Spider’s spring-loaded trap launches prey into its web

When most silk-spinning spiders hunt, they build a web and wait for prey to blunder into it. But a newly discovered species from Australia instead uses its silk to craft a spring-loaded, cone-shaped death trap, which catapults its prey into the spider’s main web. A foraging tree ant bites the base of the trap, then silk tethers release and the structure hurls the ant upward. This feat of arachnid engineering has never previously been observed.

Scientists recently discovered that this trap was the work of a spider from the rainforest of North Queensland. The spider — nicknamed “ballista spider,” after a projectile weapon that originated in ancient Greece — has gangly, orange limbs and a greenish-yellow body measuring about 0.2 inches (5 millimeters) long. It belongs to the genus Propostira but has yet to receive a species name, researchers reported Monday in the journal Current Biology.

“The snare is perhaps most effective because it releases energy so rapidly that, relative to its size, it produces thousands of times more power than muscle can generate,” said lead study author Ajay Narendra, a sensory biologist and professor in the School of Natural Sciences at Macquarie University in Sydney.

“Like a loaded spring, it stores energy slowly and releases it almost instantaneously,” Narendra told CNN in an email.

Only one species of ant is drawn to this trap and sets it off: the green tree ant, or Oecophylla smaragdina, an arboreal species that is abundant and aggressive. It attacks rivals and predators with bites and blasts of formic acid, and has adhesive pads on its feet to help it carry heavy loads up trees.

The ballista spider is the only known spider to hunt just one species of prey. Its highly specialized strategy likely evolved to take advantage of the ant’s natural aggression and then overcome its defenses; the slingshot mechanism flings the ant from the foraging trail, reducing the risk that the spider will be swarmed by other workers, the researchers hypothesized. However, it is still unknown why the green tree ant and no others approach the trap aggressively. One possibility, according to the study, may be that the spiders apply pheromones to the silk catapult that provoke only green tree ants, causing them to attack the cone and trigger its release.

“This is a remarkable discovery because it combines two aspects that are rarely seen together: extreme biomechanical performance and a high degree of ecological specialization,” said Leonardo Delgado-Santa, a professor of biology at the University of Quindío in Colombia and a researcher with the Ecdysis Research Group. He studies arachnids but was not involved in the new discovery.

“Biologists have known for some time that certain spiders can use tensioned silk to amplify power and rapidly capture prey, but this study describes a system in which the trap is specifically tuned to exploit the defensive behavior of a particular ant species,” Delgado-Santa told CNN in an email.

“The fact that the prey itself triggers the mechanism through its aggressive response makes the system especially elegant from an evolutionary perspective,” he added.

Study coauthor Gregory Anderson, a taxonomist and emeritus research fellow at the QIMR Berghofer Medical Research Institute in Brisbane, Queensland, was the first to observe the trap-building prowess of the spider. He then contacted Narendra and senior study author Jonas Wolff, a researcher at the University of Greifswald’s Zoological Institute and Museum in Germany who specializes in spider silk.

“Jonas and I immediately thought this was absolutely bizarre and needed investigation,” Narendra said.

To capture the first-ever images of this unique trap mechanism, the scientists drove thousands of miles to the remote rainforests of Australia’s Cape York Peninsula and conducted nighttime spider stakeouts with multiple cameras and infrared lights. Over several nights, they watched and filmed for nearly four hours as spiders built their conical trap strand by strand.

First, a spider would lay down an anchor point on a surface where green tree ants were likely to forage, before attaching it to a tension line connecting back to the main web. It would repeat this process until it had constructed a scaffold of 15 to 60 tension lines that are stretched taut in a conical shape, which the spider would then wrap with a thinner type of silk. Completed cones measured about 0.24 inches (6 millimeters) long, with a diameter of 0.09 inches (2.3 millimeters) at the base.

“Within seconds ants are attracted to it and bite the silk cone,” Narendra said. Biting destabilizes the cone so it detaches from the forest floor, leaf or branch, and the tension lines rapidly contract. The ant — still clutching the cone in its mandibles — is then yanked up and flung into the spider’s web.

On the first night, when an ant triggered the snare, the trap was deployed so quickly that the researchers’ high-speed camera — capable of shooting between 5,000 and 7,000 frames per second (fps) — wasn’t ready and missed the shot.

“It was over in a flash,” Narendra said. Luckily, a few images were caught by a camera shooting at a much slower speed: 25 fps. “We went through the footage on site, and at this frame rate, the snare was present in one frame, and gone in the next,” Narendra said. “We only saw the captured ant dangling in the core web. That’s when we knew we had witnessed something special.”

They later filmed a trap’s release at 5,000 fps. Acceleration of the triggered snare was more than 3,058 miles per second (4,921 kilometers per second), or about 100 times greater than the acceleration of a Formula One car, Narendra said.

The snare doesn’t just move quickly, he added. For such a tiny mass of silk, “it packs an extraordinary amount of energy,” even more than that of the famous slingshot spider, “long regarded as one of nature’s most powerful silk-powered launch systems.”

For Delgado-Santa, who researches spider ecology and behavioral adaptations, such as how artificial light affects hunting habits in some spiders, “this study provides another example of how finely tuned spider foraging strategies can become in response to specific ecological challenges.”

The researchers are also eager to delve deeper into the ballista spider’s family tree, to see what other surprises they might have in store, Narendra said.

“Other species of Propostira are found in Asia and we are keen to determine their hunting strategies.”

Mindy Weisberger is a science writer and media producer whose work has appeared in Live Science, Scientific American and How It Works magazine. She is the author of “Rise of the Zombie Bugs: The Surprising Science of Parasitic Mind-Control” (Hopkins Press).

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