Ogre-faced spiders listen with their legs

Abstract

The giant eyes on Deinopis spinosa spiders have earned them the apt if insensitive moniker ‘ogre-faced’ and are responsible for their remarkable ability to catch insects using nets spun between their forelegs. Sometimes, however, these spiders snag out-of-sight meals by quickly lunging backwards and capturing whoever was trying to sneak behind them unnoticed. A team of scientists from Cornell University, USA, looked past the mesmerizing eyes of these ogre-faced hunters to determine how they catch flying insects that they can't see. The researchers found that despite a lack of ears, the spiders hear sound through their legs and certain sounds prompt a hunting spider to strike behind itself.To determine that the spiders detect sound, the researchers measured brain activity in response to tones with frequencies ranging from 100 to 1000 Hz. The spiders’ brains responded more to tones from 150–200 Hz, 400–450 Hz and 700–750 Hz, indicating that they can hear over those ranges. Notably, those tones also overlap with the wingbeat frequencies of tasty insects like flies and mosquitoes. Next, the team set out to test if sound alone triggers predatory acrobatics in D. spinosa spiders by playing the same tones to spiders that had already spun nets and were ready to hunt. Whether in the lab or in the wild, half of the spiders that heard the researchers’ tones lunged backwards, if they moved at all. Further, spiders in the wild as well as in the lab only responded to certain frequencies – 150 Hz, 400 Hz, and occasionally 750 Hz – the same frequencies at which they observed brain responses, which also match the wingbeat frequencies of potential prey. The spiders seemed to be responding to some sounds with backward strikes, so the researchers wanted to figure out how they are able to hear these sounds without ears.Fortunately, insects have evolved some impressive strategies to tune in to the audible world. Spider legs have groups of tiny sensors that detect forces like air pressure changes and, therefore, may be sensitive to sound. To determine whether the sensors on the legs detect noises, the team recorded nerve activity in the leg while playing the same tones that had triggered brain signals. Although they found more nerve activity, they still weren't sure whether the tiny sensors were involved in detecting the pressure differences caused by sound waves or if the sound was detected by other means, such as the hairs on the leg. This time, the team gently held the leg so that the sensors wouldn't be able to detect any more pressure from sound but the hairs could still vibrate. They recorded much less nerve activity, indicating that the tiny sensors allow the spiders to ‘listen’ to the pressure of sound waves bouncing into their legs. Whereas the brain was more active at wingbeat frequencies, the leg nerves responded to higher frequencies of 1000–10,000 Hz, possibly to detect sounds emitted by predators.Like the mythical monsters these spiders resemble, their outward appearance doesn't tell the whole story. They use more than big eyes to find food: they also use their legs to keep an ‘ear’ to the air behind them to create a panoramic hunting ground.