The cause of the prime number of the periodicity of the cicada life cycle may be due to a time of near extinction during glaciation. Gould and others have posited that the prime number evolved to throw off even-numbered period predators. If you don't think about it too much, it seems to make sense. But if you ask for evidence, there isn't any. Besides, the cicadas emerge in such great numbers that predators eat their fill of them anyway no matter what their periods are.

Glaciation did not affect the eastern part of the US as it did most of the rest of the world. The cicadas took refuge there. Most insects populations like to avoid hybridization and have even-numbered years of periodicity. In the case of the cicada where only a small population congregated for mating purposes, hybridization had to occur causing the even-numbered periodicity to break. After 150 years of this hybridization, only prime-numbered periodicity remains, specifically 13, 17 and 19 years. After 500 years, only the 13 and 17 remains. This was first proposed by Warder Clyde Allee in the 1920s and is called the Allee Effect. A mathematical model was later devised by two Japanese and one American researchers that proved out hypothetically that near-extinction followed by hybridization produced prime-numbered period life cycles of 13 and 17 years.

Some entomologists have also noted that certain 17-year cicadas can emerge from the ground four years early to become 13-year cicadas and hence it is possible that the 13-year variety evolved out of the 17-year. I think it more likely that some 17ers emerge four years early simply because if they emerge any earlier or later, they will be alone--trilling away in the trees with no hope of mating because no other magicicadas have emerged yet.

This demonstrates that the periodicity isn't just a blind process--that a certain amount of choice is involved for each and every magicicada as to when it is right to emerge from the ground. Each cicada simply knows when it's time--as if by magic.