Scientists float idea on spiders' silky skills

Scientists believe they have figured out how floating spiders take flight.

Spiders fall from the sky all the time.

Most airborne spiders are small and scattered, so they often go unnoticed, except when they descend all at once. A rain of spiders drowned the Australian town of Goulburn in silk in 2015. A Goulburn resident who bravely looked up saw a several-hundred-metre tunnel of spiderlings in the air, he said. The spiders parachuted in on long strands of webbing, a behaviour that biologists call "ballooning".

Records of spider ballooning go back centuries. But scientist have struggled to understand how the arachnids generate lift. One physicist proposed that they use electrostatic forces to take to the sky. A new study in the journal PLOS Biology focuses on silk, not static. It's the most detailed examination yet of the skinny spider fibres that lasso the wind.

Herve Elettro, who studies silk and bio-inspired materials at the Swiss Federal Institute of Technology, called the research "the first rigorous experimental study" to cut through the multiple theories about spider ballooning. The tiny silk fibres, he said, "experience the right drag for the spider to be lifted with relatively little effort".

This ballooning is all about dispersal. Spiders are some of the first animals to show up on new volcanic islands. Decades ago an entomologist with the US Department of Agriculture captured a spider, using an insect trap affixed to an aircraft, nearly 5km above sea level. That was much higher than most winged insects could fly, said Moonsung Cho, a researcher at the Technical University of Berlin and an author of the new report.

Before a spider ballooned, it searched for the highest point on the domed platform, Cho said. Once there, something peculiar happened: The spider reached up with a foreleg.

Cho was careful to say that he couldn't know whether the foreleg twitch was intended. But there was a high frequency of these leg lifts right before the spiders ballooned.

Steve Yanoviak of the University of Louisville, who has studied gliding spiders, said, the authors "make a good case the spiders are probably sensing air currents".

After the foreleg lift, the spiders posed in what Cho called the "tiptoe" stance. High on their legs, the spiders stuck their rears upward, sprayed out silk and zipped into the sky. Like a kite torn from a toddler's grip, a spider was gone with the wind in seconds.

"It is plausible that spiders can sense wind speed and direction using sensory hairs by raising their legs," said Monica Mowery, a doctoral student at the University of Toronto who has studied ballooning by black widows.

It wasn't easy to study the ballooning silk, Cho said. Unless light catches the strands in just the right way, the fibres are invisible.

Cho is working on a computer simulation to mimic these flights. After that, he would like to create tiny structures that can float in the atmosphere like a spider. Just don't ask him how those structures will fall where he wants them to - nobody knows how, or whether, ballooning spiders steer themselves down.