Tens of billions of noisy cicadas will burst from the ground in the coming weeks in a rare synchronized event that last occurred 221 years ago.
Winged cicadas are expected to appear in the Midwest and Southeast starting in early May, with a double appearance of two different species of periodical cicadas. These insects spend a certain number of years underground before emerging to the surface to participate in a noisy mating ritual.
This year’s emergence is expected to be unusually large; some experts estimate that more than a trillion cicadas could cover parts of the country where two broods overlap.
Cicadas are harmless to humans, but some people find the insects’ abundance and loud mating songs annoying.
For insect enthusiasts, experiencing the emergence of two cicada hatchlings together is a once-in-a-lifetime opportunity, considering the last time this occurred was in 1803, when Thomas Jefferson was president.
“This is something no one alive today has ever seen, and no one alive today will ever see again,” said Jim Louderman, a collections assistant in the insect department at the Field Museum in Chicago. “For entomologists, this is a really big deal.”
This spring’s emergence will feature the Brood XIII and Brood XIX cicadas. The first group lives on a 17-year cycle and usually occurs in the Midwest, mostly in Illinois, but also in parts of Iowa, Wisconsin, and Indiana.
Brood XIX cicadas, meanwhile, live on a 13-year cycle and occur over a much larger geographic area that includes Illinois, Missouri, Louisiana, North Carolina, Virginia and Maryland.
A brood of seven periodic cicadas nests in North America, and they emerge every 13 or 17 years, depending on the species. A 13-year litter and a 17-year litter occasionally occur at the same time, but each specific pairing sees their cycles align only every 221 years.
This year’s event is unusual because these particular broods are in sync, but also because they are adjacent to each other with a narrow area of geographic overlap in central Illinois.
“There aren’t many places in the country where two very different offspring overlap,” said Daniel Young, a professor of entomology at the University of Wisconsin-Madison and director of the school’s insect research collection.
Thousands of species of cicadas can be found around the world, but periodic cicadas are different because they spend most of their lives underground, feeding on tree roots. After 13 or 17 years, depending on the offspring, the insects tunnel to the surface to begin a month-long frantic search for a mate.
Louderman said he hasn’t seen any reports of early cicada sightings yet. The insects usually emerge from underground when the soil temperature reaches about 64 degrees Fahrenheit.
Louderman said some could start appearing in early or mid-May. This is expected to continue into June as conditions warm. Once the insects are gone in full force, he and his colleagues will travel across Illinois to collect specimens for the Field Museum’s collection.
Cicadas are perhaps best known for the noise they make when above ground. The insects’ mating song, a high-pitched buzzing sound, is hard to ignore and can reach up to 100 decibels.
“There are so many of them that the noise is almost deafening,” Louderman said. “It’s like 1,000 chainsaws going off and being used all at once.”
The insects have several chaotic weeks to find a mate and lay their eggs before they die. From now on, people will probably see lots of insect carcasses scattered on the ground.
“They’re literally paving the sidewalks and streets; it’s just crazy,” Louderman said. “Some people are a little afraid of it.”
But cicadas don’t bite, Louderman added.
He describes himself as a “lifelong cicada enthusiast” and said his passion emerged after an early brush with insects.
“When I was 8 years old, cicadas appeared periodically and that got me hooked on insects and entomology,” Louderman said.
In the same way that this spring’s total solar eclipse has sparked curiosity for many, perhaps some young people may be fascinated by the emergence of the cicada and even decide to pursue its field of study.
Researchers at the University of Wisconsin-Madison plan to track where cicadas are spotted to map their range. Young encourages others to join this mission.
People in the Midwest and Southeast can take photos of cicadas in their area and report sightings through an app called Cicada Safari or through websites such as Bugsinourbackyard.org or Bugguide.net.
Young said he hopes people will get the chance to experience this rare and important event, especially in parts of Illinois where two cicada broods overlap.
“Not many people in human history will be in a place to witness this,” he said. “Many people might find this unpleasant, but it’s quite surprising.”
This article first appeared on NBCNews.com.