Periodical cicadas (Magicicada septendecim), the cousins of katydids and crickets, have a unique breeding schedule, and after 17 years of living underground, a large group of them are preparing to fill the skies along the U.S. East Coast, from North Carolina up to Connecticut.

Normally, periodic cicadas spend their lives in complete darkness underground, sucking the fluid out of the roots of trees and shrubs. At the end of their life, they emerge, breed, and almost instantly die, completing a lifecycle that humans have studied for centuries.

In the process, however, they annoy millions of people with their constant chirping and, of course, the piles of dead cicada bodies on the ground. While some areas may see no cicadas at all, others in the past have seen millions of cicadas in a single acre. (Listen to the cicada’s love song.)

"It can be like raking leaves in the fall, except instead of leaves, it’s dead cicada bodies," said Dan Mozgai, a cicada researcher who keeps a clearinghouse of cicada information and breeding schedules at cicadamania.com.

Consistency

Cicadas are easy to anticipate because of their extremely consistent mating behavior. Every 13 or 17 years, depending on the population, species of periodic cicadas will emerge as part of a specific brood in order to look for a mate.

The group expected this spring, known as Brood 2, are the offspring of cicadas last seen in 1996. If they follow the same tracks as their parents, they'll emerge in Connecticut, Maryland, North Carolina, New Jersey, New York, Pennsylvania, Virginia, and Washington, D.C.

The genetic mechanism that prompts periodical cicadas to emerge kicks in every 17 years (or every 13 years for other broods) when the ground warms up to 64°F (18°C).

Some researchers think the timing of a brood’s emergence is a defensive mechanism—appearing at infrequent intervals means that it’s harder for would-be predators like birds and squirrels to anticipate when the insects will be available to eat.

Others suggest that the 13- and 17-year cycles, prime numbers in mathematics, help cicadas avoid parasites. A 2004 study from the University of Campinas in Brazil suggested that a cicada with a 17-year cycle and a parasite with a two-year cycle, for example, would meet only twice each century.

But not all cicadas breed on this multiyear cycle. Some, like the tibicen cicadas, work on an annual rotation, leaving them more susceptible to predators like the cicada killer wasp (Sphecius speciosus).

The wasps know exactly when to expect the annual cicadas in late summer or early fall. The wasp lays its eggs on the cicadas, and the larvae slowly kill the cicada and feed off its carcass.

Anticipation

For the periodical cicadas, their temperature thresholds for emergence are getting harder to anticipate. During mild winters, like in 2012, cicadas from Brood 1 appeared in mid-April. Scientists think that this year, Brood 2 cicadas could wait until early May or even later.

John Cooley, a cicada researcher at the University of Connecticut, has led efforts to map where cicadas live, breed, and die. With funding from a grant from the National Geographic Society, Cooley has attempted to trace their evolutionary history since the last ice age, trying to understand how the species evolved as the world's climate changed.

"We know that they've had to change in the past because of postglacial forests that exist now," Cooley said. If scientists understand how cicadas once responded to climate change, then they can imagine how another episode of global warming might impact both cicadas and other insects.