Jumping spiders focus like a camera to hunt

Jumping spiders do not weave webs. Instead, they jump about and then pounce on their prey. For this reason, depth perception is very important to their survival. Now Japanese researchers have discovered that the spiders use an unusual technique to estimate how close or far away an object is. The technique, called image defocus, involves blurring out objects in the distance while keeping closer ones sharp. Jumping spiders are the only animals known to use it, but a camera with a good lens demonstrates the same principle — keeping a person’s face sharp in the foreground while blurring out the trees in the background. Takashi Nagata and Mitsumasa Koyanagi of Osaka City University and their colleagues write about the jumping spiders in the current issue of Science. The researchers studied each of the four layers of the retina in one of the spider’s eyes. They found that green light is never in focus in one of the layers, and that in another layer it is always in focus. “So the spider seems to capture a sharp image and a blurry image and compare the information to estimate the distance of an object,” Dr. Koyanagi said. Depending on whether a fly, for instance, looks blurry or sharp, the spider can decide how far to jump. “It’s done in real time, so studying this may help develop better computer vision,” Dr. Koyanagi added.