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Seeing green helps spiders perfect jump

Arachnid focus Jumping spiders use green light to gauge the distance of their jumps, a Japanese study has found.

The findings not only explain how the spiders so reliably hit their targets, they may help to improve computer vision.

Professor Akihisa Terakita, Dr Mitsumasa Koyanagi and Dr Takashi Nagata and colleagues of Osaka City University in Japan report their results this week in the journal Science.

There are thousands of species of jumping spider spread throughout the world, which have a remarkable ability to leap several times their body length on to their prey. The scientists looked at a particular species (Hasarius adansoni) that lives in the fields around the Osaka City University. "We often find them in our houses", says Terakita.

Most jumping spiders have four sets of eyes. The key to their athletic prowess appears to be the main eyes in the centre. In the 1980s, studies showed that the retina of these central eyes are very unusual - having four layers of photoreceptor cells instead of the normal single layer.

The Japanese scientists knew that the spiders were not using 'binocular vision' to measure distance. This is the main technique we use, where each eye gives a slightly different picture of the scene and the brain can then work out how far away things are.

The spiders were also not using 'lens accommodation' - thickening or thinning of their lenses. The spider lens is "in a rigid cuticle and therefore not able to be altered in thickness", says Terakita.

They weren't using 'motion parallax' either. This is used by insects such as the praying mantis, which sways back and forth thus giving itself different pictures of the object.

This left scientists wondering how do spiders measure depth.

'Depth defocus'

Terakita's team examined a mechanism known as 'depth defocus', where the depth (or distance to an object) is determined by measuring the fuzziness of its image.

Their first discovery was that the two deepest layers of the retina only had receptors for green light.

Whether or not an image is focussed depends on two things: the wavelength (or colour) of the light, and distance between the lens and the layer of photoreceptors. Other scientists had already noted that green light would only be sharply in focus in the deepest retinal layer. In the next layer, which is a little closer to the lens, green light would be 'defocused' giving fuzzy images.

So why would the spiders bother collecting a fuzzy image? Terakita thinks that the spiders are in some way measuring the fuzziness of the image in the nearer layer and using that to judge distance by depth defocus.

The culmination of their research was an experiment where four spiders were repeatedly tested for their ability to jump on flies in either red or green light. In the green light they jumped perfectly. In the red light they consistently fell short, jumping only about 90 per cent of the distance to the flies. The value of 90 per cent fitted nicely with theoretical calculations the scientists had done using lens equations.

'Excellent eyes'

Professor Marie Herberstein of Macquarie University, who was not part of the research, is an expert in colour and vision in spiders. "Jumping spiders have excellent eyes", she says. "Their entire life revolves around vision, unlike most other spiders which rely on vibration."

"Depth perception is important to these spiders, not just for jumping on prey, but for things like males approaching females (which can be much larger). Cannibalism is rife amongst spiders and when they are hungry, well anything that moves, they just jump on it."

"The real beauty about this paper is the fullness of their explanation", says Herberstein. "I particularly value the fact that they went on to do the behavioural study. High tech is great but in the end you have to test [the hypothesis] on the whole animal."

Terakita's team have looked at just one species of jumping spider, but both he and Herberstein suspect that other jumping spiders are also using depth defocus.

"About ten kinds of jumping spiders have had their retina structure investigated by different research groups", says Terakita. "All of them have a four-layered retina. So we think most jumping spiders have a similar system."

"How to detect depth is one of the ongoing challenges in the field of computer vision", adds Terakita. He thinks there may be much to learn from the spider's system.