system
Given the following post, respond as if you're commenting on Reddit:
human
How did the cicada's mating cycle come to be 17 years long? Wouldn't a cycle of only a few years be necessary to avoid lining up with predatory mating cycles?
gpt
It's very beneficial to take a prime number as the length of your mating cycle. If you'd take 4 years for example, any predator that has a cycle of 1, 2 or 4 would eat a large part of the population. If you take a prime number like 7, then predators with a cycle of 2, 3, 4, 5, 6 will all miss you. 

The bigger the prime number the less you will be eaten. 19 would probably be an even better cycle, and 23 even better.

If a predator's cycle happens to synchronize with that of the cicada, they will probably evolve to have a different cycle again (or they will shift their cycle by one year.) 18 years won't make it better though, so they will probably choose 19 years.