Ogre-faced spiders (Dinopsis spinosus) have massive eyes that grant them phenomenal night vision to hunt prey in the dark with their silk nets. It seems these skilled predators have yet another ace up their eight legs: very sensitive hearing.
Although they don’t have ears, these spiders have tiny hairs and join receptors on their legs that are sensitive enough to pick up sounds from at least 2 meters away, according to researchers at Cornell University.
“These spiders are wonderful creatures that have some fascinating behaviors, allowed by fine-tuned sensory systems very unlike our own. Sadly, these spiders are very overlooked, especially considering how impressive their sensory systems and behavior are. We’re hoping to establish a foundation for future work on these and other spiders in the realm of sensory ecology,” Jay Stafstrom, first author of the new study and a postdoc at Cornell University, told ZME Science.
The stick-like nocturnal predators are part of the net-casting family of spiders. Instead of waiting passively for prey to fall into a permanent web that they weaved earlier, ogre-faced spiders and other Dinopsis spiders throw a rectangular-shaped net over unsuspecting victims that pass by, sort of like gladiators.
In order to ensnare their prey, ogre-faced spiders are aided by their googly-looking eyes. But besides keen night vision, these spiders also perform elaborate backward strikes to catch their prey, which suggests that they have another sophisticated spidey sense.
Previously, Stafstrom and colleagues blindfolded ogre-faced spiders by placing dental silicone over their eyes. The blind spiders weren’t able to catch prey off the ground, but they could still hunt insects from out of the air. This was a huge hint that they employed other sensory systems in hunting besides vision.
For their new study, the researchers observed the spiders’ reactions to various tones by measuring their neural response with tiny electrodes placed in the spiders’ brains and legs. They found that the spiders could sense vibrations in the air of up to 10 kHz, much higher in frequency than the sounds produced by walking or flying insects.
Stafstrom says that it’s difficult to compare the ogre-faced spider’s hearing with other animals, but he finds it “impressive that they can hear so well, at least in terms of speed and direction.”
“If you consider trying to catch an insect as it’s flying past you, in nearly complete darkness (as they are doing this at night), the act of snatching something with a small net would seem a pretty difficult feat. We suspect that hearing is fairly widespread across other spiders, but we haven’t been able to conduct the appropriate comparative study to really investigate how widespread this phenomenon is. The two types of sensors shown to detect sound in spiders (long hairs and the metatarsal organs) are possessed by most, if not all spiders,” the researcher told ZME Science.
Since the spiders would only need to detect low-frequency tones to snatch prey, the researchers believe that their ability to detect much higher frequency may be helpful in staying alert for signs of their own predators.
“If you give an animal a threatening stimulus, we all know about the fight or flight response. Invertebrates have that too, but the other ‘f’ is ‘freeze.’ That’s what these spiders do,” says senior author Ron Hoy, professor of neurobiology and behavior at Cornell University. “They’re in a cryptic posture. 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.”
In the future, the researchers are interested in learning whether the spiders have directional hearing too, meaning whether they can tell where sounds are coming from. This would explain their impressive choreography while hunting and perhaps inspire a new generation of microphones.
“These spiders have evolved for millions of years to be really good at snatching things up with this net, and the sensory systems they possess are exquisite at allowing them to do so. Since these spiders have been adapting for so long to detect both visual and hearing information so well, it would be beneficial, as humans, to better understand how they do it. If we could truly understand how they detect and process environmental information, we would surely have some valuable insight that could be applied to creating more sensitive/accurate biosensors (like microphones) in ways that we have never imagined before. Looking at the ability of these spiders to detect the directional component of a sound is what we’re most interested in next – we expect these spiders are quite adept at determining the location of a sound source, and we’re interested in understanding exactly how good they are at it,” Stafstrom said.
The findings appeared in the journal Current Biology.