Where I live, they come out in 17-year cycles. Other parts of the US it's every 13. Why the prime numbers?
Prime numbers help cicadas avoid predators. According to the article:
Research has shown that the population of creatures that eat cicadas — typically birds, spiders, wasps, fish and snakes — often have shorter 2 – 6 year cycles of boom and bust.
So, if our cicadas were to emerge, say, every 12 years, any predator that works in either 2, 3, 4 or 6 year cycles would be able to synchronize their boom years with this regular cicada feast. In fact, they’d probably name a public holiday after it called Cicada Day.
That’s not much fun if you’re a cicada.
On the other hand, if a brood of 17-year cicadas was unlucky enough to emerge during a bumper 3-year wasp season, it will be 51 years before that event occurs again. In the intervening years, our cicadas can happily emerge in their tens of thousands, completely overwhelm the local predator population, and be mostly left in peace.
Resourceful little guys, eh?
DesignFestival than takes this concept of using prime numbers to avoid synchronizing into regular patterns and applies it to designing backgrounds and images for web pages. That's cool too, and Alex Walker has more detail on that in the original article. I just can't get over the use of prime numbers to outwit predators!