It is fascinating how the cicadas of Brood X manage to emerge all at the same time, after spending 17 years underground! Why did they wait for so long? The answer might come from natural selection.

Cicadas' predators, creatures like birds and rodents, have a typical life cycle of two to six years. While these predators gorge themselves and reproduce like mad when cicadas emerge in huge numbers, their growth is limited in time, since a whole generation of predators will die before the next cicada emergence will arise. If cicadas had a shorter evolutionary cycle, they would constantly feed their predators, whose population would then become a threat to the cicada population.

Cicada cycles often coincide with prime numbers - say, every 13 or 17 years. This allows them to minimize the chance of two broods waking up simultaneously, which would carry the risk of both broods disappearing forever, if there were a large number of predators that year.

The noise you hear consists of two components: a narrow-band frequency distribution located at around 1.2 kHz and a wider band at 6 kHz. The lower distribution corresponds to thousands of males producing their mating call, which sounds like "eeeeeeeeeehooo" when you are close enough to hear an individual specimen. At large scale and from a distance, the superimposition of all these individual calls just produces noise, as illustrated by the first slider. The second frequency distribution is the regular cicada noise, which often comes in waves. These waves are illustrated in the right-most sliders.