for National Geographic News
August 12, 2004
Grab a branch of a young acacia tree crawling with appropriately named thornbugs and you just might utter an "ouch" at the sharp prick. The tiny horned insects will not hear your vocalization with ears, but rather as vibrations in the stem.
The thornbug (Umbonia crassicornis) is one of about 3,200 species in the treehopper family that, along with at least 200,000 other insect species, communicate by making the surface they live on vibrate, a method scientifically called substrate vibration.
Thornbugs send signals into the stems of trees by shaking their bodies. Other insects pick up the vibrations via sensors in their legs, explained Rex Cocroft, a biologist at the University of Missouri in Columbia.
Cocroft studies the evolution of communication systems. He is interested in the treehopper family because their communication systems are closely related to the ecology of individual species. For example, treehopers that spend their entire development sucking sap from one stem do not communicate about food, whereas treehoppers that move from branch to branch in search of new leaves to eat do signal about food.
"Comparison among related species may help reveal how communication systems evolve to meet new challenges," he said.
Peggy Hill, a biologist at the University of Tulsa in Oklahoma, said communication via vibration "appears to be an old option" that has evolved in insects, spiders, crabs, frogs, and even mammals such as kangaroo rats and elephants.
"We are still struggling to learn the breadth and depth of the use of vibration in communication, but it appears to be much more ubiquitous than any of us would have imagined 20 years ago," she said.
The communications that thornbugs and other insects send via substrate vibrations are inaudible to human ears without the assistance of technology. To listen in, researchers record the vibrations and play the recordings back though a loudspeaker.
"This doesn't change the frequency or timing of the signals; it just makes them accessible our ears," Cocroft said.
The preferred recording device is known as a laser vibrometer. The tool detects changes in a laser light reflected from a vibrating surface, such as a tree branch, to measure the velocity of the vibrations. Other devices are essentially microphones that scientists clip onto to the branch.
Tiny insects—thornbugs are about a half inch (1.3 centimeters) long—can 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 air—the 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 chords—to get each others' attention, find mates, and share the address for a bite to eat.
According to Cocroft young thornbugs—which cluster together on the twigs of trees, where they suck sap for about six weeks as they mature—signal 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 communicating—and eavesdropping—on 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 predators—especially spiders—out 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."