@Georgiann Baldino These cicadas are a great example for the not-so-random occurrence of prime numbers in nature. The reason the breeding cycles of prey animals are often prime numbers is that those make it hardest for predators to co-evolve unless they happen to have the exact same cycle length. Prime numbers can't be made by multiplying 2 other numbers, so if a predator for example has a cycle of 3 years, they will only catch the cicada every 3*17=51 years, meaning they can't evolve to specialize on that exact cicada species because they would just starve to extinction.