Biologists have terrifying news: some spiders can fly. Of course, technically they’re just gliding, but that’s still a feat for a creature with eight legs and no wings. It’s also a big surprise for biologists.
High in the rainforest canopies of Panama and Peru, a team of biologists gathered spiders from the genus Selenops. They’re nocturnal hunters who spend their days hiding in crevices or under bark, and their bodies are remarkably flat. The largest species are about three inches wide, but less than a sixteenth of an inch thick. It’s still “an unlikely, if not truly ungainly, aerodynamic platform,” but biologist Stephen P. Yanoviak and his team thought the spiders’ wide, flat bodies might create just enough lift and drag to help them glide safely from tree to tree.
And they turned out to be right. Yanoviak and his team dropped 59 spiders from the treetops, and 55 of them glided directly to a landing on a nearby tree trunk. The researchers noted, “Each spider was tested only once, as it was not possible to recover individuals post-descent.”
When the spiders began to fall, most of them took just a tiny fraction of a second to flip themselves right side up and point their heads toward a tree trunk, in a reflexive manoeuvre that the biologists say “could be relevant to robotic design.” They glided with their front two legs extended and pointing slightly forward, while their back legs were held out and slightly back. The spiders may be using their front legs to steer, according to the biologists, who also filmed some Selenops flying in a vertical wind tunnel to get a better look at their leg movements.
The really impressive thing is that these spiders are just free falling. Some spider species use silk draglines to help them jump short distances or control a descent, and others use special webs to catch the wind, but Selenops use no cords and no parachutes. If other spiders are bungee jumpers and skydivers, Seleonops are the extreme wingsuit flyers of the arachnid world.
The four spiders that didn’t manage to glide from tree to tree probably ended up on the rainforest floor, and they probably didn’t survive there for very long.
Selenops have evolved to live in the treetops, where there are fewer predators, and their camouflage helps protect them. On the ground, many more predators lurk, and spiders from the treetops don’t blend in as easily, so it pays for the spiders to stay up in the canopy. Gliding gives the spiders a way to move from tree to tree without venturing down to the dangerous forest floor.
And because Selenops can perform their aerial feats without a net, they can recover easily if high winds blow them out of a tree, or they can make quick getaways from predators. “We observed that Selenops spp. will jump from tree surfaces to escape attacking ants,” wrote Yanoviak and his colleagues in their paper in the journal Interface.
Because gliding is such a useful adaptation for spiders who live in treetops, the researchers say other arboreal spiders may turn out to have similar abilities.