These, glittering in the sunshine, might be compared to diverging rays of light; they were not, however, straight, but in undulations like films of silk blown by the wind.
--Charles Darwin, Voyage of the Beagle, 1832
Strolling on the deck of the Beagle, Charles Darwin was captivated by spiders parachuting onto the ship from over 90 kilometers off shore via shimmering lengths of silk. The first explanation for how they do this came in 1987 from engineer Joseph Humphrey at the University of Virginia in Charlottesville. Now, scientists have righted a crucial failing of that model by taking into account one of Darwin's original observations--that the threads by which spiders travel are flexible, not rigid.
Humphrey sought to distill the behavior, called ballooning, down to a mathematical formula. Treating the thread of silk as a stiff object, he calculated that friction between the silk and the air was sufficient to ferry small spiders about 100 meters on the wind. "Like all scientists or engineers, he was trying to find the simplest explanation," says David Bohan, a quantitative ecologist at the Rothamsted Research facility in Hertfordshire, United Kingdom. But Humphrey was wrong. A straight thread can't generate enough lift to transport spiders as far as they travel--as much as 9000 meters.
So Bohan and colleagues decided to complicate things. In a paper published 12 July in Biology Letters, the team lays out a ballooning model that takes silk's gossamer qualities into account. The more flexible the thread, the greater its effective surface area, and the more drag it creates. For fine silken thread, air "acts much more like syrup than gas," explains Bohan, enabling spiders to travel tremendous distances if they encounter the right cross winds. He predicts that the model will help scientists manage the migration patterns of spiders and thereby control their populations as well as those of a number of pest groups upon which they prey.
It's a very elegant and well executed model, Humphrey says. But he points out that, because it assumes specific atmospheric conditions, it contains an oversimplification of its own. "I don't think it's an entirely universal model," he says. The next step is to test the model by observing the behavior of real spiders in wind tunnel experiments. The small size of ballooning spiders makes these observational experiments difficult, however Bohan expects results in about a year.
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