A trillion cicadas will descend on the US this spring in rare event that could leave unforgettable stench

More than a trillion cicadas could emerge throughout the U.S. Midwest and Southeast this spring as the schedules of two separate broods align for the first time since 1803.

Brood XIII and Brood XIX represent two distinct groups of periodical cicadas (Magicicada) that emerge according to 17- and 13-year life cycles, respectively. In a rare natural event that occurs once every 221 years, these two broods will synchronously tunnel through the ground to the surface starting in late April across 16 states.

The event, known as a dual emergence, could potentially lead the two broods to interbreed, experts told The New York Times. 

"Under just the right circumstances and with just the right number of individuals cross breeding, you have the possibility of the creation of a new brood set to a new cycle," Floyd Shockley, an entomologist and collections manager at the Smithsonian National Museum of Natural History, told the Times.

Periodical cicadas, which comprise seven species, spend most of their lives underground as nymphs and feed off of sap that oozes from tree roots. After 13 or 17 years starved of daylight (depending on the species), the insects burrow to the surface using their front legs and transform into adults. The males vibrate membranes on the sides of their bodies to produce a song — potentially louder than a plane in a chorus — that attracts mates, according to The New York Times. Once a pair has finished mating, the females cut slits in tree branches to lay their eggs in. 

Adult periodical cicadas survive for three to four weeks and don't live to see their eggs hatch roughly three weeks later. The newly hatched nymphs then drop to the ground and tunnel down into the soil to repeat the cycle.

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