Spiders may not have ears, but they can still hear you talking about them.
According to a new study, spiders can hear and respond to sounds more than 3 meters (10 feet) away. That would be impressive for any animal their size, but this spider sense is especially remarkable given the the arachnids' absence of ears.
In lieu of ears, spiders feel the vibrations of sound waves. Scientists already knew spiders can detect sound this way, but until now, the prevailing wisdom suggested they could only hear within very short distances. Thanks to an accidental discovery by researchers at Cornell University, however, we now know spiders have much better hearing than we thought — even letting them listen to people from across a room.
"The standard textbooks say that spiders are acutely sensitive to airborne vibrations from nearby sources, sounds about a body length or a few [centimeters] away," study co-author Gil Menda says in a press release. "We have discovered that jumping spiders can hear things from much farther away than this. Interestingly, it seems that in both cases, this 'hearing' is accomplished by sensory hairs."
Tiny hairs, like those of this North American bronze jumping spider, help the arachnids hear. (Photo: Wikimedia Commons)
Menda and his colleagues discovered this by accident while studying vision in jumping spiders, which are known for having excellent eyesight. They were using a new technique that Menda developed for recording neural activity in spiders' poppyseed-sized brains, a process that traditionally requires dissection.
That older method killed the spiders, the researchers note, since the arachnids' pressurized bodies are highly vulnerable to incisions. In the new method, however, Menda creates a tiny hole that seals like a self-sealing tire around a hair-sized tungsten microelectrode. This electrode can then record electrical spikes when neurons fire inside the living spider's brain.
"One day, Gil was setting up one of these experiments and started recording from an area deeper in the brain than we usually focused on," explains Cornell arachnologist Paul Shamble. "As he moved away from the spider, his chair squeaked across the floor of the lab. The way we do neural recordings, we set up a speaker so that you can hear when neurons fire — they make this really distinct 'pop' sound — and when Gil's chair squeaked, the neuron we were recording from started popping. He did it again, and the neuron fired again."
That had to mean the spider heard Menda's chair squeak. Intrigued, the researchers began testing how far away the spider could hear them.
"Paul clapped his hands close to the spider and the neuron fired, as expected," Menda says. "He then backed up a bit and clapped again, and again the neuron fired. Soon, we were standing outside the recording room, about 3-5 meters from the spider, laughing together, as the neuron continued to respond to our clapping."
Sound wasn't the only stimulus that got a response from these neurons, though: They fired in a similar way when Menda and Shamble shook individual sensory hairs on the spiders' bodies. That suggests the spiders "hear" with these hairs, which can feel the subtle effects of sound waves on particles in the air.
Menda identified an area of the spider brain that integrates visual and auditory input, and realized the arachnids were sensitive to frequencies around 90 hertz (Hz). That was a mystery at first, until a colleague pointed out 90 Hz is nearly the same frequency as the wingbeats of parasitic wasps that prey on jumping spiders. These wasps capture spiders and feed them to their babies, so the spiders have a clear evolutionary reason to listen out for their telltale sound.
"When we played 90 Hz, 80 percent of the spiders froze," Menda says. The spiders held still for up to a second — a normal behavior in animals that can hear, known as "startle response," that helps them hide from predators that scan for movement.
Here's a video of the spiders reacting to the sounds:
While the study initially focused on jumping spiders, most spider species have these hairs, so long-distance hearing is probably widespread. And follow-up experiments also revealed evidence of hearing in four other types of arachnids: fishing spiders, wolf spiders, net-casting spiders and house spiders.
This could shed light on how spiders' behavior is controlled by their brains, and thus inform the way researchers design experiments involving spiders. It could also have practical uses for people, the researchers add, such as inspiring hairlike structures for highly sensitive microphones in small robots, hearing aids or other devices.
It may be unnerving to know spiders can hear us, but there's no need to worry. Spiders don't want trouble from humans, and they have better things to do than eavesdrop on us, anyway. But just in case they are listening, it couldn't hurt to thank them once in a while for eating pests like roaches, earwigs, flies and mosquitoes.