Cicadas are insects about the size of a shrimp with transparent wings that crawl out of the ground every 17 years and make a deafening buzz when they mate.

The US Department of Agriculture designated all the cicadas that emerged back in 1893 and at 17-year intervals thereafter as Brood I, said Gene Kritsky, a biologist at the University of Mount Saint Joseph in Cincinnati, Ohio.

"The cicadas that will appear in 2004 belong to Brood X, which is the largest of the 17-year broods" and includes three different species, he said.

Cidadas "don't bite, and they don't attack people," said Keith Clay, an insect expert at Indiana University.

Cicadas are likely to appear in the mid-Atlantic region between Washington and New York City, as well as large swathes of the midwestern US, especially Indiana and Ohio, as well as Kentucky and Georgia in the south.

The cicadas are likely surface around the end of May, or when the soil temperature reaches around 17 degrees Celsius (around 63 degrees Farenheit). With a bit of rain to soften up the soil, they'll be coming out.

The number of insects could reach the trillions in the state of Indiana alone, Clay said.

Tens of thousands of cicadas can appear overnight and cover large areas.

Despite their large numbers, the cicadas usually cause little damage to trees and smaller plants. After so many years underground slowly sucking tree roots, the insects arrive full of energy and determined only to mate.

"They are not very active when the sun goes down, so the massive noise we'll hear in the daytime will subside, allowing people to sleep," said Clay.

Clay predicts that "there will be some crop damage, especially to orchards, but we don't expect a disaster."

The cicadas contribute to soil aeration, and their presence is a sign of good environmental health, specialists say. But why the insects appear only every 17 years remains a mystery.

Clay recently received a 300,000 dollar grant from the National Science Foundation to study Brood X.

People in the affected areas "who have just spent thousands of dollars on landscaping may want to consider throwing netting over the trees during the two to three week period when the adult cicadas are out," he added.

Any damage caused by the cicadas happens when the females use a razor-sharp appendage to slice branches and twigs open so they can insert their fertilized eggs.

The eggs, about the size of a grain of rice, hatch about four weeks later -- in early June -- just as the cicadas reach the end of their life cycles. The larvae then drop and burrow into the ground with help from its front legs searching for roots.

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