This May billions of cicadas swarmed the eastern United States. But by the end of June the bugs will be gone — and they won't be seen again for 17 years.

Cicadas are flying, plant-eating insects about the size of shrimp. They are small, stout-bodied, large-headed and have two pair of wings. Cicadas are usually green with red and black markings.

Cicadas are known for their loud, piercing song. Only male cicadas have drumlike membranes on their abdomen that vibrate, making creepy tones. From June through September, adult males sit in treetops throughout much of America producing a variety of rhythmic ticks, buzzes and whines to attract females. Female cicadas produce timed "wing flick" signals in response to male calls. The signal consists of a quick flip of the wings that creates a broad-frequency sound that can vary from a soft rustle to a sharp snap. Every species of cicada has a unique call. The frequency of the sound is related to the temperature and time of day. The loud noise of the male's chorus may repel birds, the cicada's chief predator.

Cicadas spend most of their time underground feeding on roots. But every now and then they come out. There are annual and periodical cicadas. The cicadas in a periodical cicada population are synchronized so that almost all of them mature into adults in the same year. The fact that these cicadas remain locked together in time is made even more amazing by their extremely long life-cycles of 13 to 17 years.

This year a group of cicadas called Brood X (Brood Ten) will leave their underground homes and cover trees throughout the eastern United States. In the open air, the bugs change into adults, reproduce and then die.

Brood X is the largest group of cicadas. The brood comes out only once every 17 years. There are 12 broods that have 17-year cycles like Brood X. "A brood is a class year, like the graduates of 2004, who graduated in June," said Gene Kritsky, a cicada expert at the College of Mount St. Joseph in Cincinnati, Ohio.

How do the bugs know when 17 years have passed?

"We really don't know how they count the years," Kritsky said. But this year the cicadas will leave the ground when soil temperatures reach about 64 degrees Fahrenheit (18 degrees Celsius).

People's reactions to the cicadas are mixed.

"Some people leave town and go west, where there are no cicadas. Other people plan camping trips timed in the middle of the outbreak, because they want to experience it in its full intensity," according to Keith Clay, a biologist at Indiana University in Bloomington.

Cicadas help the environment. They improve soil, provide food for other animals and prune treetops. Periodical cicadas achieve astounding population densities, as high as 1.5 million per acre. Densities of tens to hundreds of thousands per acre are more common.

Some people even like to eat the bugs! Whether they're curious or doing it for shock value, people are eating cicadas. Eating cicadas is not a new thing. People have been eating them for centuries.

"When you eat them when they're soft and mushy, when they come out of their skin, they taste like cold, canned asparagus," Kritsky said.

Cicadas are harmless. They do not bite or sting defensively, nor do they attack people. If a cicada lands on you, it does so only because it finds you to be a convenient place to land.

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Resources: World Book Encyclopedia; www.ummz.lsa.umich.edu