I vaguely remember watching the same documentary, so I don't doubt you. But surely they would emerge at the same time every 21 years? Edit: I looked it up, its

I vaguely remember watching the same documentary, so I don't doubt you. But surely they would emerge at the same time every 21 years? Edit: I looked it up, its 13 and 17 years, giving a 221 year overall cycle. I guess it only really "matters" that the years are coprime. https://en.wikipedia.org/wiki/Periodical_cicadas https://en.wikipedia.org/wiki/Periodical_cicadas