If you're arachnophobic, I hate to tell you this, but spiders can fly.
Don't panic - it's pretty much only the extra-tiny ones that take flight, which is a behaviour called ballooning. By releasing a bouquet of streamer-like silks, baby spiders ascend into the air to find new homes after hatching, and adult spiders do so to get around more easily and find mates and new food sources. Some have even crossed entire oceans using these silk parachutes to stay aloft on wind currents.
And though the behaviour is widespread, scientists haven't nailed down exactly how spiders are able to take to the skies. Moonsung Cho, an aerodynamics engineer at the Technical University of Berlin, wanted to find out, so he studied crab spiders to see when they decide to take off and how they do it. Crab spiders are decently large for spiders that fly - though still only 5mm long - so Cho thought they'd be excellent test subjects, because he wouldn't need a heavy-duty zoom to record their behaviour.
He gathered 14 of them and placed them on a small, dome-shaped structure in a Berlin park to see how they reacted to natural winds. He also studied them in the lab using controlled wind tunnels. He found that before flying away, the spiders would lay down an anchor silk strand for safety. They would then reach one of their front legs into the air to evaluate how fast the wind was blowing, and from which direction. That's the spider equivalent of licking your finger and sticking it in the air.
If the wind conditions were just right - which, for these crab spiders, meant less than 11km/h (3.3m/s) with a nice upward draft - they stood up very straight, stuck their butts in the air, and produced 50 to 60 nanoscale silks that lifted them into the skies. On average, those silks were nearly 3m long. Once they let go of their anchor strands, they were gone.
Crab spiders like this one reach out with a limb to assess the wind before deciding to fly.
"You need to see it to believe it," said Cheryl Hayashi, a spider biologist at the American Museum of Natural History who did not work on this study. "It gives you a deeper appreciation for how spiders have evolved to do this feat - they're literally sailing through the air."
From the physics perspective, this flight method works for spiders thanks to the air's viscosity compared to the extreme thinness of their silks. By looking at the silk lines under a scanning electron microscope, Cho found that many were thinner than the wavelength of visible light, which ranges from 400 to 700 nanometres.
"Most winged insects flap their wings to build a vortex of air to lift their bodies and make them float," Cho told Gizmodo. But these nanoscale silks are so thin that they use the viscosity of air to stay afloat. "From the viewpoint of spider silk, the air is like honey."
Hayashi said that it makes sense these spiders check the weather first, and aren't just spewing out silk willy-nilly when trying to fly. Spider silk is made of protein and is energetically expensive to make; the arachnids wouldn't want to waste it. In future studies, Hayashi said she'd be curious to see if other spider species poke out a leg to test the air before ballooning, too, or if this is just a crab spider behaviour.
Both Hayashi and Cho said studying the way spiders fly could inform work in the physical sciences.
"Over millions of years, these spiders fine-tuned the use of these proteins for certain functions," including flying, Hayashi told Gizmodo. "I think we could certainly get some ideas from them about aerodynamics and moving things through the air."
Specifically, Cho imagines tiny, crab spider-sized devices that could float on hurricane or tornado winds to take data, perhaps replacing the manned aircraft that currently fly into storms. Though, he admits, that is a lofty goal well beyond the limits of the current research, which was published today in the journal PLOS Biology.
And if you're still terrified of thousands of tiny, flying spiders gliding your way, Hayashi has some words of solace. "Maybe it's comforting to know that spiders can leave your area," she said. "They don't just fly toward you - spiders are shy. Spiders can fly away from you, too."