Crickets and katydids make noise by rubbing their forewings together. The upper wing has a series of serrated teeth called a file, and the lower wing has a scraper.
As the insects rub the scraper against the file, the wings amplify the sound, making it loud enough for other insects to hear.
Walker learned to identify katydids and crickets based on their songs while he was a graduate student at Ohio State University in Columbus in the late 1950s.
He was studying so-called tree crickets that inhabit weedy fields. They all looked alike, he says, but when he listened to their sounds, he heard four distinct tunes.
(Related news and audio: "Tropical Wrens Sing Complex Tunes, Researchers Find" [August 2006].)
So he put like-sounding crickets together in four different collection boxes. Sure enough, he says, closer inspection revealed physical differences that proved that each of the four was a unique species.
Since then, he says, he has discovered about 35 new insect species based initially on differences in their songs.
Today, Walker says, scientists count 258 katydid species and 127 cricket species living in the United States. He believes there are dozens more of each yet to be identified.
The count, for example, excludes about ten species he's identified but has yet to describe in scientific literature.
Worldwide scientists know of several thousand species of katydids and 400 species of crickets.
According to Walker, only male katydids and crickets strike up a tune, and when they do, they invariably have one thing on their mind: sex.
"Almost all song you hear produced by crickets and katydids is to attract females of the same species," he said.
(Related news: "Mice Serenade Mates With Complex Tunes, Study Suggests" [November 2005].)
Since the purpose of the song is to attract a mate, Walker adds, it makes sense that different species of crickets and katydids in the same area would play different songs.
That way, the females direct their attention only to males of the same species.
Richard Alexander is a professor emeritus of biology at the University of Michigan in Ann Arbor who studies crickets, katydids, and cicadas.
He said the insect serenades are "a cheap, wonderful system for males and females to get together with members of their own species."
For biologists, Alexander adds, the insects' distinctive songs are invaluable for rapidly determining their geographic ranges and ecology.
For example, a pair of experts can drive country roads at 30 miles (48 kilometers) an hour and rapidly assess species' geographic ranges and whether they are active at day or nightsimply by listening.
"Nobody can work that fast on any other group [of animal]," he said.
Such studies are a quick way to assemble a database of insect diversity on regional and continent-wide scales, Alexander says.
The differing cricket and katydid songs are also indicators of evolution, he adds.
Alexander has shown that differences in species' songs can be linked to several tiny genetic variations that distinguish one species from another.
"That's a very potent indication that species are just populations that step by step evolved away from each other," he said.
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