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Leg hairs help ogre-faced spiders hear predators, prey

Ogre spiders, named for their big eyes, use hairs on their legs to hear both high- and low-frequency sounds from several feet away. Photo by Jay Stafstrom/Cornell University
Ogre spiders, named for their big eyes, use hairs on their legs to hear both high- and low-frequency sounds from several feet away. Photo by Jay Stafstrom/Cornell University

Oct. 29 (UPI) -- Spiders don't have ears, but they can "hear" by sensing vibrations through other parts of their bodies.

According to a new study published Thursday in the journal Current Biology, ogre spiders boast especially sensitive hairs and joint receptors on their legs.

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The hairs allow them to pick up low-frequency vibrations made by unsuspecting prey, as well as high-frequency vibrations made by wing-flapping predators, researchers say.

"I think many spiders can actually hear, but everybody takes it for granted that spiders have a sticky web to catch prey, so they're only good at detecting close vibrations," senior author Ron Hoy said in a news release.

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"Vibration detection works for sensing shaking of the web or ground, but detecting those airborne disturbances at a distance is the province of hearing, which is what we do and what spiders do too, but they do it with specialized receptors, not eardrums," said Hoy, professor of neurobiology and behavior at Cornell University.

Unlike most web-building spiders, ogre spiders -- named for their giant eyes -- don't deploy passive traps. Instead, these hairy, long-legged spiders use their silk as a weapon, casting net-like webs onto absentminded insects.

Often, ogre spiders cast their nets as they hang from the fronds of Florida's abundant palms, using their sharp eyes to spot hapless, earthbound prey. But the spiders can also perform a complexly choreographed backwards strike on airborne insects. The attack strategy seems to be unaided by vision.

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"In a previous study, I actually put dental silicone over their eyes so they couldn't see," said first author Jay Stafstrom.

"And I found that when I put them back out into nature, they couldn't catch prey from off the ground, but they could still catch insects from out of the air. So I was pretty sure these spiders were using a different sensory system to hunt flying insects," said Stafstrom, a postdoctoral researcher in the Hoy lab.

For the most recent study, researchers attached electrodes to the brains and legs of ogre spiders and subjected the arachnids to a variety of tones, measuring their neural reactions to different frequencies. The tests showed ogre spiders can hear sounds up to 10 kHz in frequency.

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"When I played low tone frequencies, even from a distance, they would strike like they were hunting an insect, which they don't do for higher frequencies," says Stafstrom. "And the fact that we were able to do that from a distance, knowing we're not getting up close and causing them to vibrate. That was key to knowing they can really hear."

The ability to hear high frequencies likely helps the spiders avoid being eaten. When subjected to high-frequency tones, the spiders didn't flee. Instead, they froze.

"They're in a cryptic posture," said Hoy. "Their nervous system is in a sleep state. But as soon as they pick up any kind of salient stimulus, boom, that turns on the neuromuscular system. It's a selective attention system."

The researchers are currently conducting followup tests to see if the ogre spider can hear directionally.

"What I found really amazing is that to cast their net at flying bugs they have to do a half backflip and spread their web at the same time, so they're essentially playing centerfield," said Hoy. "Directional hearing is a big deal in any animal, but I think there are really going to be some interesting surprises from this spider."

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