Trillions of cicadas are about to emerge from 15 U.S. states. Scientists say Brood X (as in 10, not the letter) is one of the biggest for these bugs that come out only once every 17 years. After years quietly underground, the red-eyed black backed bugs emerge in a weird spectacle involving a race against everything trying to eat them, singing in the treetops, sex, death and plummeting to the ground. It sounds creepy but scientists say it is a sign of an ecosystem healthy enough for something that’s been happening for millions of years. And it’s quite a show.