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Sometimes spiders ride the wind. They spin out lines of silk that are caught by the breeze and carry them aloft. They have been reported to rise a mile or two above the earth, and perhaps even to cross oceans.

It’s called ballooning.

Moonsung Cho, an aeronautical engineer, was in Denmark the first time he saw the flight of a spider. It was autumn, when baby spiders often balloon en masse and spread to new areas.

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

The flights of spiders are well known, but not their physics, so Mr. Cho tested crab spiders both in nature and in a wind tunnel, and discovered, among other things, what holds the spiders up in the air.