Update (March 29, 2022): The paper described in this article was published today in PNAS.
o hunt for alien life, human scientists build bigger and more sensitive arrays in the hopes of picking up a radio transmission from a faraway world. It turns out that Larinioides sclopetarius, also known as the bridge spider or gray cross spider, uses a similar trick. Instead of hunting for E.T., the spider can tune in to its surroundings and hear across great distances by treating its round, orb-shaped web like a comparatively giant acoustic array, according to new research.
The bridge spider uses its web as an engineered “external ear” up to 10,000 times the size of its body, according to a preprint study posted to bioRxiv on October 18. The discovery, which has not yet been peer reviewed, challenges many assumptions that scientists have held for years about how spiders and potentially other arthropods navigate and interact with the world around them.
“Evolutionarily speaking, spiders are just weird animals,” Jessica Petko, a Pennsylvania State University York biologist who didn’t work on the new study, writes in an email to The Scientist. “While it has been long known that spiders sense sound vibration with sensory hairs on their legs, this paper is the first to show that orb weaving spiders can amplify this sound by building specialized web structures.”
Spiders—both orb-weavers and others—are perfectly capable of hearing at closer distances without their webs thanks to the tiny hairs and organs on their legs that sense vibrations as air flows past. But the majority of spider biologists assumed that they could only hear sounds in their immediate vicinity, senior study author and Cornell University neurobiologist Ronald Hoy tells The Scientist.
Lead study author and Binghamton University mechanical engineer Ronald Miles had demonstrated that spider silk is sensitive to airflow across a wide range of frequencies in a 2017 study published in PNAS. Armed with the knowledge that strands of silk and the hairs on a spider’s legs could vibrate to the same frequencies of noise, Miles and his colleagues aimed to determine whether vibrations in one could be transmitted to the other.
First, the researchers had to ensure that the spiders were actually hearing through their webs and not directly through those sensory hairs. To do so, the scientists brought spiders into the lab and waited while they wove new webs on wooden frames. Once the spiders’ handiwork was complete, the scientists perturbed the web using a carefully-engineered and directed sound stimulus that hit the web but not the spider sitting in the center.
“That is one of the hardest parts: showing that they don’t have a little hidden ear on them that’s picking it up,” Miles tells The Scientist. “To do that, we created a sound source in the air that would propagate sound over a very short distance.”
Once they heard the signal, the spiders responded by crouching, flattening out, or otherwise giving a startled response, the study authors explain. Because the web is so much larger than the spider, the paper suggests this mechanism allows the spider to hear noises it would otherwise miss, such as birds or crickets from over ten meters away.
See “A Song of Spider Silk”
The discovery that the bridge spider uses its web as an external auditory sensor is fascinating, University of California, Davis, arachnologist Lisa Chamberland, who didn’t work on the study, tells The Scientist, because it’s such a drastic departure from what scientists previously assumed about a spider’s hearing.
“I think this opens up doors for some exciting research,” Chamberland says, adding that she hopes scientists will start “looking at the evolution of sound systems across spiders.”
Plenty of questions remain not only about why the spiders started using their webs as comparatively giant acoustic arrays, but what purpose it serves.
“While I am sure the authors had fun doing this research, it is not a trivial finding,” University of Cincinnati biological sciences professor George Uetz, who studies spider sensation but didn’t work on the new paper, tells The Scientist in an email. “I think it will have a large impact on arachnology and beyond. For example, people have often speculated that spiders might use airborne vibration from prey activity to determine an optimal web location, and this is an ecological question worth exploring.” Another question, he adds, is how differences in webs among species “could impact both the process of sensing and function in prey capture and communication, which is an important evolutionary question.” Finally, there’s more to learn about how spiders “tune” their webs to specific stimuli.
The researchers speculate that spiders may increase their hearing range to help dodge predators or track prey, but the spiders in this experiment primarily responded to the noise stimuli with apparent alarm and confusion. Down the road, the study authors say they hope others in the field will pick up where they left off by studying whether the noises given off by, for example, a hungry bird or a tasty insect elicit different behaviors. A further open question is whether other species of spider, orb-weaver and otherwise, use the same giant-ear trick. For example, Uetz wonders whether the wolf spiders that he studies, which don’t spin webs, could use other objects like leaves as external ears to help them detect predatory birds.
“It will be interesting to find out whether other web building spiders (like the ones that make messy cobwebs) use webs for a similar purpose or if this evolutionary marvel is restricted to orb weavers,” Petko writes.
However, Hoy and Miles suggest that external-ear behavior may not be restricted at all—they speculate that they’ve scratched the surface of a whole world of spider and insect sensation that could change our understanding of bioacoustics at a philosophical level.
“Most [biologists] assume that insects can sense sound only when it’s really near,” Hoy says. The study demonstrates, he says, “that it’s time to look again at animals that are hairy. A simple hair is a perfectly legitimate sound system because it’s sensing flow.”