To find out how ballooning spiders lift off, Moonsung Cho and colleagues from the Technical University of Berlin put 14 large crab spiders (Xysticus) on a mushroom-shaped platform exposed to the natural breeze, serendipitously in the same Berlin park where aviation pioneer Otto Lilienthalonce tested heavier-than-air gliders. In a separate experiment, the researchers placed the spiders in a wind tunnel, where they could control wind speed and temperature.

The spiders did not release their silk balloons at random, the team found. Instead, they raised one or two hairy legs aloft, apparently testing the wind for 5 to 8 seconds. On cold and windy days, the spiders huddled on the side of the platform and most did not launch. But if conditions were right—a warm, gentle breeze no stronger than 3 meters per second—the spiders raised their abdomens and released up to 60 silk threads, forming a triangular sheet that bore them skyward, the researchers report on the online preprint server bioRxiv. The study is the first to show that spiders, like any good aviator, carefully evaluate wind conditions before takeoff.