To a jumping spider, the difference of a few millimetres can determine whether it gets to eat or not. So it should come as no surprise that these animals, which capture prey by deftly pouncing on it from a 
distance, have a very unusual depth-perception system, scientists have discovered.
Most animals judge distances either by comparing the images formed by their two separate eyes, as in humans, adjusting the focal lengths of the lenses in their eyes, as chameleons and other vertebrates do, or by moving their heads from side to side and comparing how much an image moves across the retina, which is the main way that insects do it.
But jumping spiders, which overall account for over 13% of all spider species, resort to an even more cunning trick of the light, which is described this week in the journal Science by Osaka City University scientist Takashi Nagata and his colleagues.
Working with the spider species Hasarius adansoni, the team found that they could cover all but one of the creature's eyes and it could still accurately judge distances to grab prey items.
This proved that the spiders weren't using a stereoscopic method to measure the distance. And by carefully watching the creatures before they leapt, the team also ruled out the use of any prior movements that would have given the spider clues about the distance it had to cover.
Intrigued, the team then looked at the animals' retinae, the light sensitive tissues at the backs of the eyes where light is focused by the lens and converted into electrical signals for relaying to the brain.
Curiously, the researchers found that one layer of the retina contained a pigment sensitive to green light, despite the fact that green light would not be expected to be properly focused on that region and any image formed there would be blurred. But this, it turns out, is the key to the spider's ability.
By comparing a normally-focused image detected elsewhere on the retina with the defocused image picked up by the green-sensitive layer, the spider can use the degree of blur to determine the distance with high accuracy.
The researchers proved that this was the case by watching the spiders hunt under red and green lights. Under green light the animals should perform perfectly. But under red light, which is focused inwards less strongly than green light by the lens, the spiders should be fooled into thinking things are closer than they are and jump short.
Sure enough, in trials, the spiders landed on target under green light, but in the red their leaps all fell short of the food...
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