Argiope spiders are the Picassos of the arachnid world, weaving exquisite zigzag and spiral patterns into their webs. And like any masterpiece, the designs draw a crowd. A new study reveals that insects and other prey prefer the stylish silk to more humdrum netting, but that predators, too, are more likely to pay a visit.
Scientists have been trying to decipher the purpose of patterned Argiope webs since they were first spotted in 1889. Some have suggested that the designs lure prey or deter predators, whereas others have argued that they function as a sort of detour sign that prevents birds from flying into the webs. But evidence for these theories has been lacking.
So biologists Ren-Chung Cheng and I-Min Tso of Tunghai University in Taichung, Taiwan, set up shop at a forest edge meadow in the country's Nantou County. The researchers installed cameras in front of various webs and collected more than 700 hours of video over a 2-month period. Their target was Argiope aemula, commonly known as the St. Andrew's cross spider because of the white "X" it weaves. The team filmed 56 webs with decorations and 59 without them.
The disparity had significant consequences. Decorated webs intercepted 60% more insects than the nondecorated ones did, the researchers report in the November/December issue of Behavioral Ecology. But the patterns also increased the risk of predation. Of the 18 recorded wasp attacks, two-thirds were against spiders with patterned webs. "This suggests that decorations are acting as a lure, drawing the attention of both prey and predators," says Tso, who notes that the increased risk of predation may explain why not all Argiope aemula spiders decorate. Such tradeoffs have been observed in other species--chorus frogs, for example, chirp loudly at night to attract mates, yet the calls also lure garter snakes--but this appears to be the first example of the phenomenon in a structure an animal builds.
As to why creatures find the Argiope webs so alluring, entomologist En-Cheng Yang of the National Taiwan University in Taipei notes that many insects such as honey bees have an innate preference for symmetric patterns. "It is very likely that the shape of the St. Andrew's cross gives a strong visual stimulus to insect predators and prey, but further neuroethological study is needed to clarify this mystery," he says.