Just like some people, there are arachnids out there that prefer to pick up their takeout meals, including the aptly named slingshot spider (Theridiosoma gemmosum). Scientists have just figured out how these crafty spiders know exactly when it’s time to launch themselves and their web toward prey.
Researchers at the University of Akron in Ohio conducted the study, hoping to confirm a hunch. By meticulously watching slingshot spiders as they were approached by a potential snack, the researchers found evidence that the spiders likely use air vibrations—a.k.a. sound—as their cue to strike. The findings not only shine a light on these mysterious creatures, but also suggest that sound is more important to spider hunting in general than we currently assume.
Spiders are well known for the variety of methods they use to capture prey. Some are agile jumpers that pounce on their unsuspecting victims; others build clandestine traps on the ground to set up an ambush; and some even go fishing for their food. But slingshot spiders, also called ray spiders, are especially showy in how they hunt.
These tiny arachnids (smaller than one centimeter) build webs that at first glance look like a typical static orb web. But they also weave a tension line that connects the center of the web to a rock or other nearby structure. When they’re on the prowl for food, the spiders will go to the center and pull forward on the line while facing backwards from the web, causing it to stretch and form into a cone. Once they spot a possible meal, the spider will release the line, rapidly flinging the web and itself in the direction of its prey.
While scientists have known about these spiders for over a century, there’s still much we don’t get about how they and their webs work to catch prey. Unlike other catapulting spiders, for instance, the slingshot spiders don’t seem to rely on their prey actually touching the web to spring an attack. So researchers Sarah Han and Todd Blackledge decided to look closer for themselves.
As previously observed, “slingshot spiders will release their webs in response to cues like people snapping their fingers nearby [research that co-author Blackledge had taken part in], but no one had studied this in detail, nor their response to nearby, contactless prey,” Han, a biologist studying the mechanics of spider webs at Akron, told Gizmodo in an email. “Based on this observation we set out to explore if sound figured into the spider’s web release and prey capture.”
The scientists collected spiders from the wild (they’re widely distributed throughout the northern hemisphere) and took back them to the lab, where they rehomed them in terrariums that mimicked the spider’s naturally moist environment. Once the spiders got cozy and began building webs, the researchers presented them with two kinds of stimuli: wing-flapping mosquitoes stuck to a thin strip of black construction paper (allowing them to move the bugs as they saw fit), and a tuning fork set to vibrate at a frequency matching the typical winged insect prey.
Across nearly 100 experiments with the bugs and tuning forks, the researchers observed a clear pattern: the spiders most often seemed to depend on their hearing to find the opportune moment to quickly attack (the spiders and their webs were measured accelerating up to 50g and moving at the speed of nearly three feet (one meter) per second). Two short video clips of the spider’s catapult trick, provided by the researchers, can be seen below or via this YouTube link.
During one experiment, for instance, the spider did nothing when a motionless mosquito was clearly in the cone’s capture zone, but it then immediately sprung once the mosquito’s wings began flapping. The tuning forks were enticing to the spiders as well, further supporting the idea that sound is key to their hunting. And their vision probably doesn’t play much, if any, of a role, since these spiders face away from the cone and they don’t even have the more developed eyes seen with some spiders. Unlike people, a spider’s “hearing” doesn’t come from having ears, but from tiny specialized hairs on its legs called trichobothria.
“These spiders hold their webs in tension, like a slingshot, and will release their webs in response to the sounds of approaching flying insects, releasing webs before the insects make contact with the web,” Han explained. “Spiders preferred to release their webs when insect prey were close enough to the front of the web that they would be hit by silk upon the web’s release.”
The team’s findings were published Wednesday in the Journal of Experimental Biology.
While the researchers’ work may have solved the biggest question about the slingshot spider, there are plenty more curiosities left to untangle. Though they don’t depend on having prey touch their webs, for instance, their bodies might still pick up and use sound transmitted through the web as well as through the air to determine when to strike (this could help them know when the prey is actually in front of the cone). Slingshot spiders probably also aren’t the only arachnids that primarily use sound to hunt, which leaves open the possibility that sound is much more crucial a tool to spiders than currently understood by scientists.
“A broader implication of this work is that sound may be a very important and understudied factor in how spiders perceive their environment,” Han said. “Recent studies have explored how spider webs can detect sounds from across a room. And how spiders construct and tension their webs may be influenced by the prey (and other things) they can hear.”
Han isn’t sure if she’ll personally do more work studying the intricacies of the slingshot spider and how they sense their flying prey, but she hopes that others can look into it.
