Cicadas are loud insects that spend most of their life in their nymph state, burrowing in the ground. One genus native to North America, Magicicada, contains species that remain exactly 13 or 17 years burrowing in the ground and feeding on plant roots. In the case of, for example, Magicicada septendecim, all nymphs will finally reach their adult stage every 17 years, filling the woods with millions of noisy insects all at once. These animals die as soon after they have mated and laid their eggs, ensuring another generation will emerge after spending 17 years in the ground.

Now the question:

Why exactly 13 or 17 years?

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