A cicada spends seventeen years underground, then comes up with all the others

Periodical cicadas live as nymphs below ground for 13 or 17 years, feeding on the sap of roots. Then a whole brood emerges within days, mates, and is gone in weeks.

Magicicada, in the east of North America, is divided into broods, each locked to a calendar. The nymphs drop from the eggs into the soil, find a root, and stay there for thirteen years or for seventeen, depending on the species. In the spring of the final year, when the ground warms, they come up together, climb, shed, and sing. The numbers are the defence. Birds eat until they cannot, and most of the cicadas still make it to a mate. A few weeks later the adults are dead and the next generation is in the soil, and the brood will not be seen again until the calendar comes round.

Thirteen and seventeen are prime, and one explanation is that a prime cycle is hard for a predator to match with its own. That is a hypothesis, not a result. What is measured is the cycle itself, counted across generations and mapped by brood across the continent. The insects do not know the year. The year is built into how long a nymph takes to be ready, and the ones that are early, or late, come up into a world with no mates and no cover.