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WHEATON, Ill. (AP) — The most striking thing about the cicada invasion currently sweeping the central United States is its sound: an eerie, astonishingly loud song that penetrates your ears and doesn’t let much else through.

“It’s a beautiful chaos,” said Rebecca Schmidt, an entomologist with the U.S. Department of Agriculture. “It really creates a kind of symphony.”

The songs – only those of the males – are mating calls. Each species of periodical cicada has its own distinctive song, but two stand out: that of the orange-striped Decims or Pharaoh cicada and the Cassini cicada, which is smaller and does not have orange stripes on its abdomen.

“The sound we hear most often is the Cassini sound, a buzzing trail that rises and comes down in waves,” said Jennifer Rydzewski, an insect ecologist at the DuPage County Forest Preserve, in an interview in a clearing near a clump of trees.

“And every time it comes up in a wave and comes down again, you see a bunch of them flying up in the treetops, so they call and then jump to a new branch and call again. So it’s actually different groups that come in waves.”

The other is a “constant whirring sound that is the staccato of the pharaoh,” and every now and then a single call that sounds like “eee-eee” can be heard, she said. Others have said the sound is more like “fffaaaro, fffaaaro.”

The sound comes from a white membrane in the male’s midsection that vibrates, said Schmidt and Rydzewski. The area underneath acts like an echo chamber.

“It’s a lot of the same physics as an instrument,” Schmidt said. “Think of it like a drum: You can have a fairly small drum struck by someone not hitting it that hard, and it still makes quite a bit of noise.”