Death By Color: Spiny Spiders' Bright Stripes Attract Prey

"Attraction is all casinos are about. They lure you; they want to get you there. They lure people with bright lights, cheap plane tickets, inexpensive hotel rooms, great shows and great meals," says Mark E. Hauber of Cornell's Department of Neurobiology and Behavior. "The spiny spiders work the same way."

Hauber's discovery will be described in a forthcoming issue of the Royal Entomological Society journal Ecological Entomology (September 2002), in an article, "Colouration attracts prey to a stationary predator."

Bright colors and contrasting patterns should be rare in predators that use traps, since conspicuous body color is scientifically counter-intuitive in stationary predators, says Hauber. Generally, he says, animals use "sit-and-wait" tactics in their concealed traps to capture prey, and colors and patterns only alert potential prey. Yet orb-weaving arachnids, such as the spiny spiders of Australia, are brightly colored and have contrasting patterns on their bodies. Hauber found that the more colorful their backs, the greater their chances of catching prey.

"It goes against what most scientists would have thought. Color is an attracting feature," says Hauber. "While color on animals like parrots allows them to blend into the colorful rain forest, other animals use color to attract mates. In this case, the color lures prey to the web. Perhaps the color itself may look like flowers to the insects that eventually become entrapped in the web," he says.

Hauber observed spiny spiders (Gasteracantha fornicata ) in northeastern Australia. He covered the yellow-black striped dorsal surface on the spiders' backs with ink from a black felt-tip pen. Spiders with the black dorsal surface caught less prey than spiders with their normal colorful stripes. Repeatedly he found that the blackened spiny spider always attracted and caught less prey.

"Perhaps the colors and patterns of their dorsal surface mimic the color of food -- such as flowers -- for visually oriented prey. It is also possible that the dorsal surface of the spiny spider is highly reflective in the ultraviolet part of the spectrum," he says. "Many flies, mosquitoes and gnats are attracted to bright light, and the kind of light rich in ultraviolet spectra, because these indicate the presence of field clearings adjacent to dense forests."

Hauber also learned that spiny spiders set their webs at an angle and that they sit on the underside of their webs with their backs to the ground. This suggests, says Hauber, that sun and nearby vegetation offer camouflage for the web. "Daytime web-building and hunting, along with the web placement and orientation, is consistent with behavior that attracts prey traveling from darker areas to lighter ones," says Hauber.

Funding for the research came from the Howard Hughes Medical Institute Predoctoral Fellowship program.