All known animals hear using an organ in their body, with the notable exception of a curious spider. We found that the orb-weaving spider breaks this limitation by outsourcing its hearing to its gigantic, reconfigurable, and regenerative web.
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published on Feb 10, 2023
Spiders are among the oldest and most successful land predators, with a fossil record much earlier than dinosaurs. All spiders are preeminent craftsmen, producing silks that can be even stronger than steel in strength-to-weight ratio. When woven into a web, the animal architecture functions as a huge aerial trapping net. Once a prey wanders onto the web, the spider immediately detects its presentence by sensing the small vibrations in the web using its highly sensitive legs and then quickly runs towards the prey to capture it. It turns out that the spider can even sense much smaller disturbances in its web — even the movements of surrounding air particles set into motion by a distant sound source.
In earlier research we discovered that a single strand of spider silk, owing to its thin diameter and low weight, can closely capture sound in a wide range of frequencies from infrasound to ultrasound more efficiently than any known eardrum.
Does the spider hear using its web? We performed a series of experiments on L. sclopetarius, a common species of orb-weaving spider featured in E.B. White’s “Charlotte’s Web”. By making the spider build a web inside a rectangular frame, we were able to observe the spider’s reactions to sound played from a loudspeaker 3 meters away. Spiders reacted with different movements depending on the volume and direction of the sound. For example, in response to an 88 decibels loud tone measured at the spider’s location (roughly as loud as a hairdryer), spiders responded with different behaviors including crouching, stretching, foreleg displaying, and body turning. For quieter sounds at 68 decibels, the spider’s only response was to crouch. When placing the loudspeaker in different locations to the left or right side of the spider at half a meter away, we found that the spider could localize the direction of the speaker accurately by rapidly turning toward the speaker.
To better understand the hearing mechanism, we measured the web’s response to sound using laser Doppler velocimetry, which works under the same principle as the radar speed guns used by police. After measuring the motion of a web over one thousand locations, we found that the entire web could efficiently move with sound, just like a single strand of spider silk. We repeated the motion measurement (using fewer locations) on 12 different webs and found the same result, strengthening the web’s potential as an ‘acoustic antenna’.
The most challenging part of our experiments is to determine whether the spider really hears by detecting vibrations of the web itself. Although the web strongly responds to sound, it was still possible that the spider was hearing the sound directly using its body. For example, our previous studies have shown that jumping spiders can hear using the sensory hairs on their bodies. To address this, we designed a behavioral experiment using 12 spiders. We stimulated the web with a precisely localized sound source focused on the web, by aligning a millimeter-size speaker as near as possible to the web without touching it, 5 centimeters away from the spider resting in the center of the web. The localized beam of sound made the web vibrate quietly, reaching the spider at an equivalent sound level of less than 68 decibels. It turned out that spiders responded to these minute web vibrations by crouching, just as they responded to 68 decibel sounds emitted from a loudspeaker at 3 meters away. This confirms the spider’s ability to hear solely by detecting web movements. With a hearing threshold lower than 68 decibels, spiders should be able to detect prey and predators such as crickets and birds at a distance more than 10 m away, long before they touch the web.
This outsourcing of auditory function to the web opens new perspectives on the extended cognition and hearing of animals. By eliminating the constraint imposed by body size, the sensory surface area in the orb-weaving spider is up-scaled enormously, up to 10,000 times greater than the surface area of the spider itself, much as a radio telescope can sense electromagnetic signals from space. The spider also enables the flexibility to functionally adjust and regularly regenerate the web antenna according to its needs.
Moving forward, this work calls for reinvestigation of the remarkable evolutionary ecology and sensory ecology in spiders. The design features of the web antenna also present bio-inspiration to develop new ways to detect sound.
Zhou, J., Lai, J., Menda, G., Stafstrom, J. A., Miles, C. I., Hoy, R. R., & Miles, R. N. (2022). Outsourced hearing in an orb-weaving spider that uses its web as an auditory sensor. Proceedings of the National Academy of Sciences, 119(14). https://doi.org/10.1073/pnas.2122789119