Brain in the Leg

Brain in the Leg

Thinner than thread and stronger than steel, silk is not simply for snares.

She’s full of brains. Very full. That’s practically all she is — a brain with its support system, carried on eight hairy legs.

Life is not always easy. Many large creatures are waiting to snap her up from below or snatch her from above. That’s why she was born with extra legs: during the course of her life, there’s a good chance some will get broken off, in one accident or another.

Meanwhile, she has to take care of her own meals: that means watching out for prey, knowing the right place to set out traps, and being on the alert for anything that moves. Not to mention making the trap in the first place.

All this watching and acting and calculating takes brains. But brains take up space. And, when you’re as small as an orb-weaver, there isn’t usually very much space to go around.


Halfway up the tree, she lets out a long, thin strand of silk. It spins out from her abdomen, blowing in the wind till it catches a branch on the other side.

This is her new chosen spot. The old trap wasn’t catching enough food, so she abandoned it to set a new one. Abandoned? Not quite. The silken threads, stronger than the toughest steel, are too precious to waste. She had attached them to her backside, reverse-spinning to draw them in for future use.

She lets out this recycled silk, as she walks across the first line. Behind her, a second, looser line trails down. Now, she walks on the loose line, till it sags to an upside-down triangle, and drops down from there, on a new thread, to form a ‘Y’.

And so she continues, step after step, until her web is complete. Then she sits inside, waiting patiently for her food to arrive.


Not all spiders build webs the same way. Some don’t use webs at all. The “orb-web” is the stereotypical spiderweb. But, there are others like the “tunnel-web” spiders, who make a house complete with a back door, and the “cobweb” spiders who let down straggly strings to snare unsuspecting victims, hauling them up like a fish when they get caught.

Meanwhile, wolf-spiders simply walk around, hunting for prey with their eight eyes — four to detect shapes, and four to see how they move.

It’s not easy to make a web. You have to follow an intricate design, moving and attaching and extending and measuring out just the right places. And yet, she manages to make them perfect, every time.

Instead of remembering exactly when and where to turn, she uses the half-built web as a guide for where to build next. The silken strings, it seems, is substituting for memory.


Silk is not just for webs. You can spin cocoons around eggs to keep them safe; you can let out a guiding tether so you don’t lose your way. You can —

But wait. You’ll see what I mean.

Here comes a swarm of insects. Look at her legs: the tiny hairs are vibrating at the sound. Her legs are very sensitive; they can pick up wind in the air, vibrations of motion, and even some noises like this one. And now comes a new signal: the web is shaking. Something has been caught!

She quickly moves to the spot, taking the special frame threads that are kept non-sticky to walk on. She sends out her silk, and starts wrapping her latest catch up.

Then, she spits into the food.

Or rather, she sends in enzymes that will digest it for her to eat. That’s right: she digests food even before eating it.

You mammals have it easy. Your bodies are so big, you can afford to have your long intestines and your four-compartmented stomachs. (What? You don’t have that? Well, some of you do — how am I supposed to tell the difference?).

For her, it’s easier to start the process off before filling in. There’s not enough place inside, so some bits go outside.


Some spiders don’t even have stomachs. At least, the young ones don’t Children of the tiny Phidippus clarus jumping-spider have very short intestines, because the rest of the space is taken up by their brains.

When she was little, her body used to bulge with all the brain she was carrying. She was completely full-up. But there are even tinier species, whose brains extend all the way to the insides of their legs.

Of course, everyone grows up. The stomachless jumping-spiders will soon grow stomachs, and then she’ll have to be careful not to end up inside one.

Jumping spiders are spiders that jump. Some species drop down on a bungee line, before their victims know what’s going on. And, their victims are often other spiders.

Jumping spiders have other tricks up their sleeves, too. They might vibrate her web, fooling her into thinking she’s caught something, when in fact it’s they who have caught her. That’s not as easy as it sounds: different things vibrate differently.

Listening to her web, she can make out when it’s torn and needs repairing. She can make out exactly where something’s fallen, and whether it’s a leaf, a large animal, something to eat, or a male orb-weaver come calling. She can even attach a ‘signal line’ to monitor the web when she’s not on it.

It’s like another sensory organ, extending where her real body can’t reach. There’s no place inside, so some bits go outside.


There — another vibration. Something’s happening in the bottom part of the web. But — what is this?

Snip. Snip. Snip. Something is cutting away at her beautiful orb, making it hang down in strands like a cobweb. That something, incidentally, is the community of bacteria, invertebrates and cells known as biologist Hilton Japyassú, of the Federal University of Bahia, who is presently conducting a study.

Cobweb spiders are a completely different kind of spider, and cobwebs are a completely different kind of web. The two kinds diverged millions of years ago, and work in completely different ways.

And yet, when an insect comes and gets stuck on the scientist-snipped web, she begins pulling it up just as a cobweb-spider would.

How does she know how to act? How could the behaviour have evolved in her brain, if her species don’t usually encounter that kind of web? One theory is, it comes form the structure of the web itself.

Are bits going outside, because there’s no place inside?


Thinking happens in the brain. Or so most people assume. After all, the brain is the decision-maker: it’s what gives orders to other parts of the body, and processes all the information that comes in.

But is that the only part of thinking — what scientists call “cognition”? What about your retina, that pre-processes images before sending them on to your brain? Pencil and paper you use while calculating? Your iPhone which remembers phone-numbers for you?

The theory of “extended cognition” was proposed by philosophers Andy Clark and David Chalmers in 1998. Your brain is only the centre of thought, they say, but other parts of the world also help you in thinking.

The question is, how do we decide if something is part of the thought-process? Pencil and paper help our math, but maybe we load the information into our brain, do all the thinking there, and then dump the result back onto paper.


The experiments Japyassú’s team conducted have re-opened the ‘extended cognition’ debate. But this time, it goes beyond philosophy. There are ways we can actually put the idea to scientific test.

Anzan’ is the art of calculating using an abacus. But anzan experts don’t need an abacus at all: they just imagine it in their heads, moving their fingers over phantom beads to make calculations.

Conventional theory has it that everything we think of is represented in the brain, just like an imaginary abacus. Extended cognition, on the other hand, means there would be things out there that are not copied in the brain, but still react to and influence the thought process.


She has rebuilt her web now, in normal orb style. Turns out the spot wasn’t a good place for cobwebs. With all the time spent on rebuilding the web, she’s now especially hungry. She’s tense and alert for the slightest sign of food.

Or rather, her web is.

In her hunger, she has draws the strings tighter, making them more sensitive to tinier movements. Once she’s had her fill, she’ll loosen them out to avoid chasing unnecessary prey. In this way, the decision to pounce or not happens in the web itself, before the information even gets to her brain.

It works the other way round too. Tighten her strings manually, and she’ll end up acting more tense and alert. Loosen them, and she’ll relax.

This is one of the ways mammal scientists are conducting experiments; trying to ask her where her cognition extends. Could she tell you, if only you knew her language?

Perhaps she’d give you an answer that sounds very like the question itself:
Where do you think?


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