Periodical Cicadas

Most insects live for only a few weeks or months. Periodical cicadas, found in the eastern United States, are a remarkable exception. These insects spend either 13 or 17 years underground as young nymphs, feeding on liquid from tree roots. Then, in the spring of their final year, millions of them come out of the ground at almost the same time. Within a few weeks, the adults climb trees, shed their skins, mate, lay eggs, and die.

Why do so many cicadas appear together? The most widely accepted explanation is a strategy known as predator satiation. Birds, squirrels, and other animals eat huge numbers of cicadas, but there are so many insects that the predators simply become full. As a result, most cicadas survive long enough to reproduce.

Scientists have also wondered why the cycles last exactly 13 or 17 years. Both are prime numbers, which can be divided only by one and by themselves. One hypothesis is that prime-numbered cycles make it unlikely that cicadas will emerge in the same year as a predator whose population rises and falls in a shorter cycle, such as every two or three years. This idea is interesting, but it has not yet been tested directly.