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Cicada Resting

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This month, trillions of periodical cicadas are set to emerge from the ground in eastern United States for the first time in 221 years.

The rare double cicada brood will include the two largest cicada broods, Brood Xlll and XIX, which will simultaneously emerge en mass after having lived underground for 17 and 13 years, respectively.

“Billions, even trillions, of cicadas are going to emerge at the same time across 17 states,” Chris Simon, a professor in the Department of Ecology and Evolutionary Biology at the University of Connecticut, told Live Science.

Historically, there hasn’t been a simultaneous underground exodus of this size since the year 1803, when Thomas Jefferson was president, and won’t happen again for another 220 years.

It is a rare occurrence for two specific periodical broods of different life cycles to emerge at the same time and overlap in location.

“The co-emergence of any two broods of different cycles is rare, because the cycles are both prime numbers,” Cooley said. “Any given 13- and 17-year broods will only co-emerge once every 13 x 17 = 221 years.”

Brood Xlll is expected to emerge in the Midwest, mostly around north-central Illinois, Ohio and Iowa. Brood XIX will cover parts of Illinois, as well as a wider geographic area in the southeastern U.S., including Louisiana, Virginia and North Carolina.