A study recently published in the Proceedings of the National Academy of Sciences demonstrates for the first time that spiders can externalize hearing to their web . These animals, and more specifically orb-weaver spiders, use their webs as enlarged auditory arrays to pick up sounds, possibly giving them early warning of approaching prey or predators.
We know that both humans and most vertebrates hear thanks to the eardrums , which convert the pressure of sound waves into signals that are then encoded by the brain. However, it is not so clear what happens in smaller animals such as arthropods and insects. Can you hear? And if yes, how do they do it?
Spiders do not have hearing as such. They have been known to respond when something vibrates on their web, for example a potential victim. In the current experiment, the researchers show for the first time that spiders turn, crouch or squat in response to sounds in the air.
The threads that make up the cobwebs are extremely fine and sensitive, so much so that they are capable of detecting the movement of air particles that vibrate and that make up a sound wave, which is different from the way the eardrums work.
Spiders can detect tiny movements and vibrations through the sensory organs of their tarsal claws , which are located at the tips of their legs and which they use to grasp their webs. Orb-weaver spiders are known for making large webs, creating acoustic antennae with a sound-sensitive surface area up to 10,000 times larger than the spider itself.
In the study, the researchers used the anechoic chamber at Binghamton University in New York, a completely soundproof room located in the Innovative Technologies Complex. Collecting orbital spiders from windows around campus, they had them weave a web into a rectangular frame so they could place it wherever they wanted.
The team began by using a pure-tone sound from 3 meters away at different sound levels to see whether or not the spiders responded. Surprisingly, they found that spiders can respond to sound levels as low as 68 decibels. In the case of a louder sound, they found even more types of behaviors.
They then placed the sound source at a 45-degree angle, to see if the spiders behaved differently. They found that the spiders not only locate the sound source, but can distinguish the direction of sound entry with 100% accuracy.
To better understand the spiders’ hearing mechanism, the researchers used laser vibrometry and measured more than a thousand locations on a natural spider web, with the spider sitting in the center under the sound field. The result showed that the spider web moves with sound with almost maximum physical efficiency over a very wide frequency range.
“Of course, the real question is if the web moves like this, does the spider hear with it?” said Ron Miles, a member of the Department of Mechanical Engineering in the Thomas J. Watson School of Engineering and Applied Sciences at the University of Washington. Binghamton. Miles has been researching insect and arthropod hearing for decades with the goal of revolutionizing microphone technology. “That’s a difficult question to answer,” he says.
“There might even be a hidden ear inside the spider’s body that we don’t know about,” said Junpeng Lai, another of the researchers involved in the study.
So the team placed a mini-speaker 5 centimeters from the center of the web, where the spider sits, and 2 millimeters from the plane of the web — close to, but not touching, the web. This allows the sound to reach the spider both through the air and through the web. The researchers found that the sound wave from the mini-speaker was significantly dampened as it traveled through the air, but easily propagated through the web with little attenuation. The sound level was still around 68 decibels when it reached the spider. Behavioral data showed that four of the 12 spiders responded to this web-borne signal. Those reactions showed that the spiders could hear through the webs.
The researchers also discovered that, by bending and stretching, the spiders may be changing the tension of the silk strands, fine-tuning them to pick up different frequencies . By using this external structure to hear, the spider might be able to customize it to hear different kinds of sounds.
Future experiments could investigate how spiders take advantage of the sound they can detect with their web. In addition, the team wants to see if other types of web-spinning spiders also use their silk to externalize their hearing.
“It’s reasonable to assume that a similar spider on a similar web would respond in a similar way,” said Ron Miles. “But we can’t draw any conclusions on that, as we have tested a certain type of spider that turns out to be quite common.”
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Reference:
Zhou et al. 2022. Outsourced hearing in an orb-weaving spider that uses its web as an auditory sensor. Proceedings of the National Academy of Sciences. DOI: 10.1073/pnas.2122789119
