Many spiders make their webs attractive to insects by using special silks that reflect ultraviolet light, according to a study by two scientists working at Yale University.

What Is Ultraviolet Light?

Ultraviolet light is the component of sunlight that causes suntans and sunburns. We can't see ultraviolet light, but insects can.

Ultraviolet light seems to provide at least two important signals to insects. First, it signifies open space, because the only natural sources of ultraviolet light are the sun and the sky. Second, ultraviolet-reflecting bars, crosses, circles, and other patterns on flowers serve as targets that attract pollinating insects.

Reflecting Silk

The authors of the study, Catherine Craig and Gary Bernard, found that many spiders make webs entirely of silk that reflects ultraviolet light. Apparently these webs attract insects because the webs look like patches of open sky to insect eyes.

Craig and Bernard did laboratory experiments showing that fruit flies are two to three times more likely to fly toward a simple web reflecting ultraviolet light than toward a web not reflecting it.

Decorating In Special Ultraviolet-Reflecting Silk

Craig and Bernard also found that many spiders, especially those who forage near flowering plants, make non-ultraviolet-reflecting webs, then sometimes decorate the webs with special ultraviolet-reflecting silk laid down in flower-like patterns.

The bodies of these spiders also reflect ultraviolet light. The two scientists observed webs built by a type of large garden spider in Panama and found that decorated webs with spiders present caught fifty-eight percent more insects per hour than undecorated empty webs.