Hearing is a fundamental sense of many animals, including all mammals, birds, some reptiles, amphibians, fish, and arthropods. The auditory organs of these animals are extremely diverse in anatomy after hundreds of millions of years of evolution, yet all are made up of cellular tissue and are morphologically part of the bodies of animals. New research shows that hearing in a species of orb-weaving spider called the bridge spider (Larinioides sclopetarius) is not constrained by the organism’s body but is extended through outsourcing hearing to its proteinaceous, self-manufactured orb web.
A spider orb web is an enormous, reconfigurable, regenerative, and highly sensitive acoustic antenna. Image credit: Zhou et al., doi: 10.1073/pnas.2122789119.
A single strand of spider silk is so thin and sensitive it can detect the movement of vibrating air particles that make up a sound wave.
This is different from how ear drums work, by sensing pressure from sound waves; spider silk detects sound from nanoscale air particles that become excited from sound waves.
“The individual silk strands are so thin that they’re essentially wafting with the air itself, jostled around by the local air molecules,” said Professor Ron Hoy, a researcher in the Department of Neurobiology and Behavior at Cornell University.
“Spiders can detect miniscule movements and vibrations via sensory organs in their tarsi — claws at the tips of their legs they use to grasp their webs.”
Orb weaver spiders are known to make large webs, creating a kind of acoustic antennae with a sound-sensitive surface area that is up to 10,000 times greater than the spider itself.
In the study, Professor Hoy and his colleagues used a special quiet room without vibrations or air flows.
They had an orb-weaver build a web inside a rectangular frame, so they could position it where they wanted.
The researchers began by putting a mini-speaker within millimeters of the web without actually touching it, where sound operates as a mechanical vibration.
They found the spider detected the mechanical vibration and moved in response.
They then placed a large speaker 3 m away on the other side of the room from the frame with the web and spider, beyond the range where mechanical vibration could affect the web. A laser vibrometer was able to show the vibrations of the web from excited air particles.
The authors then placed the speaker in different locations, to the right, left and center with respect to the frame.
They found that the spider not only detected the sound, it turned in the direction of the speaker when it was moved. Also, it behaved differently based on the volume, by crouching or flattening out.
“This finding opens new perspectives on animal extended cognition and hearing — the outsourcing and supersizing of auditory function in spiders,” they said.
“The study calls for reinvestigation of the remarkable evolutionary ecology and sensory ecology in spiders — one of the oldest land animals.”
“The sensory modality of outsourced hearing provides a unique model for studying extended and regenerative sensing and presents new design features for inspiring novel acoustic flow detectors.”
The results appear in the Proceedings of the National Academy of Sciences.
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Jian Zhou et al. 2022. Outsourced hearing in an orb-weaving spider that uses its web as an auditory sensor. PNAS 119 (14): e2122789119; doi: 10.1073/pnas.2122789119
Source link: https://www.sci.news/biology/acoustic-spider-web-10675.html
