I read once that there are several species of cicadas that emerge in multi-year intervals. The 17 year cicadas may be the most well known, but there are some that have a 13 year reproductive cycle, for example. The interesting thing about all the species that have this is that the cycle is always a prime number of years (SOURCE).
The 17 year cicadas have hatched in Akron OH, where Deborah and I are now visiting my parents. When you go outside, there is this constant post-apocalyptic droning sound coming from the tree tops. It's actually quite loud.
Why prime numbers? The link above explains it pretty well, so go read it.