How Ultra-Sensitive Hearing Allows Spiders to Cast a Net on Unsuspecting Prey
Sounds trigger the ogre-faced spider to backflip and shoot a silk trap on other insects
The ogre-faced spider’s huge eyes and nocturnal vision were thought to be its greatest hunting tool, but hearing may play a bigger role in capturing prey than previously thought. The net-casting spider relies on sight to capture prey in front of it, but can also ambush prey behind it. How the predator does that remained a mystery to scientists, until now.
A study published today in Current Biology reveals that the spiders strike behind them with amazing accuracy after hearing lower-frequency tones. They can also detect higher-frequency sounds, which researchers suspect warn the spiders of incoming predators. Though the study’s subjects were ogre-faced spiders, researchers suspect other net-casting spiders also rely on acoustic signals to hunt.
“There's an incredible world of behavioral adaptations out there among spiders, a loathed and feared group of animals,” says Ronald Hoy, who studies insect communication at Cornell University and co-authored the recent study. Researchers say this finding is an important step forward in understanding spidey-senses, which Hoy says are “overlooked and underestimated.”
Not all spiders follow a Charlotte’s Web-style strategy to get their meals. Jumping spiders stalk their prey like cats, pouncing on their insect meal. Net-casting spiders like the ogre-faced spider combine a sit-and-wait strategy with lightning-fast attacks. By day, they are stationary and resemble sticks or palm fronds. By night, the inch-long spiders come to life, building a special web of non-sticky silk that they use to suspend themselves. For the remainder of the night, they dangle with their gigantic eyes fixated on the ground. When they see a tasty insect scuttle below, they leap downward, trapping their prey with a sticky, spider-man-style net grasped between their front four legs.
Ogre-faced spiders also employ a second, more cryptic move to catch flying prey behind them. The spiders make two explosive movements, simultaneously backflipping from their dangling position and casting that net from their front legs to ensnare prey—all of which happens faster than an eyeblink.
To test the spiders’ hearing, researchers used electrodes to record how the arachnids’ brains responded to a range of sounds. Once they knew the frequencies that the spiders could hear, they wanted to see if the sounds would trigger a prey-capture behavior. When they played lower-frequency sounds to the spiders in the lab and in the field, the spiders cast their nets.
“They'll respond to those frequencies as if an insect is flying past and it's trying to snag a meal,” says Jay Stafstrom, who studies net-casting spiders at Cornell University and authored the study with Hoy. “Those frequencies match up really nicely with known wingbeat frequency patterns from prey items that these spiders are hawking out of the air.”
Ogre-faced spiders don’t have ears to hear, but they can detect flying prey with sensitive hairs and finely tuned sensors on their legs. Their tiny hairs can sense even subtle vibrations in the air, and their sensory organs, a series of slits in the spider’s leg joints, can detect minute strains on their exoskeleton. Previous research revealed jumping spiders can similarly pick up auditory signals with these special leg sensors, but this is the first time the ability has been documented in net-casting spiders.
When it comes to snagging airborne prey, ogre-faced spiders aren’t taking a shot in the dark. Stafstrom and Hoy found that the spider’s strikes were accurate and directional. Because their web is limited by the span of their front four legs, being able to direct their net is key to capturing a meal, says Stafstrom.
Whether the spiders rely solely on sight for forward strikes and use hearing for backward strikes is not yet clear, but Stafstrom says their research suggests that’s the case. In an earlier study, he temporarily blinded ogre-faced spiders and found they could no longer catch prey off the ground but were still able to snag airborne prey.
“I had no idea that the auditory signals were going to be so important in something like prey capture,” says Lisa Chamberland, who recently completed her PhD on net-casting spiders at the University of Vermont and was not involved in the study. Her research focuses on untangling why these spiders evolved such over-sized eyes, which are more light-sensitive than an owl’s.
The fact that net-casting spiders use hearing isn’t a complete shock, says Chamberland, as she recalls a quirk of working with net-casting spiders in the field. Scientists would make a low grunting sound as a trick to get the spiders to cast a net, which she says makes a lot of sense in light of this recent discovery.
Damian Elias, who studies jumping spiders at the University of California Berkeley and was not involved in the study, says many animals “hear” by picking up vibrations on the ground, called substrate-borne signals. This is closely related to how human ears pick up sound vibrations in the air. Like many spiders, ogre-faced spiders detect sound this way, but Elias says that spiders that use substrate-borne signals are often assumed to be deaf to airborne signals. “What this paper shows is that idea, and that assumption, is wrong,” says Elias.
The same sensors on the spider’s legs that detect vibrations on the ground also pick up vibrations in the air, which Elias says will shift how he and others think about spiders’ senses.
“It’s just pushing the boundaries of how we understand spider physiology and spider neurophysiology to work, and that's really exciting to see,” says Elias.
Next, Hoy and Stafstrom plan to investigate if ogre-faced spiders use their high-frequency hearing to pick up on predators. “Now we know that they can hear, but in opening up the world of hearing, we opened up yet another mystery,” says Hoy.