Some spiders wait for prey and tickles to come into their net. But the ogre-faced spider (Deinopis spinosa) uses its sense of hearing to bring its net to the prey.
Hanging upside down, the spider weaves a rectangular web between its legs. When an insect flies behind the hanging arachnid, the spider sways backwards, launching the web toward the prey. This back-hunting technique is a clue that spiders can hear an unexpectedly wide range of sounds, according to researchers online on Oct. 29 in Current Biology.
“A couple of years ago, we didn’t really have a great idea that spiders could hear,” says Jay Stafstrom, a sensory ecologist at Cornell University. But now, he and his colleagues have analyzed several species of spiders and most can hear the use of specialized organs in the legs, according to him. This includes jumping spiders, which respond to low frequencies (SN: 15/10/16). Surprisingly, ogre-faced spiders can also hear fairly high frequencies, Stafstrom says.
Stafstrom and colleagues inserted microelectrodes into the brains of 13 ogre-faced spiders and then reproduced tones of varying frequencies from a speaker while monitoring the spider’s auditory nerve cell activity. Activity peaks revealed that spiders can hear airborne sounds between 100 and 10,000 Hz, though not at all frequencies, the team found. (Humans generally hear between 20 Hz and 20,000 Hz.)
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Nerve cells in amputated spider legs – where the sensory cleft is located, the organ that responds to sound vibrations – have also responded to the wide range of frequencies. This finding confirms that spiders listen with their legs, researchers say.
The team wondered how the spiders would respond when they heard sounds of varying frequencies in the wild. Thus, the scientists took the speaker away from the natural range of spiders in Gainesville, Florida, and found 25 of the pending hunters waiting prey in the dark. Of these, 13 reacted at frequencies of 150, 400 or 750 Hz. And everyone reacted the same way, with a blind, delayed strike.
“Obviously they can pick things up from the air just by using sound,” Stafstrom says. And because spiders attack only at low frequencies, they’re probably using the lower end of their hearing to hear prey and hunting. As for the higher frequency range, “they don’t seem to use it in a food search context,” he says.
An ogre-faced spider hangs from its net as it waits for prey in the dark. When he hears the prey flying on his back, the spider swings backwards and, like a fisherman throwing a net, throws a rectangular web at his food.
Still, the fact that spiders can detect higher frequencies means these sounds are likely to be important to them, says Jayne Yack, a neuroethologist at Carleton University in Ottawa who did not participate in the research. Spiders can use their sense of hearing for a number of things, including listening to predators, she says.
In fact, those higher frequencies fall into the same range of sounds that predators, including birds, produce when they move or call, so it makes sense for spiders to hear those frequencies, says Damian Elias, a biologist at the University of California, Berkeley who did not participate in the study. However, the tricky part is detecting a behavioral response to those higher sounds. Unlike the sling in the net, the reaction when listening to a predator can be simply to be hidden.