While some parts of the South are dealing with (or bracing for) record floods, others are anticipating another kind of flood: a flood of cicadas. A brood that emerges every 13 years started appearing late last month in southern Alabama, and the insects have since appeared in Georgia, Tennessee, Mississippi, and a number of other southeastern states. The cicadas will mate with each other *en masse* before dying, which frankly seems like a pretty hasty end after 13 years underground. But why every 13 years (or, with some other species, 17 years)? How did Mother Nature cook up that number?

Several years ago, students at the University of Wisconsin-Madison used mathematical and spatial models to compare a number of possible explanations. They concluded that predator satiation (appearing in large enough numbers to ensure the population survives any predators) and the effects of competition between different broods of cicadas were the two strongest factors in the evolution of "cicadas' periodicity." However, the models were not able to explain why the periodicity settles on prime numbers. (Scientists assume prime numbers play a role in some way because when cicadas that normally emerge every 17 years are tricked into surfacing earlier, they appear in 13 years.) One possibility, the authors say, is that the counting mechanisms "needed to synchronize emergence constrain periods to prime numbers." Whether Mother Nature just enjoys messing with zoologists' heads was not addressed.