If you haven't seen them yet, you may have heard them: the periodical cicadas that are out in full force in nearly 20 U.S. states across the Southeast and Midwest.

These 17 states, which range from Oklahoma to Wisconsin to North Carolina and more, are seeing the trillions of cicadas emerging this year in a rare, double brood event.

The two broods this year, the 13-year Brood XIX located mainly in the Southeast and the 17-year Brood XIII in the Midwest, have not emerged together in 221 years and are not expected to do so again until 2245.

Thanks to warm soil temperatures and ideal conditions, cicadas from both broods have already made their way above ground and the emergence is already in full swing.

Here's where you can find cicadas above ground this year.

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2024 cicada map: Check out where Broods XIII, XIX are projected to emerge

The two cicada broods were projected to emerge in a combined 17 states across the South and Midwest. They emerge once the soil eight inches underground reaches 64 degrees, beginning in many states in April and May and lasting through late June.

The two broods last emerged together in 1803, when Thomas Jefferson was president.

Where are the cicadas already out in 2024?

Adult periodical cicadas from Brood XIX are now completing its emergence as the brood is out in full force in states across the Midwest and Southeast, according to Cicada Safari, a cicada tracking app developed by Mount St. Joseph University in Cincinnati, Ohio.

They have been spotted on the app in Alabama, Arkansas, Georgia, Illinois, Kentucky, Maryland, Mississippi, Missouri, North Carolina, Oklahoma, South Carolina, Tennessee and Virginia.

Brood XIII has started to emerge in Wisconsin, Iowa and Indiana and throughout the Chicago area and around central and northern Illinois around Peoria and Champaign, according to the tracking service.

What is a brood?

According to the University of Connecticut, broods are classified as "all periodical cicadas of the same life cycle type that emerge in a given year."

A brood of cicadas is made up of different species of the insect that have separate evolutionary histories. These species may have joined the brood at different times or from different sources. These different species are lumped together under the brood because they are in the same region and emerge on a common schedule.

How long will the cicadas be above ground?

How long cicadas live depends on their brood and if they are an annual or periodical species.

The two periodical broods this summer are Brood XIX, which have a 13-year life cycle, and Brood XIII, which have a 17-year life cycle.

Once male and female periodical cicadas have mated and the latter has laid its eggs, the insects will die after spending only a few weeks above ground − anywhere from three to six weeks after first emerging.

That means many of this year's periodical cicadas are set to die in June, though some could die off in late May or July, depending on when they emerged.

The nymphs of annual cicadas remain underground for two to five years, according to the Missouri Department of Conservation. These cicadas are called "annual" because some members of the species emerge as adults each year.

Why do cicadas make so much noise?

You'll have to thank the male cicadas for all that screeching. Male cicadas synchronize their calls and produce congregational songs, according to Britannica, which establish territory and attract females. There is also a courting call that they make before mating.

The periodical 13-year and 17-year brood cicadas are the loudest, partially because of the sheer number of them that emerge at once.