Like a base jumper leaping off a high precipice into the abyss below, a flat, brownish spider dives from the top of a tall tree and careens through the air, spiraling downward before smashing into the tree's trunk.
No, this leaping arachnid isn't a daredevil; it has just figured out the most efficient way to move around in the forest canopy, which is where it lives in order to avoid the hazards of the forest floor, some 100 feet (30 meters) below. Selenops banksi is one of several spiders in the genus Selenops that can leap off treetops to get around, according to a new study, published today (Aug. 18) in the Journal of the Royal Society Interface.
In addition to these arachnids, lots of small insects (including many species of ants and bristletails) are known to engage in similar behavior — leaping from the tippy tops of trees with total confidence, even though they don't have wings to help them along, said Stephen Yanoviak, an associate professor of biology at the University of Louisville and lead author of the new study. [Video: See the "Soaring" Spider's Skydiving Moves]
Yanoviak first observed this behavior in ants about a decade ago, and ever since then, he has been chucking wingless arthropods off the tops of trees to see if they, too, can glide. But he wasn't expecting any fancy flying from S. banski, a common rainforest spider that Yanoviak said he typically avoids. They're not known to be dangerous to humans and are quite shy, but these half-dollar-size spiders are big enough to freak out a tree-climbing researcher.
Leaps of faith
Selenops, often called "flatties" because of their exceedingly thin bodies, blend in well in tropical forest environments, the researchers said. The exoskeletons of these critters, which are found in the rainforests of Peru and Panama (as well as other countries), are often lichen-colored, which helps them stay camouflaged against tree trunks. These spiders are also incredibly fast: Their running speed and the speed with which they attack prey place them among the fastest animals in the world, according to researchers who study these critters.
For their study, Yanoviak and his colleagues were able to collect a few Selenops specimens (59 of them, to be precise) in plastic cups. Then, they took the spiders high up into the forest canopy and turned the cups upside down. The spiders came flopping out and performed their impressive gliding maneuvers.
"They immediately right themselves, which means they turn dorsal side up [back facing the sky], and they essentially sail over towards the tree trunk — kind of like a Frisbee that's not spinning," Yanoviak said. The spider glides down headfirst for about 16 to 26 feet (5 to 8 meters), before hitting the trunk of the same tree from which it just leaped, he added.
The spiders almost always (93 percent of the time) maneuver in this way, gliding down a bit and then thwacking into the side of the tree, the researchers found. The landing doesn't seem to hurt the spiders; because of their low mass, they don't accelerate very quickly during the jump — they fall at a rate of about 9.8 feet (3 m) per second, Yanoviak said.
If a spider finds itself heading off course, it moves its front pair of legs rapidly to reposition itself, the researchers discovered. Overall, the whole jumping process is "pretty impressive," Yanoviak said.
Balloon not included
Spiders are no strangers to unconventional methods of transportation. Many small spider species engage in a behavior known as "ballooning," in which they climb to the top of a blade of grass or some other high point and stick their spinnerets into the air. Then, they release a strand of silk that catches on the wind and pulls them up, off the ground and further downwind. [10 Things You Didn't Know About Spiders]
Ballooning spiders usually only travel a few meters at a time in this manner, and the behavior is often used as a dispersal method, allowing a brood of newly hatched spiders to spread out after birth to increase their likelihood of finding prey. But it's also possible for ballooning spiders to get swept up into a strong wind and be pushed many miles away from their starting location, the researcherssaid. Spiders are nearly always among the first species to colonize a new habitat (such as a fire-ravished forest or a valley in the wake of a volcanic eruption), which may have a connection to this ballooning behavior, they added.
And recent research has shown that spiders also know how to sail. By contorting their bodies and legs into various yogalike postures, these creatures can both walk on water and sail over it, according to a studypublished last month by researchers in Japan.
But S. banski doesn't need any fancy yoga moves, nor does it need any silk, to carry out its daily base-jumping activities. And it isn't gliding around the forest canopy to disperse babies or even to look for food. In fact, the rainforest's "flying" spiders likely take to the skies in order to avoid becoming someone else's supper, Yanoviak said.
"I think this is yet another example of how organisms that live in treetops, especially wingless organisms, are faced with specific challenges that this environment presents — namely, falling is a problem," Yanoviak said. "If they fall out of a tree and into the leaf litter on the forest floor, then something is going to eat them almost right away."
S. banksi has a long list of predators — including other spiders, lizards, birds, frogs and ants — so staying off the ground is the spider's best bet for avoiding all of these animals, the researchers said.
In the future, Yanoviak and his fellow biologists would like to learn more about how Selenops species evolved to jump like tiny skydivers and how they avoid plummeting straight down onto the forest floor. The scientists suspect that the spiders control the direction of their fall with their front legs and that they use their eyes to zero in on a landing site, but they plan to do more research on exactly how this whole process comes together, Yanoviak said.
Understanding wingless animals that can somehow "fly," is an important step toward understanding how winged animals evolved. Because certain critters, such as the jumping bristletail (belonging to the order Archaeognatha),have no ancestors with wings and yet still developed these advanced gliding behaviors, it's possible that the behavioral mechanisms for controlling flight may have existed before the evolution of wings, Yanoviak said.
And the gliding spider, S. banksi, offers another example of how animals evolve to survive in challenging environments (like rainforest canopies) without doing something as drastic as sprouting a pair of wings.
"There are no spiders with wings, and there never will be spiders with wings, as far as we know, because they are so amazingly successful without ever having to have wings," Yanoviak said.