Hungry spiders tune up web jiggliness.

An Asian spider spins webs of different designs depending on how hungry it is, reports a Japanese researcher.

A well-fed Octonoba sybotides spider adds silk bands along web spokes, Takeshi Watanabe of the University of Kyoto has observed. When the spider gets hungry, however, it arranges the bands so that they spiral toward the web's hub.

The two web types responds differently to incoming prey, Watanabe argues in the March 22 PROCEEDINGS OF THE ROYAL SOCIETY OF LONDON B. Hungry spiders set their webs to quiver at the ping of even a tiny insect, he contends. Satiated spiders tune down the sensitivity so webs vibrate only for a big prize.

The report joins a recent surge of research on an old question: Why do spiders decorate their webs? At least 78 species add scrawls of silk, perhaps bars or an X, that have no purpose clear to people. A description in 1889 called them stabilimenta, as if they added stability, notes George Uetz of the University of Cincinnati. Since then, researchers have speculated that silk squiggles evolved as hiding spots, warnings for birds, lures for prey, devices to slow bee leaning, and even sun shields.

"It s like `Certs is a candy mint; Certs is a breath mint," Uetz says. "What's interesting about this paper is, here's yet another point of view."

Watanabe's suggestion echoes the original notion of a structural function, notes Marie Herberstein of the University of Melbourne in Australia. When she recently reviewed the literature on web decorations, she found little testing of their mechanical effects.

Now, however, Watanabe has analyzed web tension by comparing the straight-line distance between points to the length of the silk strand linking them. Webs with spiral stabiltmenta have tauter strands, he reports.

Watanabe's earlier work had established that when a spider in a laboratory case satiates itself with two fruit flies a day, it builds its daily web with linear instead of spiral decorations.

In the new tests, he moved spiders to webs that their neighbors had built. An underfed spider responds slowly to prey when placed on the looser web of a well-fed neighbor. Conversely, that well-fed spinner's hunting speeds up when it's on the hungry spider's taut web. Tautness, not hunger alone, sparks spiders' quickness, Watanabe argues.

Todd Blackledge of Ohio State University in Columbus says that he's intrigued by Watanabe's demonstration that individual spiders adjust web sensitivity to detect prey of different sizes. It's the first evidence he's seen for that, he notes. Blackledge asks whether the spiral stabilimenta themselves tighten the web or just mark a taut structure.

The experiments also convinced Herberstein that spiders adjust their webs' tautness, she says. However, she adds, "I do not think spiders evolved stabilimenta to tune web tension."

The decorations evolved independently at least nine times, but only in diurnal spiders, Herberstein notes. If tuning had driven the evolution, wouldn't nocturnal other purposes.