Spider eyes

This is a jumping spider: Salticus scenicus, the zebra spider, so named for its black and white stripes.

They are tiny. The males are only about 5mm in length while the females, like the females of most spider species, are larger, maybe up to 9 mm long. But both male and female have extraordinary eyes.

A zebra spider’s cephalothorax (the fused head and thorax) is oddly rectangular, and its front-facing portion (is it anthropomorphic to call it a face?) is almost entirely occupied by two pairs of eyes, the central pair much larger and just a little further forward than the outer pair. Two more pairs of eyes are set further back on the sides of the cephalothorax. That’s four pairs of eyes all together.

The large central front-facing pair of eyes (the anterior median eyes in science-speak) have high resolution, colour vision. All three pairs of lateral eyes have less focused, black and white, wide-screen vision that give the spider an all-round, 360° view.

When it is hunting, a zebra spider just sits and waits while its lateral eyes provide a fairly unfocused monochrome picture of the surroundings, But the lateral eyes also act as sensitive motion detectors; when they find movement that might indicate potential prey, the spider turns to face it, bringing it within range of the central pair of forward-facing eyes that can provide an HD, coloured image detailed enough to identify.

When it knows that what it is facing is edible, the spider shuffles backwards and forwards to adjust the focus of all four of its anterior eyes and to bring the prey into binocular focus. Distance, size and range all accounted for, this is when the zebra spider jumps.