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Spiders Hunt With 3-D Vision

With their keen vision and deadly-accurate pounce, jumping spiders are the cats of the invertebrate world. For decades, scientists have puzzled over how the spiders’ miniature nervous systems manage such sophisticated perception and hunting behavior. A new study of Adanson’s jumping spider (Hasarius adansoni) fills in one key ingredient: an unusual form of depth perception.

Like all jumping spiders, the Adanson’s spider has eight eyes. The two big ones, front and center on the spider’s “face,” have the sharpest vision. They include a lens that projects an image onto the retina—the light-sensitive tissue at the back of the eye. That much is common in animal vision, but the jumping spider’s retina takes things a step further: It consists of not one but four distinct layers of light-sensitive cells. Biologists weren’t sure what all those layers were for, and research in the 1980s made them even more enigmatic. Studies showed that whenever an object is focused on the base layer, it is out of focus on the next layer up—which would seem to make the spider’s vision blurrier rather than sharper.

That led to a “long-standing mystery,” says Duane Harland, a biologist who studies spider vision at AgResearch in Lincoln, New Zealand, and who was not involved in the new study. “What’s the point of having a retina that’s out of focus?” The answer, it turns out, is that having two versions of the same scene—one crisp and one fuzzy—helps spiders gauge the distance to objects like fruit flies and other prey.

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