Spider webs: luring light may be a trap.

Spider webs: Luring light may be a trap

In the first studies of the spectral properties of spider silks, researchers have found that some of these silks reflect ultraviolet (UV) light and that this property lures insects to the webs, says coauthor Catherine L. Craig, evolutionary ecologist at Yale University in New Haven, Conn.

The work "provides an unexpected new insight into the factors that shape the evolution of spider web design," says biologist Stephen Nowicki of Duke University in Durham, N.C. Comparing webs from evolutionarily early and later specieis, the scientists found that the optical properties of spider webs change with the evolution of the web, Craig says.

In contrast to the traditional view of spiders as passing foragers, the studies also show that "spiders are doing more than we imagined to increase their probability of capturing prey," Nowicki says. Unlike humans, insects can see ultraviolet light and are known to use this sense to locate UV-reflecting flowers and liquics, which may be important food resources or mating sites, Craig explains. But, until now, no one realized that spiders" webs have UV-reflecting properties that turn their prey's UV-detecting ability into a liability -- for the insect.

Craig and Gary D. Bernard at the Yale University School of Medicine studied the spectral properties of silks from different spider species by directing a monochromatic beam of light at the silk and measuring the relative amounts of the colors reflected back. They found that the silks from the earliest "ancestral" spiders, which spin silks for only domestic purposes such as lining burrows and covering eggs, selectively reflect ultraviolet light and that the prey-capturing silks of the more recently evolved primitive aerial web weavers, Uloborus glomosus, have an even more enhanced UV-reflectance peak.

When drosophila fruit flies were given a choice between a glomosus web illuminated with white light containing a UV component and one brightened with nonUV-containing light, the majority flew to the ultraviolet-reflecting web. This work indicates that although UV reflectance in spider silk did ot evolve for the purpose of capturing prey, its prey-luring advantage seems to have caused natural selection to preserve and enhance the property, Craig told SCIENCE NEWS.

When the researchers looked at the catching silks of the more recently derived garden spider, Argiope argentata, they found that the main portions of these webs do not reflect ultraviolet light, but that the decorations added to their webs do. They then discovered that, in nature, decorated webs capture 58 percent more insects than do undecorated webs, suggesting a novel, prey-attracting function for the designs.

Althouh other scientists have proposed mechanical functions for the designs and recent data from Nowicki and his team suggest that the features function to warn birds of a webhs presence, "the strongest point about our hypothesis is that it applies to all situations where you find these [decorative] structures," Craig says. The new studies are scheduled to appear in a forthcoming issue of EOCOLOGY.