Crab Spiders Are Taking to the Air in 10-Foot Silk Parachutes
Crab spiders, carried gracefully along by the wind beneath their spun silk parachutes, get to live the fantasy of any imaginative child who’s ever held a kite string and dreamed of flying. As these spiders, which each weigh about as much as a small pinch of salt, unfurl an impossibly thin strand of silk, the lightest current of wind catches their micro parachute and lifts them into the air. They travel hundreds of miles and have even been found miles up in the air, higher than pretty much any other bug. Scientists and civilians have observed this wild behavior before, but now for the first time, researchers have investigated the physics behind how spider ballooning actually works.
In a paper published Thursday in the journal PLOS Biology, a team of researchers presents the results of the wind tunnel testing and outdoor observations that helped them figure out exactly what’s going on when crab spiders sail away on their gossamer parachutes. They say it’s all about “hairpin vortices,” formed by air turbulence around the spiders’ silk.
To figure it out, researchers caught crab spiders and filmed them as they tested the wind and let loose their balloons in both a wind tunnel and in a public park. By measuring wind speed, they found that spiders typically only took flight in a gentle breeze, about six miles per hour. They also investigated the balloon itself by catching the silk as spiders unraveled it. What they found was that each balloon consisted of anywhere between 6 and nearly 15 feet of silk strands that were between 121 and 323 nanometers wide — narrower than the wavelength of visible light. The unique qualities of this delicate parachute are what make the spider crab’s flight possible.
“Most winged insects flap their wings to build a vortex of air to lift their bodies and make them float,” Moonsung Cho, a Ph.D. candidate in aeronautics at Technical University of Berlin and first author on the study, tells Gizmodo. But crab spiders let the air’s viscosity do all the work for them. “From the viewpoint of spider silk, the air is like honey.”
This stickiness and thickness of air in relation to the spider silk is the key to the crab spider’s whole deal. As the sticky air flows around the spiders’ fine threads of silk, it creates turbulence. So instead of using their own energy — like chumps — the spiders let the air do the hard work of creating the turbulence that carries them aloft. The researchers suspect the updrafts that lift the spiders only occur at lower wind speeds, which is why the spiders don’t balloon when the wind is gusting.
And to this point, they observed a curious behavior: Before the spiders lift their abdomens into the air to let loose their parachute juice, they often lift a single leg. Cho and his colleagues stop just short of saying the spiders are testing the wind speed with this leg check, but the number of spiders they saw doing it did raise the possibility. Perhaps further research will show whether this is really what’s going on.