Crab spiders belong to the Thomisidae family and have been known to soar to the skies, but the mechanism which allows these relatively large spiders to take flight has long remained a mystery.
The secret was finally cracked by scientists at the Technical University (Technische Universität) of Berlin, Germany, who uncovered that these spiders are able to paraglide on even the smallest breeze and spin “parachutes” from nanoscale fibers that let them float away, carried by the wind, Science Daily reports.
A new study on these tiny arachnid aviators revealed that crab spiders actually sense weather conditions and have learned to use the wind to their advantage. This behavior, known as ballooning, is quite common among juvenile spiders, as well as in small adults that don’t grow beyond three millimeters (0.11 inches) in length — and serves as a means to find food, a mate, or refuge from attackers.
But crab spiders, and particularly those of the Xysticus species (imaged above) that were observed during the study, are considerably larger, measuring five millimeters (almost 0.20 inches) in length and weighing up to 25 milligrams.
Puzzled by the arachnids’ ability to take flight despite their fairly imposing size, the German team followed a bunch of crab spiders out in the field to record their ballooning skills.
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Gizmodo reports that the team gathered 14 crab spiders and placed them on a small, dome-shaped structure in a park in Berlin, then waited out to see how the arachnids react to the wind.
Aside from studying the ballooning behavior of these spiders in nature, the scientists devised a series of wind tunnel experiments in the lab, which unraveled the details of how crab spiders ride the wind and travel for hundreds of kilometers through the air.
The first step that gets a spider aviator off the ground is evaluating the wind conditions with its front legs, by repeatedly raising one or both of them into the air and orienting toward the direction of the wind.
“From our observation, it seems obvious that spiders actively evaluate the condition of the wind with their front leg and wait for the preferable wind condition for their ballooning takeoff,” the authors write in their paper, published yesterday in the journal PLOS Biology.
The second step to arachnid paragliding is making the actual “parachute.” While researchers previously believed that crab spiders weave one or two silk fibers to catch the wind, notes New Scientist, it turns out that they actually spin between 50 and 60 nanoscale fibers, around 200 nanometers thick and roughly three meters (or 10 feet) long.
— New Scientist (@newscientist) June 15, 2018
Study lead author Moonsung Cho, an aerodynamic engineer at the German university, chimed in on his team’s discovery.
“The fibers are very hard to observe with our naked eyes. This is why, until now, we have not been able to explain the flight of ‘ballooning’ spiders.”
According to Science Daily, the nanoscale fibers that the crab spiders use to weave their “parachutes” can be even twice as long, reaching close to seven meters (almost 23 feet) in length.
The interesting thing about these spiders’ ballooning behavior is that the arachnids start weaving their nanoscale silk fibers before the wind speed reaches 3 m/sec (7 mph), which offers a clear indication that they have learned to choose the best weather conditions for flying.
The third and last step is liftoff — launching from a blade of grass by cutting themselves loose from a separate silk line which ties them down while flight preparations are being made.
“Ballooning is likely not just a random launch into the wind, but one that occurs when conditions most favor a productive journey,” concludes Cho.