Before Soaring Through the Air, Tiny Crab Spiders Study the Wind

A recent study sheds new light on spider flight

A crab spider spins out fine silk fibers for its aerial dispersal. The image is displayed as a negative to make the silk easier to see. Moonsung Cho, Technical University of Berlin.

Some spiders can travel vast distances by releasing their silks into the wind and soaring through the air. This process is known as “ballooning,” and spiders do it for several reasons: to find food and mates, to search for promising spots for a new colony and, in the case of spiderlings, to escape cannibalism at their birth sites. Ballooning has helped turn certain spiders into great travellers, in some instances allowing them to cross oceans.

While scientists have long known about ballooning, the mechanisms of this remarkable arachnid ability are not well understood. So, as Jessica Boddy of Gizmodo reports, an aerodynamics engineer recently set out to discover precisely how some spiders are able to float through the air on strands of silk.

Moonsung Cho, an aerodynamics engineer at the Technical University of Berlin, gathered 14 crab spiders of the Xysticus genus and put them on a dome-shaped structure in a park, hoping to see how they would behave in the wind. The specimens that Cho used in his experiment only measured between 0.12 and 0.24 inches long, but crab spiders are actually larger than most ballooning arachnids, making them good candidates for study. After observing the critters’ behavior in the park, Cho studied them in a wind tunnel in a lab.

The results of his experiment, published recently in PLOS Biology, reveal that crab spiders do not just hurl themselves randomly into the air, but are instead careful, deliberate fliers. The ballooning process occurred in several phases. First, the spiders would anchor themselves with a strand of silk to make sure they did not get blown away before they were ready to take flight. Then they would stay still, taking stock of wind conditions through sensory hairs on their legs. If the wind seemed favorable—blowing at speeds of less than 7 miles per hour with light updrafts, according to a press release announcing the new study—the spiders would raise one or two of their front legs and hold them up, sensing the motion of the wind.

In the final phases, the spiders would raise their abdomens, point their bodies in the direction of the wind and spin up to 60 ballooning silks that stretched an average of 10 feet long. The strands formed a triangular sheet and propelled the spiders through the wind like “thin, silky kites,” as Yasemin Saplakoglu of Live Science writes. The safety line that anchored them in place before takeoff broke mechanically when it stretched between 10 and 16 feet long, according to the study.

Cho also examined the spiders’ silk parachutes under a scanning electron microscope and found that the strands measured between 121 to 323 nanometers—substanitally thinner than the silks of other spiders species, which can spin strands "on the order of four micrometers" in thickness, according to a 2011 study. Crab spiders do not spin webs, but with their superfine silks, they can use the viscosity of the air to stay afloat. “From the viewpoint of spider silk, the air is like honey,” Cho tells Gizmodo’s Boddy.

While the new study cleared up some mysteries about ballooning, it raised other intriguing questions about spider flight. The spiders kept their legs outstretched throughout the flight—why? And how do the critters’ silk strands form a triangular sheet without getting tangled together? These questions, the study authors note, “could be interesting topics for future research.”

Editor's note: This article originally misstated the widths of the spiders' ballooning fibers.

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