Why (some) cicadas only come out once every 17 years

Periodical cicadas spend 99.5% of their life underground.

Most of them live in large broods that go underground for cycles of 17 or 13 years — though some have different timings.

During these periods, they live as nymphs that feed on fluids from the roots of trees.

When the time is right, the individuals in a brood emerge, all at once, and live overground for only a few weeks.

Cicadas likely developed this evolutionary strategy to avoid predators. Using prime numbers for their emergence makes it difficult for predators to adapt.