Why prime numbers? Because only prime numbers won’t ‘overlap’ with the life cycle of any potential predators. For example, if a cicada has a 13-year life cycle, then any predators with life cycles between 2 to 12 years will miss the window for hunting cicadas, since 13 is not divisible by anything other than 1 and 13. So if a predator has say, a 6 year cycle, it will miss the cicadas with 13 year life cycles, since 6 x 2 = 12 and 6 x 3 = 18. The second generation of the predators will be too early by a year, and the third generation will be too late by five years. Only the cicadas with 13 and 17-year life cycles will avoid being ‘overlapped’ by potential predators.