a lot of people the sight of a spider emerging from a hiding place is just too much, and the only thoughts which pop up behind their shocked expressions involve screaming, thing-throwing, or evacuating the room.

There’s an odd flipside to this fear though: if the same spider were dead, with the same eight legs curled serenely into a spider-ball, all most people would feel is mild disgust. It seems the intrinsic horror of spiders is not in their spidery shape, but in the spidery way they move.

I live in the UK, which only has harmless spiders, so for me at least there’s no point at all being scared of them. So what is it about their stuttering gait which fills people with so much terror? It’s normally too fast to see very clearly, but watching slowed down videos or large spiders who travel at leisurely paces reveals that a spider’s walk is not at all like our own.

There’s something too jerky about their limbs, which seem to snap and whip forward more like a clockwork toy than a familiar animal. They seem more like a frightening relic left behind by a deranged Victorian inventor than fellow creatures. Seeing a spider lurching around a room you can almost hear the hiss and click of pistons and valves.

There’s a very good reason they seem so ‘other’, and it’s all down to the mechanism behind how they move. Spiders propel themselves in a way totally alien to us mammals, one which relies on such unfamiliar parts and dynamics that the resulting gait looks awkward and uncomfortable to us.

The business of moving around is, at its heart, just getting some sort of appendage, be it a leg, a fin, or a wing, to move backwards and forwards repeatedly. As long as you have some sort of process which can generate a force on this body part in one direction, and then in the opposing direction, you’re in business.

Before we see how a spider does it, let’s have a quick look at how people, tortoises, fish, and other animals with backbones can generate their less jarring movements, using muscles.

Muscles attach each of their two ends to a different part of the skeleton. When they’re tensed, the muscle pulls the two skeleton parts closer together, which can create all sorts of movement, like this bicep curl:

Here you can see one set of muscles lifting up the lower part of the arm, and an opposing set of muscles pulling it back down again. This system of opposing pairs exists for practically every joint in the body, and works perfectly if you have an internal skeleton and lots of room to fit a sheath of overlapping fronds and layers of muscle. But spiders are as different from us on the inside as they are on the outside, and don’t have this luxury.

First off, they’re hollow. You might remember from biology in school that their skeletons are on the outside (they come off and everything whenever spiders and insects shed their skins!), leaving their insides like a balloon full of water.

There are a few advantages to this, one being that they don’t even need veins and arteries, as the blood-like liquid inside them nourishes all of the organs it bathes. But it's a different advantage of being a spindly water-bed which we’re going to look at today - how it allows you to move in ways which us mammals can only have existentially confusing daydreams about.

This liquid isn’t still, it’s dynamic and moves. By squeezing its sides in and adjusting leg-valves which can block off its legs, a spider can manipulate the liquid’s pressure and direction of flow, squirting it from one part of its body cavity into another at will. To get a grip on how they can actually use this to move, think about what water pressure can do in a syringe with a piece of flexible tubing on the end.

When you push on the syringe, the water squeezes out into the sealed tube. Unable to escape from the sealed end, the pressure makes the tube stiff, and raises it up.

(For anyone sniggering right now, you’re showing not immaturity but a fine spirit of scientific observation. Blood undergoes exactly the same hydraulic process to keep a penis erect.)

This is the principle behind how spiders flex their legs outwards or forwards. When they want to move a leg out, they increase their internal water pressure by using muscles in their body to squeeze and constrict their internal cavity, forcing the liquid into anywhere it can escape to. By closing all the leg-valves except those in the leg it wants to extend, a spider forces each leg out like blowing up a series of well-coordinated balloons. You can see what's going on here in an unfortunate spider who, for clarity's sake, has had all but one leg removed:

So this is how spiders extend their legs - like a digger forcing hydraulic fluid through pipes to move its bucket-arm, spiders force liquid into their legs to inflate them outwards. Jumping spiders show just how powerful this process can be, pushing fluid into their back legs quickly enough to ping their tiny frames a hundred body lengths or more.

Unlike a person's legs, spider legs are thin enough to put space at a real premium. But while there isn't enough room for opposing pairs of muscle, there is enough room for a single set, and it’s these which are used to directly retract each leg once it’s needed back in its original position.

Here you can see the way a well-placed muscle pulls the end of the leg back in towards the spider’s body, and doesn’t need to take up much room at all. So with this partnership between water pressure and muscle power, spiders can function well with just a single muscle per joint.

Interestingly, and on an entirely unrelated note, this need to save space also means our hands are kind of screwy. Our slim fingers, like understuffed sausages, don’t have nearly enough room to fit bulky wodges of muscles. In fact, human fingers don’t contain any muscles at all. Instead, the whole arrangement is pulled around like a puppet on strings by muscles inside your forearm slunk around a series of pulleys inside your hand and digits. When the muscles in the forearm pull on the strings, they yank your fingers around.

If you don’t believe me (or want to see something cool), roll up your sleeve to your elbow right now. Turn your palm towards yourself, now watch your forearm as you repeatedly clench a fist then outstretch all of your fingers as far as they’ll go. The shapes which seem to be shuttling backwards and forwards just underneath your skin are the strings, which go all the way from where you see them to that last crook of your fingers.

But while we have two sets of muscles for each joint, one to move it backwards, the other to move it forwards, we’ve now seen that spiders function in an entirely different way. The weird jerkiness in their walk is because while the back-stroke is pulled by reasonably familiar muscles, the outward movement of each leg is like the squeezing of a syringe.

There’s a final upshot to this hybrid engine. When an organism has run, jumped or scuttled its last, and the time has come to join the great menagerie in the sky, one of the first things to happen to their mortal remains is rigor mortis, where all of the muscles contract and go stiff.

For familiar animals working, like we do, with opposing pairs of muscles, death brings a sort of stalemate. The opposing pairs of muscles pull against each other, the body goes rigid, and not much else happens.

But after a spider has lived its last day clearing up pesky flies from the corner of the garage, it only has one set of muscles to go stiff and contract. As death lets down the water pressure inside their body cavity, the legs are able to slacken like a deflating life ring. So as the single set of muscles in each egg stiffen and contract, they draw the legs inwards just as they always did, but this time completely unopposed by hydraulic pressure. And that's why a dead spider’s legs are always curled up.

Have a look the next time you come across a dead spider. If you’re not scared, of course.