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Jumping spiders zoom in with green light

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February 10, 2012

It’s well known that jumping spiders have excellent vision, far surpassing that of other spiders and even dragonflies, whose clearness of vision ranks at the top of the insect world. In a study appearing in the January 27, 2012, issue of Science, researchers in Japan describe how jumping spiders likely use green light and image defocus — comparing a blurry image with a focused image — to enhance depth perception, enabling them to jump with great precision on prey.

The Japanese study involved bathing jumping spiders in green and red light (for this study, Hasarius adansoni). Spiders in green light made very accurate jumps toward prey, while those in red light consistently jumped short of their mark.

The scientists believe that the deepest layer of a jumping spider’s four-layer retina is sensitive to green light, while the next layer up is only partially so. This enables the spider to contrast imagery from the two layers and fine-tune its jumping distance. (Notice how the distance blurs when you focus on something close at hand, giving you a sense of depth.)

If you peer at a jumping spider closely, you’ll see it positioning its body to watch your every move with its four sets of eyes. Evidence suggests that the big eyes in front have receptors for various colors and are very sensitive to UV light.

In the diagram below, notice how the big front eyes of the spider extend deep into its cephalothorax in a tubular fashion — like telephoto lenses — focusing light onto the retina.

Unlike human eyes, which can move to focus on something we want to see, a jumping spider’s eyes are fixed in place. Yet a jumping spider’s retina is different from ours; it can move to inspect something on the edge of its field of vision.

If a jumping spider should ever select your living space as its preferred hunting ground, know that many people delight in their company, describing them as “friendly” and “intelligent.” Place your hand beside a jumping spider, and it will inevitably jump aboard, saving its bite for the insects it hunts by day.

Bottom line: A jumping spider’s visual acuity is the sharpest of all spiders. The anterior medial eyes function as telephoto lenses, focusing light upon a four-layer retina. The retina can move to focus on objects at the edge of the spider’s field of vision. Scientists in Japan report in the January 27, 2012, issue of Science that the bottom two layers of the spider’s retina are sensitive to green light and enable the spider to use image defocus to enhance depth perception.