Periodical cicadas usually have 13- or 17-year life cycles. Their development is so synchronized that practically no adults are present in the 12 or 16 years between emergences. When these cicadas do come out of their underground homes, they appear in huge numbers and create a cacophonous, throbbing din during their brief period of mating frenzy in the open air.

Curiously, 13 and 17 are both prime numbers, evenly divisible only by themselves and 1. The fact that periodical cicadas emerge after a prime number of years could be just a coincidence. Or it might reflect some sort of evolutionary pressure that leads to prime-number cycles.