Humans and other vertebrate species hear using eardrums that can turn soundwave pressure into signals to the brain. A new experimental study led by Binghamton University has investigated the auditory processing of orb-weaving spiders and found that these small creatures are using their webs as extended auditory rays to capture sounds. Such a procedure is possibly giving spiders advanced warning of incoming predators or prey.
It has long been known that spiders readily respond when something vibrates in their webs, such as potential pray. In these new experiments though, researchers discovered for the first time that spiders turned, crouched, or flattened out in response to sounds travelling through their web.
Spider silk is so thin and sensitive that it can easily detect the movements of vibrating air particles that make up soundwaves. “The spider is really a natural demonstration that this is a viable way to sense sound using viscous forces in the air on thin fibers,” said study senior author Ronald Miles, a professor of mechanical engineering at Binghamton. “If it works in nature, maybe we should have a closer look at it.”
According to professor Miles and his colleagues, spiders can detect minuscule movements and vibrations through sensory organs located on their tarsal claws at the tips of their legs, which they use to grasp their webs. Since orb-weaving spiders build large webs, they are creating “acoustic antennas” with a sound-sensitive area that is often 10,000 times larger than the spiders themselves.
To better understand spiders’ auditory mechanisms, the scientists used laser vibrometry to measure over one thousand locations on a spider’s web, while the spider was located in the center of the web. They placed a mini-speaker five centimeters away from the center of the web and two millimeters away from the web plane, allowing the sound to travel to the spider both through the air and through the web.
The results were amazing: while the soundwave from the mini-speaker died out significantly while travelling through the air, it propagated readily through the web with very little attenuation, providing evidence that the web moves with sound almost at maximum physical efficiency across an ultra-wide frequency range.
The researchers also found that, by stretching and crouching, spiders are changing the tension of the silk strands, tuning them to pick up different frequencies, and thus customizing their external auditory “organ” to hear different sorts of sound.
Further research is needed to investigate how spiders make use of the sounds they can detect using their web and to find whether other species of web-weaving spiders also use their silk to outsource their hearing.
The study is published in the journal Proceedings of the National Academy of Sciences.