Life - Zoologger: How a blurry-eyed spider pounces on target

Species: Hasarius adansoni
Habitat: Hopping all over the tropics, including parts of north Africa, Europe, south Asia and Japan

For most of us, blurry vision is a bad thing, if only because it means we're going to have to spend a lot of money on a new pair of glasses. For one jumping spider, though, it's how it catches dinner.

Adanson's house jumper, as the name implies, is a jumping spider. It springs on unsuspecting prey insects from several centimetres away and swiftly dispatches them.

To pull off these leaps, it has to be an excellent judge of distance. And for that, paradoxically, it has part of its visual field permanently out of focus. It's the only animal known to judge distance in this way.

Stalk, jump and bite

The Adanson's house jumper is a cosmopolitan species – meaning it lives all over the place. It hunts during the day, pouncing on insects and other prey, although like many jumping spiders it may also take the occasional drink of nectar.

To cope with its agile lifestyle, it must have excellent eyesight. How it works is not obvious, though. Lab tests have shown that it has top-class colour vision, but that doesn't help it judge distance.

Other animals have all sorts of ways to work out how far away an object is, the most obvious being simply to have two eyes with overlapping fields of vision and compare what they see.

There are also ways to judge distance using just one eye. Chameleons do it by changing the focus of the eye's lens, bringing the object in and out of focus. Alternatively, some insects move their heads from side to side to see how the object appears to move relative to the background.

According to Mitsumasa Koyanagi of Osaka University in Japan and colleagues, though, the Adanson's house jumper does it in a way that has been predicted from theory but never seen in a real animal before.

I can see you…

The spider has two pairs of forward-facing eyes: the central principal eyes are flanked by anterior lateral eyes. If the latter pair are blinkered, the spider can still judge distance, so the principal eyes must be able to do it alone. The visual fields of these eyes don't overlap, though, and they can't adjust their focus, so they can't be using any of the known methods of judging distance.

Koyanagi found a clue in a 1981 study of a related jumping spider, which like the Adanson's house jumper has four layers of light-sensitive cells in its retinas. Bafflingly, the second-deepest layer is full of receptors sensitive to green light, but green light is always out of focus on that layer, so the image there must be mostly blurred.

The same is true of the Adanson's house jumper's principal eyes. That means the blurry image on the second layer contrasts with the sharply focused image on the layer below. As the spider closes in on its prey, the defocused image will get blurrier still, allowing the spider to gauge the distance.

Koyanagi confirmed that this is how the spiders work by testing them under pure green and pure red lights. Under red light, the total absence of green should trick the spiders' perception of its defocused images, making all objects seem closer than they are, so the spiders' jumps should fall short. That was exactly what happened.

None like me

No other known animal judges distance like this, but then, no other group of animals has this retinal structure. All jumping spiders do, though, and so Koyanagi says they may all share the rangefinding ability.

Humans do something similar when we look at photos in which the subject is sharply focused against a blurred background, but that only tells us that the subject and background are at different distances: it doesn't tell us how far we are from the subject. However, there are microscopes that determine depth this way, and engineers working on computer vision have long been interested in the idea.

The Adanson's house jumper, it seems, got there first.

Journal reference: Science, DOI: 10.1126/science.1211667

http://www.newscientist.com/