Believe it or not

By Jonathan Sarfati

Spiders’ “out-of-focus” eyes help them see depth


Jumping spiders don't use webs to catch their prey, but pounce on them accurately. Researchers have long wondered how tiny eyes could provide the depth perception needed, or how a brain smaller than a house fly's could interpret it.

Biologists from Osaka City University in Japan have solved the first problem. The spider's retina has four distinct layers of light-sensitive cells. However, this means that light that is focused on one layer will be out of focus on the others. Atheists would point to "bad design" of an eye that produces unfocused images.

Also, the spiders are most sensitive to green light, which just happens to be the peak wavelength from the sun; pure coincidence no doubt. But they don't detect red light very well. The spider's eye lenses also refract (bend) red light less strongly.

The researchers covered up the six secondary eyes of Adanson's jumping spiders (Hasarius adansoni), and left the two main eyes. Then they let them try to pounce on fruit flies in both green and red light. Their pounces were almost always successful in green light, but were about 10% short in red light.

The reason is that both colours focus sharply on the first layer, but the second layer has a fuzzier image in green light than in red. The sharper red image tricked the spiders into thinking that the flies were closer than they were. Thus the very fuzziness of the second image is no design flaw, but is actually an ingenious design, vital for the spiders' depth perception.

In nature, there is no confusion, precisely because they see so weakly in red, so the green image swamps the red.

But now, spider researchers must solve the second problem: how their tiny brains process the difference in image clarity and fuzziness into depth perception.

Reference: 3-D Vision for Tiny Eyes,, 26 January 2012.

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