Periodical cicada: The mathematical mystery that waits underground

In eastern North America, one kind of cicada spends 13 or even 17 long years living quietly underground, then emerges all at once — the periodical cicada. Its life is like a mathematical puzzle that has fascinated scientists for over a century. Let's meet this "waiter of time".

Why 13 and 17?

The periodical cicada's young stay underground for 13 or 17 years, and over a few weeks billions crawl out and turn into flying adults. Scientists noticed that 13 and 17 are both "prime numbers" — divisible only by 1 and themselves. That means no predator's life cycle can synchronise easily with them; a predator breeding every 4 years, or another every 6, can hardly line up with 13 or 17. This is a clever way to keep predators from "lying in wait".

Why they come out together

Billions of cicadas appearing at once looks risky, but it is a clever tactic called "predator satiation": however much the predators eat, they cannot finish the swarm, and plenty of cicadas survive to leave offspring. That overwhelming number is exactly how the periodical cicadas have lasted.

Its "broods" and its life

The emergences in different years are numbered as different "broods" by scientists — for example "Brood X of the 17-year cicadas". They spend years below ground sipping root sap, and the adults live just a few weeks: sing, mate, lay eggs, done. That brief, deafening summer is a natural wonder unique to the American East.

A note for grown-ups

You could talk with a child about why it hides for so long: waiting all those years means predators can never "wait for" it. Ask a child to think — if it came out every year, would it be eaten more easily? That is a gentle, playful way to think about strategy and survival.