It all started with a squeaky chair and a bit of new technology. Gil Menda of Cornell University was helping his colleague, Paul Shamble, set up a visual experiment with spiders using a new method that can detect their neural activity. “We usually ask the spider a question, and the brain answers us,” said Menda of the technique, which allows researchers in the Hoy Lab at Cornell to measure how neurons inside spiders' brains respond to certain stimuli.
The setup was designed to make a popping sound whenever neurons fired — and when Menda moved his chair, he unexpectedly heard a long sound from his research rig. He asked Shamble to clap his hands from across the room.
They both ended up outside the recording room, listening as the speakers (and the spiders' neurons) fired again and again. A new experiment was born.
For years, scientists thought that jumping spiders and their kin couldn't really hear. Though they have acute visual systems that let them judge both distances and depth to hone in on prey and escape predators, they lack ears and eardrums. Instead, they hear using supersensitive sensory hairs that were thought to vibrate only in response to the movement of air molecules or shifting surfaces nearby.
The fact that the clap-happy spider could sense sound so far away triggered the researchers' own spidey sense. Could the creatures in fact hear not just faraway sounds, but different frequencies, too?
The researchers describe what happened next in an article published Thursday in the journal Current Biology. They put spiders in a recording arena engineered to have minimal acoustic reflections or echoes. Then they played sounds at different frequencies and recorded the spiders' responses.
When the team played sounds that matched the frequency of the beating wings of wasps — common jumping spider predators — the spiders froze, a typical response to such a threat. Further experiments tested neural feedback of spiders that were exposed to sounds at different frequencies. The team confirmed that the sensory hairs were causing the responses with tests that produced neural responses when a single sensory hair was shaken back and forth.
Remember how spiders were once thought to only hear close-by sounds? The new experiment blew that concept out of the water. Now, Menda and his colleagues think that jumping spiders can hear sounds up to three meters away — over 350 body lengths. And the sounds they seem designed to hear correspond to those produced by flying insects that could do them harm.
“This is real, and it's not only with jumping spiders,” said Menda, who's moved on to testing the same responses in other kinds of spiders. So far, at least five species seem to have the response — which makes sense, given that all spiders have sensory hairs. The other spiders Menda has studied are all sensitive to different frequencies, which opens up evolutionary questions about whether spiders' newly discovered hearing skills developed due to environmental threats.
The findings could change the ways in which researchers work with spiders in the future. Menda points out that until the new finding, researchers had no idea the sounds they made could influence spider experiments. Now, spider behaviorists will need to change their ways to avoid sullying their results with sound.
Studying spiders' sense of sound could help humans one day, too: Menda said that a better understanding of how spiders hear could lead to a better sense of how they evolved, inspire micro-robotics, and even help researchers make more sensitive microphones and hearing aids that mimic spiders' extraordinary senses.
Has spending time making spiders freeze with sound changed Menda's take on the eavesdropping eight-legged animals? Absolutely. “They can hear everything we do,” said Menda, who used to bring a wary eye to “Spider-Man” movie sessions with his kids. “I used to laugh at [the movies], because spiders can't hear sounds. Well, guess what? They can.” So next time you've got something to say about a spider, watch your words — they may not have ears, but they definitely can listen in.