Scientists have discovered a type of nocturnal hunting spider in South America, about two inches across, that is able to steer while falling - much like a wingsuit flyer - in order to return to the tree from which it fell.
The spider joins a small number of non-flying insects - ants, bristle-tails and some insect larvae - known to have the ability to manoeuvre while falling instead of dropping like a rock, according to Robert Dudley, a professor of integrative biology at the University of California, Berkeley.
"My guess is that many animals living in the trees are good at aerial gliding, from snakes and lizards to ants and now spiders," Dudley said.
The spider, from the genus Selenops, is the only arachnid they found that was able to do this. Other arachnids - scorpions, pseudoscorpions, whip scorpions and even other types of spiders - merely plummeted to Earth.
Dudley and Stephen Yanoviak, a professor of biology at the University of Louisville in Kentucky, have been studying gliding insects in tropical forests for more than a decade, after discovering a group of ants that unfailingly land on a tree when accidentally brushed off a branch.
This led them to toss from a tree every non-flying arthropod they could find to see which animals glided.
"As far as adult arthropods are concerned, only ants, bristletails and spiders use directed aerial descent," Yanoviak said.
"However, the wingless immature stages of various insects that are winged as adults can also glide really well. These include cockroaches, mantids, katydids, stick insects and true bugs," Yanoviak said.
Dudley is interested in directed aerial descent because controlled gliding, he thinks, may have been the predecessor to flying, as animals learned how to use their arms and legs to gain lift in addition to manoeuvring in freefall.
"This type of aerial behaviour preceded the origin of wings," he said.
The 59 individual Selenops spiders they studied were all well-adapted to skydiving.
They are "wafer thin," Dudley said, and flexible; they manoeuvre by spreading their legs wide in order to use lift and drag to steer themselves towards the tree trunk when they fall.
If they fall upside down, they are able to right themselves in midair. The biologists occasionally saw spiders bounce off the trunk, recover and manoeuvre back to the trunk a second time for a successful landing.
By studying these unusual types of animal behaviour, Dudley said, biologists may be able to supply engineers with novel ideas for robots that can right themselves when falling.
The study was published in the journal Interface of the Royal Society.
