Nocturnal spider favors artificial lights.

Spiders that spin their webs near lights at night to feast on bedazzled insects may be responding to more than the abundance of easy meals. For the first time, say researchers, a laboratory test has shown that a spider that feeds at night has an inborn preference for building webs near artificial light.

Webs of the spider Larinioides sclopetarius hanging along a footbridge over the Danube Canal inspired Astrid M. Heiling of the University of Vienna to study arachnid light preferences. More spiders cluster on handrails with built-in lights than on those without illumination, she reports in the June BEHAVIORAL ECOLOGY AND SOCIOBIOLOGY. In the lab, even spiders raised without artificial light built their webs near it on the first exposure.

Heiling's work adds to a growing appreciation of the sophistication of web building, comments Mark A. Elgar of University of Melbourne in Parkville, Australia. "It really shows these aren't just some kind of sit-and-wait predators that lead uninteresting lives," he says.

The species that Heiling studied lives near European waterways and has been reported at North American sites including New York City docks. Among adults, only females, which grow to 14 millimeters long, spin webs. Males, which are smaller, snitch prey from the females' webs.

The spiders can't break through human skin, Heiling reassures people who might blunder against them. "There is no reason for arachnophobia," she says.

Surveying the Danube footbridge from May to October, Heiling found that lit handrails averaged more than four times the spider density of the unlit ones.

To test light preferences, Heiling placed spiders at the dividing point of a two-chambered box, lit on one side and blacked-out on the other. She kept the temperature equal in the halves. Eighteen out of 20 females Heiling collected from the bridge spun webs in the lighted portion.

She also raised spiders without artificial light and fed them in both darkness and daylight so they would not associate light with food. When put into the artificially lit test box, 14 out of 15 spiders chose to build webs in the light side.

The behavior may have evolved as spiders came to select sites near reflections of moonlight on water, she speculates.

A test by other investigators had found that another nocturnal orb-web spider avoids light. However, Elgar says he wouldn't be surprised if other nocturnal species turn out to favor city lights. "Where you find a streetlight, you find the odd spider hanging about," he notes.

His studies and others are bringing to light ways that spiders adjust their webs to maximize meals. Orb-web species fine-tune the total area and the mesh size of their webs. For example, hungrier spiders tend to build bigger webs.

A spider with a web that gets repeatedly bashed switches to a more tranquil location. Also, some spiders active in daylight decorate their webs with insect-attracting, UV-reflective silk squiggles. When spinning a web in a shady spot, they use more of this tinsel. A well-fed spider also invests more in decoration. "If you're well-heeled, you tend to eat more expensive food," Elgar muses.

Building good webs demonstrates real hunting prowess, he argues. A cheetah's chase to bring down dinner by sheer muscle power is "spectacular," he says, but "to our way of thinking, that's not very sophisticated."