Tiny insectsthornbugs are about a half inch (1.3 centimeters) longcan produce very low frequency sounds more commonly associated with larger animals. "The reason for this has to do with the constraints of broadcasting a signal through airthe signaler must be large enough in relation to the wavelength of the sound to be able to couple the sound to the air efficiently," Cocroft said.
Small animals and insects are thus restricted to making high-frequency airborne sounds. But this constraint does not exist for animals that communicate via substrate vibration. As a result, tiny insects can make very low frequency substrate vibrations.
"Although not all small insects that communicate with vibrations use low frequencies. Many of them do, and this leads to very surprising results for human listeners," Cocroft said.
By listening in on these vibrations and observing treehoppers in the field, Cocroft and his colleagues have learned that the insects use substrate vibrations for many of the same reasons other animals, including humans, use vocal chordsto get each others' attention, find mates, and share the address for a bite to eat.
According to Cocroft young thornbugswhich cluster together on the twigs of trees, where they suck sap for about six weeks as they maturesignal to alert their siblings and mother to the approach of predators.
"Once a few individuals start signaling, the rest of the offspring contribute signals of their own in synchronous waves. When the mother perceives these waves of signals, she responds by walking over to the nymphs and trying to find and drive away the predator," Cocroft said.
After the predator leaves, the mother also signals. Researchers are less certain as to the function of the mother's signal but theorize that it may let her offspring know she is still present and they should stay put.
If the predator wins in the encounter with the mother, the offspring may elect to leave the group and make a go of it on their own, Cocroft said.
Mother or not, thornbugs leave their nest branch about a month and a half after they hatch. Males go in search of female mates, and females go in search of a place to lay their eggs.
Young-adult thornbug vibrations take on a different meaning. "Males produce advertisement signals; if a female is receptive, she answers the male signal with a simpler one of her own," Cocroft said.
Once the male "hears" the female, he begins to locate her position as the pair signals back and forth in a mating duet. If the male encounters another male, "they may exchange a series of rather different signals," Cocroft said.
After mating, the female will lay abut 200 eggs into the tree branch and spend the remaining six weeks of her life caring for and protecting her young.
Peace and Quiet?
With at least 200,000 different insect species communicating via vibrations, there can be several different species communicatingand eavesdroppingon the same surface.
Cocroft said that, in general, insects are able to pick up and respond to the vibrations made by individuals of their own species, telling the signals apart by the pitch or the rhythm of the timing.
"There are also a lot of predatorsespecially spidersout there capable of sensing and locating a source of vibrations. So a signaling insect also probably runs a risk of attracting a predator," Cocroft said.
With multiple vibrations on any given surface, Hill said, insects, like humans, find peace and quiet by filtering out all but the most critical incoming messages. For example, when an insect is ready to mate, mating becomes the primary focus.
"If it does not mate within a tiny window of time, it will die without contributing to the gene pool," she said. "In that scenario, motivation is high to process signals linked to mating, while those sent out by any other species are just so much noise."
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