It turns out spiders may be some of nature's best little pilots.
Using a technique called “ballooning,” they release sail-like trails of silk that lift them up and off into the wind. In some cases, they drop just a few metres from their takeoff site; in others, they get caught in jet streams that take them across oceans. In all cases, they go where the wind takes them.
Aerodynamics engineer Moonsung Cho from Berlin's Technical Institute recently set out to find out how spiders seem to “fly.” Were they randomly shooting out webs or acting more deliberately, he wondered.
He started by gathering 14 crab spiders. They were chosen because they're on the heavier side of spiders that show this behaviour, weighing more than five grams.
Cho then exposed them to windy environments. First in a Berlin park that contained a natural breeze, and then in a lab, where he could manipulate wind speeds.
Setting the spiders on an open platform, he took video of their flights to observe the quick motions that are difficult to see in real-time.
The behaviour Cho observed was “highly developed,” he says. He gathered that there are “big numbers of spiders doing this accurately.”
Before takeoff, the spiders prepared, like any good pilot would do.
Sticking out a front, hairy leg, the spiders tested wind speeds. In his lab, Cho was able to manipulate speeds and found that the spiders typically didn't take off until speeds were lower than three metres per second.
Once they were ready, they raised their abdomens and released about 50 to 60 threads of silk. Looking at the threads under a microscope, Cho observed that each one was no more than 320 nanometres (one billionth of a metre) thick.
The wavelength of visible light ranges from 400 to 700 nanometres—thus “the thinness of a spider's ballooning silk is smaller than the wavelength of light,” says Cho.
Next for Spider Flight?
What impresses Cho, he notes, is just how many spiders are exhibiting this complex behaviour around the globe. Charles Darwin noted spiders landing on his ship 97 kilometres from the coast of Argentina during a 1832 voyage. Today, ballooning spiders blanket a town in Australia, leaving a film of silk.
In the future, the Berlin-based researchers think further study of how the spiders fly could advance human innovation.
Cho and his university advisers hope to apply the findings to biomechanics. His work so far was published on the preprint server bioRxiv, but more research is forthcoming, he adds.
Cho thinks spiders could be an example of low energy transportation, but emphasises such an innovation is years away and that his current work is simply to better understand the spiders themselves.