Spiders don’t have ears, so they build them out of webs

If a tree falls in the woods and there are only spiders around, can they hear it? That’s the question Ronald Miles from the Department of Mechanical Engineering at Binghamton University, and colleagues, set out to answer.

Sound is nothing more than our brain’s interpretation of vibrations in the environment around us. In humans those vibrations are picked up by the eardrum and converted to electrical signals which the brain can understand. Some animals use other auditory processes, like the ear stones inside the bodies of fishes. Insects and spiders hear using fine hairs on the surface of their bodies. Meanwhile, other species appear to have no auditory organs or systems at all. Cue the humble jellyfish, there's just nothing at all going on in there.

Regardless of how an animal goes about hearing the world around it, the sense is typically limited to the physical components of the body. However, according to a recent study published in the journal Proceedings of the National Academy of Sciences, spiders might have leveled up their sense of sound by outsourcing it to their webs.

"We found that spider webs respond to sound really well. If spider webs respond to sound, maybe the spiders can hear it, because they can detect the motion of the web with their feet," Miles told SYFY WIRE.

Strands of a spider’s web react to sound waves in the air in much the same way as your eardrum. If a spider can take in and interpret those changes in pressure and velocity, then a web would act as a gigantic antenna for capturing sound. That stimuli could then be delivered from the webbing to the spider’s legs. Spiders may not have eardrums, but it appears they are capable of building them at a truly massive scale.

For the study, researchers used Larinioides sclopetarius, a common species of orb-weaving spider made popular within the pages of E.B. White’s Charlotte’s Web. The spiders were gathered on Binghamton's campus and brought into the lab where they built webs inside wooden frames which could be directionally manipulated with respect to a sound source.

The team pointed speakers at the spiders either face on or at 45 degrees to the plane of the web. The spiders were video recorded at 60 frames per second for 10 seconds split evenly before and after sound initiation, in order to capture both a silent control period and any response.

"We played all kinds of sounds. We played insect sounds, birds chirping, natural sounds. Mainly, though, the data for the behavioral experiments were gathered using simple, pure tones," Miles said.

While responses varied across individual spiders and according to the frequency and volume of the sound source, scientists did note a response which differed from the inaction observed during the silent controls. Experiments uncovered that spiders behaved in directed, intentional ways depending on the type of sound they were presented with. For instance, at 88 decibels spiders crouched, stretched, or flattened their bodies resulting in a change to the tensile nature of the web. Scientists speculate this might be a way for spiders to tune their web antenna to better capture certain sounds.

Spiders were also seen turning their bodies toward the source of the sound, indicating that not only are spiders capable of picking up sound from the web, but that they can get some directional information. This was uncovered during the 45-degree tests. In those cases, sound doesn’t hit the whole web all at once, instead striking different portions of the web at different times. The behavior of the spiders provides evidence that they are capable of picking up on those subtle cues.

It was possible, of course, that spiders were picking up sound cues directly from the sensory hairs on their bodies and were not actually using the web as an outsourced acoustic instrument. So, the team designed an experiment to find out for sure.

"The hairs are really fine and thin and are driven by viscous forces in the air. At small size scales, the air acts like a fluid. Spiders hear the motion of the air back and forth because it drives those hairs. It's the same sound, it's just detected in a really different way," Miles said.

Using a small speaker, only millimeters in size, scientists initiated a sound close to the web without touching it. The sound waves were subtle enough that they would interact with the edge of the web but would decay before they reached the spider perched in the center. The spiders reacted with the same sorts of body motions they'd seen in the previous tests, turning toward the sound source or tensing the web to better collect information.

Having the ability to create a sound capturing instrument out of webbing could give spiders the ability to tune out background noises which are of little importance to them and focus in on only the types of sounds which are useful. It also means if their antenna is damaged, they can simply build a new one, something we don’t have the luxury of when our hearing degrades. Mirroring the function of the web could also give us a new way to engineer sound capturing devices for human use.

"We're really arrogant animals and we think about how we're made and think that's the only way to make things. When we design something, we're thinking about ourselves and how our ears work. Our ears do incredible things but it isn't necessarily true that it's the best way," Miles said.

It’s no wonder Peter Parker had an early warning system built-in to his power set. Whether he knew it or not, he had the benefit of enhanced sense of sound perfected over hundreds of millions of years of evolution, at least when he was web slinging.