I saw the 17-year cicadas in New York in 1996 - now they're back (20 may 13)
Why do cicadas pupate and hatch after 17 years? Why is there another species whose breeding season is every 13 years? They are prime numbers. It would be difficult for a predator to evolve to take advantage of these cycles to feast on cicadas. An organism with a four-year cycle would have some chance of parasiting a cicada whose population suddenly expanded each 16 years. Not 17. Or 13.