How do spiders control eight eyes? What kind of vision would that be like? Do they have good vision or just simple contrast change?

A.C.

West Tisbury

A. Most spiders aren't known for their vision. They're known for their remarkable mechanoreception -- the ability to sense the tiniest of vibrations and perturbations. This, of course, helps web-spinning spiders find entangled prey.

But a few spiders, most notably jumpers, have excellent vision. These spiders don't spin webs. Instead they hunt for prey, prowling and pouncing on unsuspecting victims much like lions, says Wayne Maddison, an evolutionary biologist at the University of Arizona who studies the creatures. That strategy isn't possible for the more myopic species. To find tiny prey amid foliage, they must be able to detect moving objects. Then, to effectively leap onto on their prey, the spiders must be able to judge distance. They have different eyes for different tasks.

Jumping spiders have two big eyes in the center of their faces. These eyes are similar to ours. They have excellent resolution and can judge distance by comparing the difference in the image produced by each eye. It's thought that these spiders, just a few millimeters in size, can see well at distances up to a foot. Such vision approaches that of primates.

The other six eyes, which are smaller and arranged in rows on the face and upper body of the spider, are used as motion detectors. Because some of the eyes are fixed in the hard outer shell of the spiders, the spider must move its whole head to see objects. They can also rotate their retinas without moving their eyes.

Because of their excellent vision, jumping spiders can be a lot of fun to watch. ``They react to the world a lot like we do,'' says Maddison. ``Most mammals run around bumping into things and smelling them, but jumping spiders look around before they jump. When you watch them, it's familiar.''

If you place one on your hand, it will follow your movements with an attention unusual for an invertebrate, writes W.J. Gertsch, author of ``American Spiders.''

``Fine eyesight has made them the outstanding spider extroverts,'' Gertsch writes.

But what does the world look like to a spider? And how is this rich visual information integrated? That, says Maddison, is still unknown, primarily because it's difficult to study a spider's nervous system -- work usually done with electrodes inserted into an animal's visual nerves. With a spider, that's technically difficult.

``Whether they put together an overall image in the brain, we just don't know,'' says Maddison. ``It's hard to say what the mind's eye of a jumping spider is.''

By watching the animal's behavior, however, it is possible to get some idea of what their world is like. For instance, it's assumed that they judge distance well because they jump so accurately. It's also assumed that they can see color because colorful plumes, feathers and fringes on the legs and bodies of male spiders are a crucial part of the animals' leg-waving courtship rituals. Many male jumping spiders are ``as colorful as birds of paradise,'' says Maddison.

In the Boston area, he says, you can find jumping spiders with pink abdomens and others with apple green legs. ``It's amazing what people can see in their own back yards,'' says Maddison, if they just focus on a smaller scale.