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Right now, something magical is underway across vast stretches of the eastern US: The soil is beginning to erupt with trillions of periodical cicadas that have been growing underground for either 13 or 17 years, waiting for this exact moment to crawl out of the Earth together. 

This particular burst of life is incredibly rare. The two groups that are emerging, known as Brood XIX and Brood XIII, appear together just once every 221 years. Brood XIX are 13-year cicadas, and are showing up in southern Illinois, Missouri, and parts of the southeast, whereas Brood XIII are 17-year cicadas that live in northern Illinois. 

That makes this event more rare than a total solar eclipse in North America. And depending on who you ask, it’s just as spectacular.

Cicadas are, in fact, spectacular bugs. Although they have tiny insect brains, they can count and sense temperature; males have built-in drums that make noises so loud they can damage human hearing; and in just a few short weeks they transform entire forest ecosystems. 

So yes, while they may be loud and, to many people, extremely icky, periodical cicadas are an ecological wonder — and one that we arguably take for granted. It’s worth getting to know these bizarre bugs a bit better, starting with these three incredible facts:

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Eastern North America is the only place in the world where you find periodical cicadas — groups of cicadas that emerge from the ground every 13 or 17 years, depending on the brood, or group. Slightly different kinds of cicadas, known as annual cicadas, appear every summer and have a global distribution. 

For all of those years, periodical cicadas are several inches or more under the soil, sucking sap from the roots of plants using a straw-like mouth. Then, at the same time and in the right year, they all surface together. 

And it’s important that they do: Bursting from the ground all at once overwhelms their predators, which include birds and squirrels (and dogs). There are simply too many cicadas to eat, so plenty of them survive and can seed the next generation. 

But how do they pull off such a stunt of time-keeping? The key may be in the root sap, known as xylem fluid, that cicadas drink while they’re underground. 

Throughout the year, as trees grow and shed leaves through the seasons, the flow and makeup of that fluid changes. It functions like an internal clock. Cicadas can likely detect those changes to keep track of the years, according to Martha Weiss, a cicada researcher at Georgetown University. 

The bugs then use their ability to sense soil temperature (among other things) to know when in the year they should emerge. 

The numbers 13 and 17 are important for these bugs, though it’s not totally clear why. Scientists suspect that surfacing at this unusual cadence makes it hard for their predators — which include basically everything in the forest — to anticipate their emergence. Birds and other animals that eat them typically won’t experience more than one eruption of a particular brood in their lifetimes.

“Their ability to track [the emergence] is almost impossible,” said John Lill, a biologist at George Washington University who studies cicadas. 

On occasion, however, periodical cicadas seem to get their math wrong. Some come out of the ground at nine years, whereas others will emerge at 21 years, Weiss said. It likely has to do with their size: If the young cicadas are growing faster than normal, they might come up four years early, whereas if they’re developing more slowly, they may wait another four years to emerge. 

The reason they choose four-year intervals is a mystery. “There’s something magical about four years and four-year intervals, and we don’t know what it is,” Weiss says. 

Remarkably, because climate change is extending the growing period for trees in temperate regions, it might make cicadas develop faster, potentially turning some 17-year cicadas into 13-year cicadas, according to Weiss.

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During a cicada emergence, the forest is chaos — everything from large and small birds to lizards and squirrels shovels the bugs into their mouths. 

While great for hungry animals, this isn’t good for cicadas, which couldn’t be easier targets. They’ve got bright red eyes, which pop against the forest palette. They’re also extremely, irritatingly noisy, making them easy to locate. Plus, they have no defense — no bite, no sting, no venom. 

Adding to their woes: They’re not even good fliers, Lill said. 

Although periodical cicadas only appear for a few weeks, their blip above ground can rewire entire forest food webs, with knock-on effects that last for years.

Consider birds. They have so much to eat during cicada eruptions that they may actually produce more chicks in the months that follow. A study from 2005 linked cicada emergences to a population bump in a number of species including red-headed woodpeckers and common grackles.

In fact, the birds are eating so many cicadas that they’re consuming much less of everything else — including caterpillars. That means caterpillars get a rare reprieve from the constant threat of attack, at least from birds. 

Scientists, including Zoe Getman-Pickering, an ecologist at University of Massachusetts Amherst, actually measured this caterpillar effect during the Brood X emergence in 2021. Remarkably, the number of them roughly doubled in the forests she studied during the emergence, relative to the two following years.

The large number of caterpillars also impacts trees, because these larvae eat leaves. 

Read more about that — and other cool cicada science — here. 

—Benji Jones, senior reporter

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