four pairs of them, each pair with a different task.

The result is fantastic vision that allows them

to stalk and hunt prey, and make some spectacular jumps.

But how exactly do those eyes work?

Scientists know they interact with each other,

but in what way?

To give a spider an eye test, researchers

had to attach the creature to the apparatus.

They held it down with a plastic and paraffin film

and made an opening to dab some wax on the spider head.

Then, they attached a hat straight out of Dr. Seuss,

and put the spider in a custom-built eye tracker —

one of only two such machines in the world.

Held in place on a trackball, the spider

watched video images.

Ultraviolet light penetrated the spider's head

to illuminate what was going on in there,

and researchers aimed a camera at the spider's

two main eyes — the big ones up front.

Flexible tubes from the eye to the retina

allow the spider to look here and there.

See them moving?

Here, the image that the spider is seeing

is superimposed over the retinas.

They look like boomerangs.

You can see them following the black dot across the screen.

But the retinas have a small field of vision.

They mainly pick up fine detail,

so they have to know where to look.

That's the role of the most forward pair of small eyes.

They pick up motion and alert the main eyes.

When the small eyes were painted over,

the main eyes were in the dark, like in this example.

See, the retinas don't track the dot.

Figuring out how the spider's tiny brain

manages this eye-to-eye communication

is next on the agenda.

And in case you were wondering,

the spiders are freed at the end of the experiment.

Off comes the hat, off goes the spider.