Cicadas will hear the call of spring. And then you’ll hear their mating calls, too.

For well over a decade, periodical cicadas do very little. They hang out in the ground, sucking sap out of tree roots. Then, following this absurdly long stint in the soil, they emerge, sprout wings, make a ton of noise, have sex, and die within a few weeks. Their orphan progeny return to the ground and live the next 17 or 13 years in darkness.

Several species of periodical cicadas appear in the Eastern US — sometimes ahead of schedule — but it’s a different 17- or 13-year crew that wakes up each time. (There are also, separately, some annual cicadas that emerge every year.)

This year, though, will be a rare event. Two groups, or “broods,” are waking up during the same season. There will likely be billions, if not trillions, of the insects.

There’s the 17-year-group called Brood XIII, which is concentrated in Northern Illinois (brown on the map below), and the 13-year clutch, Brood XIX, which will emerge in Southern Illinois, Missouri, Arkansas, and throughout the Southeast (see them in light blue on the map below).

It’s the first time since 1803 that these broods have emerged together.

US Forest Service via Wikimedia Commons

Emerging in these humongous annual batches is likely an evolutionary strategy. There are so many cicadas swarming around all at once that their predators, such as birds and small mammals, can’t make a meaningful dent in their numbers. As Vox’s Benji Jones explains:

Their defense strategy is to flood the forests so that predators, from blue jays to squirrels (and, during these eruptions, everything in between), become so full that they literally can’t stomach another bite. That leaves plenty of insects to mate and lay eggs that will become the next generation of periodical cicadas.

There are many mysteries about cicadas: Why do their alarm clocks use prime numbers? For that matter, how the hell do they keep time? What is clear is that they’re coming soon, and in huge numbers, and it won’t happen like this again for a long, long time.

Update, May 3, 11:42 am ET: This piece, originally published in 2021, has been updated for 2024, most recently with details about the previous emergence of two cicada broods in the early 19th century.