27.01.2012

Eyesight "plays a major role" in how spiders jump

Eyesight "plays a major role" in how spiders jump

By Adrian Galbreth

The way in which a species of spider is able to jump is down to the arachnid using green light to gauge the distance of their leap, a new breakthrough study has suggested.

Research published in the journal science by experts from Osaka City University in Japan and carried out by Professor Akihisa Terakita, Dr Mitsumasa Koyanagi, Dr Takashi Nagata and colleagues, suggest that the findings may have wider implications.

The experts examined the species Hasarius Adansoni, which lives in the fields around the Osaka City University and has four sets of eyes, with the key to their ability believed to be the main eyes in the centre.

Previous studies proved that the retinas in these central eyes is very unusual, as they have four layers of photoreceptor cells instead of the normal single layer, though the Japanese scientists were aware that the arachnids were not using 'binocular vision' to measure jumping distances, like humans do, or other methods such as lens accommodation or motion parallax.

Professor's Terakita's team therefore examined a mechanism known as 'depth defocus', where distance to an object is determined by measuring the fuzziness of its image, with initial results revealing that the two deepest layers of the spider's retinas only had receptors for green light.

The ability to focus on an image often depends on the wavelength of the light, as well as the distance between the lens and the layer of photoreceptors, with previous studies showing that green light would only be sharply in focus in the deepest retinal layer, while in the next layer green light is 'defocused' to produce 'fuzzy' images.

According to the researchers, spiders are able to measure the fuzziness of the image in the nearer layer and use it to judge distance by depth defocus.

To prove this theory, they conducted an experiment which involved four spiders being repeatedly assessed on their ability to jump on flies in either red or green light, with the result being that they jumped perfectly in green light but only jumped 90 per cent of the distance to the flies in red light.

According to the specialists, the findings may not only explain how the spiders so reliably hit their targets, but could also help to improve computer vision in subsequent studies.

by Adrian Galbreth


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