If you haven’t been scared away by the thought of spiders jumping, you’re in for a treat of fascinating biology. Jumping spiders have a new entry in their list of terrifying skills, which includes living a vagabond life without a web, pouncing on prey, and dancing to court the opposite sex. Paul Shamble and other members of the Hoy Lab at Cornell University has recently discovered that a species of jumping spiders, originally considered deaf and dependent highly on vision, can actually hear from far distances of 3 meters or greater!
This discovery was a serendipitous one. While studying the spiders’ neural responses to visual stimuli, the scientists noticed that thumping their chairs led to heightened activity in some neurons in their brains. This observation was striking because jumping spiders were thought to be deaf. Jumping spiders lack tympanic membranes like those in human ears. Previously, scientists thought that tympanic membranes were necessary for insects and spiders to hear at distances as far away as the chairs in the laboratory. With the new observation, the scientists guessed that jumping spiders actually can hear airborne acoustic stimuli at far-field distances (i.e. further than one wavelength of the sound; around 3 meters for a 100 Hz tone). How can they detect sound without ears? The scientists hypothesized that they actually hear through hairs on their legs.
Do jumping spiders care about acoustic stimuli at all? If airborne sounds don’t influence their behaviors at all, studying their ability to hear might be pointless. The scientists performed a very simple experiment to answer this question: they put the jumping spiders in a chamber and observed how speaker-generated tones affected their locomotion. Almost every time a tone of 80 Hz (around the lowest note an adult human man can sing) or greater was played, the spiders froze as if they were trying not to be seen.
Having confirmed that spiders do respond behaviorally to sound, the scientists honed in on what the neurons did in response to sound. Because spiders have highly positive internal body pressure, carelessly sticking in an electrode pops their bodies like balloons. Fortunately, Hoy Lab has developed a highly specialized technique involving gluing the spider onto a 3D-printed, spider-shaped mold, carefully carving a tiny hole on their heads, and inserting a thin tungsten electrode into their brains.
With an electrode in the spider’s brain after the painstaking procedure, the scientists could ask whether neurons respond specifically to airborne acoustic signals and whether they prefer certain sounds more than others. By playing speaker-generated tones at a range of frequencies and amplitudes, the scientists found that some neurons near the arcuate body, a brain area of multisensory integration, fired in response to sounds. Furthermore, they found that the neurons, despite some variability, generally had their peak sensitivity around 80-100 Hz. Matching their observation with the chairs, the neurons responded to the sounds despite the sound source being 3 meters away, which is considered “far-field”. Interestingly, the 80-100 Hz range that they hear most sensitively is exactly the wing-beat frequency of a kind of wasps that preys on the jumping spiders!
Hold on a moment. Could it be possible that the spiders still are deaf to airborne sounds but that they might be picking up vibrations of the ground induced by the airborne sounds? To test this possibility, the scientists repeated the experiment in an anechoic chamber, whose walls and floors almost completely absorb all sounds. Even here, jumping spiders and their neurons responded to airborne sounds.
Even though no equivalent to human eardrums have been found in jumping spiders, they do have hairs that can sense vibrations. The scientists thus sought to test whether the neural responses to airborne sounds could be replicated by physical stimulation of a hair on the spiders’ foreleg. They stimulated the hair with a rod that could vibrate precisely at defined frequencies. Using this method, they could compare whether the neural responses to airborne stimuli and physical stimuli of same frequencies matched. They found that, indeed, the neural responses were similar, suggesting that the leg hair is the jumping spiders’ sensory organ for airborne acoustic stimuli. This was especially surprising because, as mentioned earlier, no insects or spiders that use hair to hear had been observed to hear at far-field distances!
If indeed the leg hairs are they primary sensory devices for detecting airborne sounds, removing those hairs should significantly curb the neural responses. When this additional experiment is performed, the results will solidify the Hoy Lab’s claim that the hairs are the acoustic sensory organs by showing that the hairs are not only sufficient but necessary to induce the sound-driven neural responses.
In this work, Hoy Lab extended a serendipitous observation to show that jumping spiders can hear airborne sounds through their leg hairs and that they can hear at far-field distances. They carefully isolated their stimuli to be airborne, so that their findings were not confounded by a flaw in experimental design. Most importantly, they challenged the preconceived notions that jumping spiders do not hear and that hearing through hairs in arthropods is limited to near-field range. While arachnophobes may be terrified to hear that jumping spiders might hear over long distances using their hairy legs, biologists can look forward to learning more about how insects navigate the natural world!
For more, including videos of the spiders in action, check out the video abstract of this paper.
Shamble, Paul S., et al. "Airborne acoustic perception by a jumping spider." Current Biology 26.21 (2016): 2913-2920.