Proxy vampire: spider eats blood by catching mosquitoes.

An extensive battery of arachnid-food tests has shown for the first time that a spider can have a special taste for vertebrate blood, say researchers.

The jumping spider of East Africa doesn't have the mouthparts to get vertebrate blood directly, says Ximena J. Nelson of Macquarie University in Sydney, Australia. But it often catches female mosquitoes bloated with a recent blood meal.

Now, laboratory tests show that this spider (Evarcha culicivora) actually prefers the engorged mosquitoes to other prey such as midges, report Nelson and her colleagues in an upcoming Proceedings of the National Academy of Sciences.

Jumping spiders, the largest spider family, with some 5,000 species described so far, have six to eight eyes and unusually good vision. They don't hunt with webs but sneak to within a few centimeters of their quarry and then pounce. It's "very catlike," says Nelson. The strike takes less than 0.04 second. Some of the jumpers specialize in hunting ants or even the dangerous challenge of bagging other spiders.

E. culicivora has had a scientific name only since 2003. One of its codescribers, Robert Jackson of the University of Canterbury in Christchurch, New Zealand, noticed that it abounds in and near houses in Kenya. To see why, he and his colleagues offered the spiders a smorgasbord of prey and found that they readily attacked mosquitoes.

That's not the same thing as having a clear preference for them, so Jackson and his colleagues, including Nelson, did a detailed analysis of the spiders' dining choices in the lab. The researchers put the spiders in a transparent box with dead-end tunnels leading off each side. Outside each glass tunnel, the scientists displayed prey, dead but mounted on corks in lifelike poses. On tests with 1,423 spiders, more than 80 percent of the arachnids settled into a tunnel with a blood-fed mosquito, rather than some other prey species nearby.

In other tests, around three-quarters of the spiders preferred female blood-fed mosquitoes to males, which don't eat blood, or to sugar-fed females.

To test whether olfactory signals also contribute to spiders' prey preferences, the investigators wafted the odors of various prey into the two arms of a Y-shaped test chamber. The spiders found the scent of blood-fed female mosquitoes most alluring. Preliminary work by the group also suggests that the preference is innate, since tests on lab-reared spiders that had never tasted blood yielded similar results.

"A variety of recent findings are suggesting that spider foraging isn't as simple as previously thought," says Eileen Hebets of the University of Nebraska in Lincoln. One study has even shown that, given a choice, spiders tend to balance their diets with protein if they're getting too much fat.