Slingshot spiders rely on good vibrations to catch supper

Entomologist Sarah Han has always been into spiders.

"I grew up in California and there are a lot of black widows," says Han. "I would keep them as pets. One day, one of them escaped. That's a mistake you only make once."

The black widow proceeded to build its characteristic cobweb right under its cage. Han noticed the messy upper part where the spider hung out and an array of vertical threads glued to the ground. "When an insect walks by," explains Han, "it will dislodge these trap lines and get pulled into the air a little bit, which is really cool."

The black widows were an early demonstration of a fundamental principle about the spider world. "There's a huge diversity of webs and the ways that spiders use these webs to try to catch prey," she says.

In research published in the Journal of Experimental Biology, Han and her colleague showcase another instance of that diversity. Scientists knew that the tiny ray spider can fling its web to snag prey out of the air rather than waiting for insects to fly into the silky strands. But Han deduced the spider launches its web in response to the vibrations of airborne insect wings.

A silky slingshot

The ray spider is smaller than a grain of rice. It spins its web as a classic set of concentric "circles" with spokes radiating outwards. But then, the spider strings a thread from the center of the web to a nearby rock or twig. It then grabs the middle of the web with its back legs and then pulls itself along that tension line with its front legs.

"That is what turns that web from a flat shape into a cone shape," says Han. The conical web is now spring loaded. When an insect flutters by, the spider releases the tension line, flinging the web forward and entangling the prey.

"So they're not just passively sitting there," Han says. "They're using the web like a slingshot without the insect ever touching the web."

This is unusual, and Han wanted to know how the ray spider figures out when to sling its web.

During her Ph.D. at the University of Akron, she collected ray spiders from the local parks. "I would just go around peering into cracks and crevices," she recalls. "I would see that tiny little spider. Then I would grab it."

Han also grabbed a bunch of spider prey. "I was just standing around in various places waiting for mosquitos to land on me and then I would catch them," she says.

Back in the lab, she glued those mosquitos to little paper strips. "We started calling them mosquito lollipops because it was just like this mosquito flying around at the end of the stick," she says. "So I would then slowly move it in towards the spider," all while recording everything using high speed video.

Three out of four times when Han moved the tethered mosquito towards the front of the capture cone, the spider launched its web at an acceleration up to 51 g's.

"It's just like instantaneous practically — a fraction of a fraction of a second," says Han. An insect "would never really see it coming."

Han then tried the same experiment with a tuning fork that vibrated at a frequency akin to an insect beating its wings — just much stronger. The ray spiders flung their webs when the tuning fork was farther away than the mosquitos had been.

Han concluded that ray spiders release their webs in response to airborne prey vibrations, determining both the direction and distance to those vibrations to capture the insect at the right moment.

A little inspiration

"There still are a lot of questions in exactly what's going on," says Han. For instance, she isn't sure how the spider detects the vibrations.

"The vibrations are hitting the silk," she says. "They're also probably stimulating the spider's body, its sensory hairs. So through one of those or some combination, the spider becomes aware that this insect is approaching the web."

Han also can't be sure how the spider deduces when an insect is close enough to the web to capture it.

Nevertheless, she says this is the first time to her knowledge that scientists have documented spiders activating their whole webs to attack prey before they hit those sticky filaments.

"Spiders may be using their webs as greater sensory devices than we previously thought," suggests Han. "The web is kind of like the spider's ear."

"It's exciting to see it in print now because we've been talking about [the idea] for years," says Symone Alexander, a chemical engineer at Auburn University who wasn't involved in the research. She says that spiders are masters of designing webs that allow them to detect stress and strain precisely.

"These spiders — the geometry of their web is slightly different and it's tensed in a different way," Alexander says. "Can we use that as inspiration for building these sensing systems in airplane wings or other materials?"

In other words, Alexander is hoping that scientists can continue to invite the spider-verse to improve the human-verse.