for National Geographic News
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.
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