Spiders of the genus Selenops (Araneae: Selenopidae) have a unique ability to control their gliding fall, as if they were skydivers, according to a team of scientists led by Prof Robert Dudley from the University of California, Berkeley.
Selenops banksi on Barro Colorado Island, Republic of Panama, as typically encountered on a tree trunk during daylight hours. Scale bar – 1 cm. Image credit: Stephen P. Yanoviak et al.
“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,” said Prof Dudley, senior author of a paper published in the journal Interface.
He and his colleagues, Prof Stephen Yanoviak from the University of Louisville and Dr Yonatan Munk from the University of California, Berkeley, are interested in directed aerial descent because controlled gliding, they think, 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,” Prof Dudley said.
In their new study, Prof Dudley and co-authors studied the gliding behavior in the tropical/subtropical spider genus Selenops.
“Fieldwork was conducted on Barro Colorado Island, Panama, and at the Amazon Conservatory for Tropical Studies field station located 67 km northeast of Iquitos, Peru on various dates between March 2006 and December 2012,” the scientists wrote.
They dropped 59 such spiders from either canopy platforms or tree crowns; the majority directed their aerial trajectories towards and then landed upon nearby tree trunks.
“Fifty-nine individual spiders were successfully drop-tested, and most of these (55, or 93%) exhibited directed aerial descent towards a tree trunk, followed by a successful landing.”
“The remaining 7% of study spiders either landed on the ground or on understory vegetation following apparently undirected parachuting.”
“By contrast, opportunistically tested representatives of other common arboreal and ground-dwelling spiders (e.g. hersiliids, ctenids, mygalomorphs, lycosids) fell with legs extended and exhibited dorsoventral righting, but showed no obvious directional control when dropped.”
Lead author Prof Yanoviak said: “this study raises many questions that are wide open for further study.”
“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?”
Prof Dudley added: “by studying these unusual types of animal behavior, biologists may be able to supply engineers with novel ideas for robots that can right themselves when falling.”
_____
Stephen P. Yanoviak et al. Arachnid aloft: directed aerial descent in neotropical canopy spiders. Interface, published online August 19, 2015; doi: 10.1098/rsif.2015.0534
