[M_Six]

This year’s dual emergence is a once-in-a-lifetime event. While any given 13-year brood and 17-year brood can occasionally emerge at the same time, each specific pair will see their cycles aligned only once every 221 years. What’s more, this year’s cicada groups, known as Brood XIII and Brood XIX, happened to make their homes adjacent to one another, with a narrow overlap in central Illinois.

https://www.nbcnews.com/science/scie...ada-rcna134152