Some tropical rainforest species spend their entire lives in the tree canopy. If a storm, a misstep or an encounter with a predator causes those creatures to lose their footing and plummet to the forest floor, they could suffer fatal injuries or quickly get eaten by ground-dwelling animals.
At least one type of spider has evolved a heroic solution to this potentially deadly problem. Should they teeter from their canopy perch, species from the Selenops genus assume a Superman-like pose—aiming head first with two front legs extended, and navigate their falls. The maneuver is the first evidence that some spiders can control their aerial descents, researchers from the U.S. and Panama report today in the Journal of the Royal Society Interface.
The spider gliders are commonly referred to as flatties, because they look as though they’ve been squashed. The team collected 59 flatties of various species from tree canopies in the lowland jungles of Panama and Peru. After weighing the spiders and photographing them in the lab, the scientists brought the arachnids back to the canopy walks where they were collected.
The researchers then performed drop tests—turning the plastic boxes containing the spiders upside down and seeing what happens as the animal tumbles out. Most spiders were dropped in windless conditions, but some were subjected to an artificial wind tunnel that the researchers set up in the trees.
The researchers filmed each fall using a high-speed, vertically mounted camera and also tracked the spiders visually. They took measurements of where the spider landed and calculated its aerial performance using the glide index—the ratio of the horizontal distance travelled to the total vertical distance dropped. They also created several digitized spider models from the video footage so they could further study how the animals manipulated their bodies through space.
More than 90 percent of the drop-tested spiders successfully glided to a tree trunk several feet below. When flatties fell, the arachnids first righted themselves so their bellies faced downward, akin to the falling strategy of cats. Then they aimed their heads toward their target and extended their forelegs to maneuver through the air. They angled their hind legs in a backward direction, giving their body a torpedo-like shape when viewed from the side.
The spiders in the wind tunnels also managed to orient themselves toward tree trunks and make successful landings—they just did it with a bit more leg flailing than the ones falling in windless conditions. By comparison, other spiders of different species that the team randomly found around the canopy walk showed no gliding behaviors and simply fell to the forest floor.
While entomologists knew that some ants and other insect species possess similar fall-orientation abilities, this is the first known example of a spider demonstrating this type of adaptational skill. Also, ants usually glide to a target abdomen-first, steering with their backward-facing heads. As the team writes, this suggests that species pursued different evolutionary paths to arrive at a common solution to a shared problem.
Studying how flatties and other wingless animals manage to control their falling trajectories could provide insight into the origins of animal flight, as well possible guidance for designing gliding robots. As the authors write: “These spiders represent a remarkable evolutionary adventure in the animal conquest of the air.”