Enduring Question of Cicada

Cicadas and prime numbers sound unrelated, but they're connected in a way that has puzzled biologists and mathematicians for a while.

• Cicadas are insects in the Hemiptera order, known for their loud songs and odd life cycles.
• One group, the periodical cicadas, has life cycles that run on prime numbers. (A prime is a natural number above one that divides only by one and itself, like 2, 3, 5, 7, 11.)
• Their two main cycles are 13 and 17 years, both primes.
• They come out of the ground all at once, breed, lay eggs, and die. The eggs hatch, the larvae burrow underground and feed on plant roots, and they come back up 13 or 17 years later.
• Nobody fully knows why the cycle is prime. The leading hypothesis is predators. A prime cycle rarely lines up with predators' cycles, which are usually shorter and not prime, so predators can't count on cicadas as a regular meal, and more cicadas survive.
• A prime cycle also makes it less likely that broods (groups that come out in different years) interbreed, which keeps genetic diversity up within the population.