In southern U.S. states one of the largest broods of cicadas live, called the Great Southern Brood. Scientists don’t know how cicadas synchronize themselves across states for breeding and hatching simultaneously. They do, however, know why they do it. Since they hatch so infrequently, no predator can live solely off of eating cicadas. Plus, by emerging all at once, they have a better survival rate. Predators can’t eat a lot of them if they come out in masses. In southern U.S. the cicada breeds in 13 year cycles and in eastern U.S. they breed in 17 year cycles.
The reason is that both of these are prime numbers. The numbers can’t be broken down any further. This is a major advantage, because it makes them more elusive and therefore harder for predators to track and eat. It is a phenomenal evolutional tactic for survival. It is also extremely fascinating.