These spiders can hear – even if they don’t have an ear

Ogre-faced spiders, named for their massive eyes, hide during the day and hunt at night, swaying on Florida palm fronds and throwing silk nets at insects on the ground and in the air. In addition to their incredible night vision, these spiders can also hear their predators and prey, the researchers report in the journal. Current biology October 29, 2020. Having no ears, spiders use hair and joint receptors on their legs to pick up sounds at least 2 meters away. The results suggest that spiders can hear the low-frequency sounds of insect prey as well as the higher-frequency sounds of bird predators.

“I think a lot of spiders can actually hear, but everyone takes it for granted that spiders have a sticky web to catch prey, so they’re only good at picking up nearby vibrations,” says lead author Ron Hoy , professor of neurobiology and behavior at Cornell University. “Vibration detection works to detect tremor in the strip or the ground, but detecting these aerial disturbances from a distance is a matter of hearing, which we do and what spiders do too, but they do.” with specialized receptors, not eardrums.

This high-speed video shows the back strike of an ogre-faced spider. Credit: Sam Whitehead

Instead of passively waiting for prey to fall into a web and get stuck, Ogre-faced spiders use their webs as a weapon. After spending the day completely still, merging into the surrounding palm fronds, they emerge at night to swing close to the ground and throw their webs like a net over reckless insects. While they use their sharp night vision to catch prey on the ground, they can also catch bugs in the air by performing an elaborately choreographed back strike, which doesn’t seem to rely on vision.

“In a previous study, I put dental silicone on their eyes so they couldn’t see,” says first author Jay Stafstrom, postdoctoral researcher at Hoy Lab. “And I found out that when I put them back in the wild, they couldn’t catch prey from the ground, but they could still catch insects from the air. So I was pretty sure these spiders were using a different sensory system to hunt flying insects.

While this study hinted that spiders might be able to hear, this one showed just how well they can do it. By observing the spiders ‘reactions to different tones and measuring their neural response with electrodes placed in the spiders’ brains and legs, the team determined that the spiders could hear sounds up to 10 kHz in frequency, although louder than the sounds of a walk. or flying insect.

“When I was playing low frequencies, even at a distance, they would hit 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 it from a distance, knowing that we are not approaching and making them vibrate. It was the key to knowing that they can really hear.

Hearing these higher frequencies may not be helpful for hunting, but it can help them stay alert when hiding from their own predators.

“If you give an animal a threatening stimulus, we all know the fight or flight response. Invertebrates have it too, but the other “f” is “gel”. That’s what these spiders do, ”Hoy says. “They are in a cryptic posture. Their nervous system is in a state of sleep. But as soon as they pick up some kind of salient stimulus, boom, it activates the neuromuscular system. It is a selective attention system.

While these results clearly show that spiders can detect sounds well, researchers are next interested in testing their directional hearing – if they can tell where sounds are coming from. If they can hear directionally as well, that might help explain their acrobatic hunting style further.

“What I found really amazing is that in order to cast their net at the flying insects, they have to do a half backflip and spread their web at the same time, so they’re basically playing center field,” Hoy says. “Directional hearing is a big deal in any animal, but I think there are really going to be some interesting surprises from this spider.”

Reference: “Ogre Spiders Webcast Use Auditory Cues to Detect Airborne Preys” by Jay A. Stafstrom, Gil Menda, Eyal I. Nitzany, Eileen A. Hebets and Ronald R. Hoy, October 29, 2020, Current biology.
DOI: 10.1016 / j.cub.2020.09.048

This work was supported by the National Science Foundation