New spider: unusual suspect steals web.

The latest crime report, from Orchid Island in Taiwan, describes a new species of spider that snatches and eats pieces of other spiders' handiwork.

Little is known about this novel form of thievery, report I-Min Tso and Lucia Liu Severinghaus from the Academia Sinica in Taipei, Taiwan. Two other species, both South American, are known to steal silk. In the July ANIMAL BEHAVIOUR, the Taiwan researchers describe the third thief, which they have named Argyrodes lanyuensis.

"The whole creature looks like a drop of mercury with some appendages attached," Tso says. It shimmers along the webs of giant wood spiders, Nephila maculata. These webs stretch more than a meter across.

"When I was sitting under a tree having a break, I found in front of me two tiny silvery spiders doing something weird on one web," Tso recalls. Still munching on a cookie, the researcher leaned over to watch and realized the spiders were eating, too. They wadded silk into a ball to devour while the web owner, a much bigger spider, "seemed to pay no attention."

After observing the spiders' natural behavior and moving them around in experiments, Tso and Severinghaus concluded that the thieves reduced their hosts' webs by 21 percent on average. The thieves also stole prey from the hosts, but only occasionally. Less than 3 percent of the giant wood spider's diet comes from prey small enough for the little spiders to handle, the researchers report.

The thieves get away with stealing silk because they're small and they move stealthily, Tso observes. Some keep taking silk all day long.

Before laying eggs, the silk-stealing spider drops out of her host's web and spins a web of her own. She also covers her eggs with silk that she spins herself.

The report did not particularly surprise Jonathan A. Coddington from the Smithsonian Institution in Washington, D.C. "In general, Argyrodes, the genus, is always pestering Nephila," he says. Naturalists have documented many parasite-host relationships between the two genera, including prey-napping, egg-stealing, and stalking. "It's kind of like the Serbs and the Bosnians," Coddington says.

Fritz Vollrath of the University of Aarhus in Denmark points out that many spiders eat their own webs, which get tatty after a day of insect collisions, and then spin new ones. He discovered silk stealing in South American Argyrodes when he saw a spider dash into a smaller species' web. "I think it was after the owner," Vollrath says. "The owner just rushed out like a rocket," leaving the invader to eat the whole web.