As if arachnophobes didn’t already have enough to worry about, biologists working in Panama and Peru have discovered a nocturnal hunting spider capable of steering while in free fall—an unprecedented adaptation in tree-dwelling spiders that’s offering fresh insights into the evolution of flight.
The spider, which belongs to the genus Selenops, joins a very small number of non-flying insects, such as ants, bristletails, and even some insect larvae, known to have the capacity to maneuver while falling. A research team led by Robert Dudley from the University of California, Berkeley, say it’s the only known arachnid capable of directed free fall. Other arachnids, such as scorpions, pseudoscorpions, whip scorpions, and other types of spiders, simply fall to the Earth like a rock.
“My guess is that many animals living in the trees are good at aerial gliding, from snakes and lizards to ants and now spiders,” noted Dudley in a statement. “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.”
Indeed, the adapted behavior appears to serve as an evasive maneuver that allows the spider to escape from potential predators. As the researchers note in their study, which now appears at the Royal Society journal Interface, “[This] discovery of targeted gliding in selenopid spiders further indicates strong selective pressures against uncontrolled falls into the understory for [tree-dwelling species].”
Credit: Yanoviak et al., 2015/Interface
To test their abilities, the researchers dropped 59 Selenops spiders from either canopy platforms of tree crowns in Panama and Peru. The vast majority (93%) directed their aerial trajectories towards nearby trunks. After landing, they re-oriented themselves and walked head-first towards specific targets.
Credit: Yanoviak et al., 2015/Interface
The scientists say that this type of behavior may have preceded the origin of wings. The spiders are exceptionally thin, and they exploit the powers of lift and drag by spreading their legs wide open. They’re even able to right themselves in midair when they turn upside-down. The biologists also witnessed spiders who bounced off a tree trunk, only to recover and resume the glide back down to the surface.
“Overall, these spiders represent a remarkable evolutionary adventure in the animal conquest of the air,” conclude the authors in their study. “The discovery of gliding behaviour in Selenops spp. raises multiple avenues for further investigation...we suggest that there are many other as yet undescribed examples of controlled aerial behaviour in non-winged [animals].”
Read the entire study at the Royal Society journal Interface: “Arachnid aloft: directed aerial descent in neotropical canopy spiders”.