Orb-Weaving Spiders Use Their Webs to Detect Sound

(Photo: Paul Harrison/Wikimedia Commons) Spiderwebs’ primary purpose may be to capture unsuspecting prey, but they have a second important function: to act as eardrums for the spiders that build them. A new study reveals that some types of spiders use their webs to pick up soundwaves, thus determining how they react to their environment.

The finding was born from an unexpected dilemma: Ron Miles, a mechanical engineer, has dedicated a significant portion of his career to figuring out how to build more effective microphones. To do this, he and his team regularly study animals without eardrums, which provide key insights into how air movement and pressure changes are perceived together. This particular part of Miles’ research involved bridge spiders, a type of orb-weaving spider which are not known to have ears at all.

Miles and his colleagues at Binghamton University and Cornell University gave 60 bridge spiders their own 12 square inch containers in which to construct webs. Once the webs were built, the researchers took individual spider and web pairs to a soundproof room equipped with echo-reducing foam. Then they directed speakers at the spiders’ webs and played tones mimicking various predators and prey, all the while testing different angles and distances from which the sounds came. 

(Photo: Germán Kaser/Unsplash)

The spiders responded by flattening their bodies, turning to the side, or extending their front legs. They also turned toward the speaker when it played sounds from the spider’s side. Even sounds that were undetectable to the researchers themselves (and came from a tiny, coin-sized speaker) elicited action from the spiders, who moved strands of silk in response.

According to the researchers, this reveals that orb-weaving spiders (the type you’ll typically find weaving spiral-shaped webs in your garden) “outsource” their hearing to their aerodynamic silk webs. A single strand of silk was shown to be capable of moving with a velocity virtually identical to that of the surrounding air, allowing the spiders to weave webs that could function as “huge acoustic antennae.” 

“By outsourcing its acoustic sensors to its web, the spider is released from body size constraints and permits the araneid spider to increase its sound-sensitive surface area enormously, up to 10,000 times greater than the spider itself,” the study reads. It was published in Proceedings of the National Academy of Sciences. “The spider also enables the flexibility to functionally adjust and regularly regenerate its external ‘eardrum’ according to its needs.”

Because microphones are traditionally made to sense only pressure changes, the insight gleaned during Miles’ team’s study may be used to design microphones that pick up on sensing the flow of air in a given sound field. 

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