In a few weeks, a natural spectacle will take place across much of the United States — one that is not found anywhere else in the world. Billions of cicadas that have spent years patiently growing in complete darkness will finally emerge, perfectly in sync, for a raucous party in the sun.
It's been 17 years, and the periodical insects, also known as Brood X, are back.
When the world last glimpsed the cicadas, Facebook was brand new, theaters were showing "Spider-Man 2," and the 2004 Summer Olympics were underway.
Since then, they've been underground, eating.
"They're in the dark, they're feeding on roots, just living their best lives until the time is right," said Matt Kasson, an associate professor at West Virginia University who studies cicadas and the fungi that zombify them. "And that's when they decide, you know what, it may be time to go up and find a partner."
Beginning in late April or early May, once the ground is warm enough, billions of Brood X cicadas will be seen across a dozen states, stretching from Illinois to the west, Georgia to the south, and New York to the northeast. The young cicadas, called "nymphs," claw their way out of the ground and climb up to shed their skins one last time and transform into adults. They will have only a few weeks to sing, mate and begin the cycle again.
There are seven species of North American periodical cicadas, all in the genus Magicicada. Four species live on a 13-year cycle, and three for 17 years.
This year's emergence is a group containing all three 17-year species: Brood X, so named because it was the 10th in an arbitrary naming system designed in 1898 by the entomologist Charles Lester Marlatt.
However, the group's other name, The Great Eastern Brood, is far more descriptive.
Most of the world's over 3,000 species of cicada don't make quite as dramatic an entrance. While they take two to five years to grow up, at least some adults of these annual species show up every summer, and in much smaller numbers.
Periodical species, where all of them consistently have the same, extralong life cycle that culminates in a mass emergence, are incredibly rare. With just one recently discovered, exception — the 8-year train millipedes in Japan — cicadas are the only insects that have evolved to live this way.
Why? For Magicicada, part of the answer lies under sheets of ancient ice.
"During the glacial periods [of the past few million years], we think that they probably extended their life cycles, because the growing season was too short to complete development at their previous time," said Chris Simon, a cicada researcher and professor at the University of Connecticut.
The second reason has to do with their survival strategy above the ground. They don't have one.
In the natural sciences, it's called "predator satiation." After a certain point, even the hungriest predators wont physically be able to eat any more. But without a big synchronized group, the plan falls apart.
"That favored individuals that came out together, because they survived better," Simon said.
Not all nymphs grow at the same rate. For example, one feeding from a tree that had a bad year will need more time. But if all the cicadas wait the full 17 years, it allows the unlucky ones to catch up and bolster the group's numbers.
"Then, there's a kind of feedback loop," Simon said. "If they come out on other years … they'll get eaten by predators. And they also won't be able to find mates."
Despite their seemingly endless numbers, the periodical cicadas are far from invincible.
Two broods, XI and XXI, have gone extinct, and a third, Brood VII, is currently declining. Cicadas need trees to lay eggs in and feed upon, so deforestation is devastating for them.
A warming climate — and a longer growing season — might also favor a shorter lifespan, leading to 17-year species permanently switching to a 13-year lifespan, as happened in the midwestern species M. neotredecim.
Periodical cicadas are a crucial part of the ecosystem. The nymphs are food for animals living underground, and the adults feed every carnivore in the area. Even the many uneaten cicadas give back to the trees — as they decay. They become "basically fertilizer," Kasson said.
Simon and Kasson said they hope that people will choose to venture out and observe Brood X while they have a chance.
Thousands of small animals buzzing about may feel overwhelming or uncomfortable to some. But the cicadas are harmless and, according to Simon, a bit goofy.
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"Cicadas are very gentle; they don't bite or sting. They're not attracted to people; they're attracted to noises," she explained.
For the cicada-curious, it's easy to get involved.
More information about Brood X and periodical cicadas in general can be found on the website Cicada Mania, and on the University of Connecticut's Cicada Homepage. Simon's colleagues have also designed a smartphone app, Cicada Safari (available both for iOS and Android), that allows anyone to be an amateur entomologist.
By uploading photos of the cicadas and recordings of their songs, members of the public can help map out the range of Brood X, track their population, and discover firsthand what Simon calls "one of the seven wonders of the natural insect world."
CORRECTION (March 17, 2021, 12:19 p.m. ET): A previous version of this article misstated how many species of North American periodical cicadas have been identified. There are seven total species, four of which live on a 13-year cycle, not six total and three on a 13-year cycle.