While jumping spiders are known to have great vision, a new study proves for the first time that spiders can hear at a distance.
The discovery runs counter to standard textbook wisdom that claimed spiders could only detect nearby sounds.
A study published online in Current Biology describes how researchers used metal microelectrodes in a jumping spider’s poppy-seed-sized brain to show that auditory neurons can sense far-field sounds, at distances up to 3 meters, or about 600 spider body lengths.
In further tests, researchers stimulated sensitive long hairs on the spider’s legs and body—previously known to pick up near-field airflow and vibrations—which generated a response in the same neurons that fired after hearing distant sounds, providing evidence the hairs are likely detecting nanoscale air particles that become excited from a sound wave.
“We are the first and only lab that has successfully and fully been able to tap into what the spider’s brain is listening to,” says Ron Hoy, professor of neurobiology and behavior at Cornell University and the senior author of the study.
Squeaky chair fires neurons
Gil Menda, a postdoctoral researcher in Hoy’s lab, developed the technique for recording the jumping spider’s neural activity.
Because spiders’ bodies are under pressure, a cut causes the arachnid to quickly bleed and die. That rules out the standard technique for studying spider neurology: dissection. Instead, Menda’s method creates a very tiny hole that seals, like a self-sealing tire, around a hair-sized tungsten microelectrode. The electrodes record electrical spikes when neurons fire.
Menda was actually studying visual perception in jumping spiders when his chair squeaked, and neurons suddenly fired. The discovery led him to test reactions to different audio frequencies. He identified an area of the brain that integrates visual and audio stimuli. He also learned the spiders were sensitive to high frequencies and very specific low frequencies, at 90 Hz.
“All the team sat there together and we were thinking, ‘why are they so sensitive to those frequencies?'” Menda says. It turned out 90 Hz is near the same frequency as wing beats of parasitic wasps, the jumping spider’s biggest enemies, which provision their nests with jumping spiders for their young to feed on.
With the help of Ron Miles, professor of mechanical energy at Binghamton University, whose lab has a special quiet room without vibrations, Menda, Miles, and undergraduate coauthor Katherine Walden ran experiments with a special cage that allowed them to eliminate vibrations. They used high-speed video and recorded the spider’s behavior when exposed to pulses of sound.
“When we played 90 Hz, 80 percent of the spiders froze” for up to a second, before they turned and jumped, says Menda. By freezing, a classic behavior called a startle response for animals that hear, spiders assess the situation to avoid detection from wasps that scout for movement, before escaping.
Five other spiders, too
The technique opens up studies that link neurology with behavior in all spiders, Hoy says. Menda has since found evidence of hearing in five different spider species: jumping spiders, fishing spiders, wolf spiders, netcasting spiders, and house spiders.
The first evidence of far-field hearing in spiders came from Charles Walcott, Cornell professor emeritus of neurobiology and behavior, who in 1959 discovered house spiders could detect audio stimuli from a sensory organ called a lyriform in their legs. When German researchers tried to duplicate his experiments in another species of spider, however, they couldn’t, and they claimed Walcott’s findings were an “artifact,” Hoy says.
Future work by Hoy’s lab will investigate audio perception from lyriform organs and will better investigate audio neurons in the brain. The findings could have applications for using hairlike structures for extremely sensitive microphones, such as in hearing aids.
The National Institutes of Health and the National Science Foundation funded the work.
Source: Cornell University