Skydiving Spider Steers While It Falls

A recently discovered nocturnal spider about two inches across can steer while falling, like a wingsuit flyer, in order to return to the tree from which it fell.

Biologists found the spider in South America forests while working in Panama and Peru. It joins a small number of non-flying creatures, ants, bristletails, and some insect larvae, that have the ability to maneuver while falling instead of dropping like a rock.

Robert Dudley, professor of integrative biology at the University of California, Berkeley and one of the authors of the study, says:

“My guess is that many animals living in the trees are good at aerial gliding, from snakes and lizards to ants and now spiders.

If a predator comes along, it frees the animal to jump if it has a time-tested way of gliding to the nearest tree rather than landing in the understory or in a stream.”

That’s good news for the spider, because forest floor predators or fish would quickly make a meal of a grounded tree-dweller.

Gliders and Flyers

The spider, from the genus Selenops, is the only arachnid they found that is able to do this. Other arachnids, such as scorpions, pseudoscorpions, whip scorpions, and even other types of spiders, can only plummet to earth.

Researchers have studied gliding insects in tropical forests for more than a decade after discovering a group of ants that unfailingly landed on a tree when accidentally brushed off a branch. So they decided to toss from a tree every non-flying arthropod they could find to see which animals glided.

Coauthor Stephen Yanoviak, professor of biology at the University of Louisville in Kentucky, said:

“As far as adult arthropods are concerned, only ants, bristletails, and spiders use directed aerial descent. 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.”

Dudley says he is interested in directed aerial descent because he thinks controlled gliding may have been the predecessor to flying, as animals learned how to use their arms and legs to gain lift in addition to maneuvering in freefall.

“This type of aerial behavior preceded the origin of wings,” he says.

Lift and Drag

The 59 individual Selenops spiders the researchers studied were all well-adapted to skydiving.

They are “wafer thin,” Dudley says, and flexible; they maneuver by spreading their legs wide in order to use lift and drag to steer themselves toward the tree trunk when they fall. If they fall upside down, they’re able to right themselves in midair.

The biologists occasionally saw spiders bounce off the trunk, recover, and maneuver back to the trunk a second time for a successful landing.

“This study, like the first report of gliding ants, raises many questions that are wide open for further study.” Yanoviak says. “For instance, how acute is the vision of these spiders? How do they target a tree? What is the effect of their hairs or spines on aerodynamic performance?”

By studying these unusual types of animal behavior, Dudley says, biologists may be able to supply engineers with novel ideas for robots that can right themselves when falling.

Stephen P. Yanoviak, Yonatan Munk, Robert Dudley
Arachnid aloft: directed aerial descent in neotropical canopy spiders
J. R. Soc. Interface 2015 12 20150534; DOI: 10.1098/rsif.2015.0534.

Photo: Stephen Yanoviak/Univ. of Kentucky.