Hordes of large insects called cicadas have emerged from the ground to swarm over the eastern United States. But as quickly as they have appeared in the past week, they will be gone by the end of June and will not return again for 17 years. In the meantime, the loud whine of the male cicada provides the backdrop to the sounds of everyday life and their discarded shells a crunch underfoot.

From the U.S. midwest to the Atlantic coast, male cicadas produce a shrill noise unmistakable during the daytime.

Cicadas are flying, plant-eating insects the size of shrimp. Their loud roar hints at their huge numbers. Indiana University biologist Keith Clay explains just how dense they are. "It's been documented up to near four million per hectare, weighing nearly three-thousand kilograms. So they're extremely abundant, more abundant in terms of biomass than any other animal on earth ever documented," he says.

Anyone unfamiliar with this periodical U.S. phenomenon should not get the idea that swarms of cicadas darken the skies or that a pedestrian has to fight his way through them. Instead, they perch on shrubs, fences, telephone poles or anything nearby, mate, and lay their eggs in trees.

The eggs will hatch in early June just as the insects reach the end of their life cycle. The larvae will drop and burrow into the ground, spending the next 17 years eating tree roots, going through five juvenile stages, molting each time as they enlarge, and emerging as the next adult swarm in 2021.

The United States also has cicadas that appear every 13 years. These long life cycles are unique among the 2,000 known species in the tropical and temperate regions of the world. Other species have shorter cycles that are not calibrated with each other so that some swarm appears every year.

Indiana University geographer John Odland says the calibrated 13 year and 17 year U.S. outbreaks of several species are apparently an evolutionary adaptation that helps the cicadas survive predators. "By all of them emerging simultaneously, they really overwhelm the capacity of predators to eat them. They are individually pretty helpless in the face of predators. But there are so many of them that any animal that can eat cicadas will probably be eating very little else," he says.

The frigid temperatures of the ice ages may have also played a role in the long life cycles of U.S. cicadas. The insects like warm weather and emerge from the ground only when the temperatures reach 18 degrees Celsius. But University of Connecticut biologist Christine Simon says the ice age periods probably kept them from emerging as frequently as they once had. "During the time when the climate was much colder, there were shorter growing seasons so they might not have been able to complete development in their normal lifetime, whatever it was at that time. So because they couldn't finish development, they had to stay underground longer," she says.

Researchers say that cicadas probably were not as dense as they are today, centuries ago, when the continent was entirely forested. But with areas of trees now patchier, the insects are forced into smaller areas.

This gives rise to the fear and disgust among many in suburbia. But cicadas are generally harmless. The underground nymphs can suck a little life out of some trees and make them more susceptible to diseases or bad weather. The adult females split tree twigs to insert their eggs. Yet, the mass emergence also aerates the soil, prunes the treetops and adds nutrients to the environment, so many nutrients that cicada cook books have also emerged.

However, most Americans probably prefer the crunch of a cicada to be under their feet rather than between their teeth.