Sixteen US states, stretching from Maryland to Oklahoma and Illinois to Alabama, are bracing for the arrival of trillions of noisy cicadas. What makes the event especially rare is that two broods are emerging simultaneously. Brood XIX, comprising four species that surfaces every 13 years, and Brood XIII, consisting of three species that appears every 17 years. This unusual coincidence last occurred 221 years ago, in 1803, and won't happen again until 2245.