Some cicadas wait 13 or 17 years underground, emerging only in years divisible by a prime
Periodical cicadas spend 13 or 17 years as nymphs feeding underground before emerging en masse to breed and die within weeks. Both life-cycle lengths are prime numbers, and biologists have proposed this makes it harder for predators, or parasites with their own multi-year cycles, to reliably synchronise with the cicadas' emergence, since a predator cycle of 2, 3, 4, 5, or 6 years can only coincide with a prime-numbered cicada cycle very rarely.