Cicadas emerge from the underground only periodically. Depending on the species, that period will be 7, 13, or 17 years, which are all prime numbers. Why does this make sense?
Suppose there are some predators (like birds, and the Cicada Killer Wasp) that attack cicadas, and that the cicadas emerge every 12 years. Then the predators that come out every two years will attack them, and so will the predators that come out every 3 years, 4 years and 6 years. But according Mario Markus, “if the cicadas mutate to 13-year cycles, they will survive.”
The other advantage is that cicada species with different intervals will rarely compete with each other for food.