How Spiders Fly – The New York Times

Sometimes spiders climb the wind. They spin silk lines that are caught by the breeze and carry them in the air. They have been reported to climb a mile or two above the ground, and perhaps even to cross the oceans.

It's called the balloon.

Moonsung Cho, an aeronautical engineer, was in Australia the first time he saw the theft of a spider. It was fall, when baby spiders often pack in bulk and spread to new areas.

He was completely taken by the phenomenon and was the subject of his studies towards a doctorate at the Technical University of Berlin.

The spider flights are well known, but not their physical, so Mr. Cho tested crab spiders in the wild and in a wind tunnel, and found, among other things, what holds spiders in the air.

They do not actually have balloons, but rather many strands of silk that they make up to six feet long. And these silk threads, from 100 to a few hundred nanometers wide (a human hair just has a diameter less than 100 nanometers), are so thin and light that they are suspended in the air like a wire, or a hair, in molasses.

Compared to a silk thread, the air is like a thick fluid, so the effect of gravity is easily thwarted by what you might call the viscosity of the air. In a breeze, the flowing air carries the silk threads with it, as the spider goes up below.

Mr. Cho and his colleagues also reported that spiders test the wind, lifting a front paw to judge its strength. They prefer a light breeze, about seven miles per hour.