They spend 17 years hidden beneath the forest floor – then all emerge together in one extraordinary, glorious eruption

Somewhere beneath the forest floor of the eastern United States, a clock is ticking. It isn’t made of gears. It doesn’t have hands. And it doesn’t measure hours, minutes or seconds. Instead, it counts years. 

For 13 or 17 years, the nymphs of the periodical cicada live underground, feeding on fluid from plant roots. Then somehow, when the time is right, they all climb to the surface together and transform into adults.

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The really strange thing is the number they count to. 

Thirteen and 17 are both prime numbers; numbers that can only be divided by themselves and one. North America has seven species of periodical cicada, and they all have either a 13-year or a 17-year cycle. 

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So, the question is, why? 

One possibility is that prime-numbered cycles reduce the chances of different cicada populations repeatedly emerging together. A 13-year and a 17-year cycle only coincide once every 221 years (13 × 17 = 221), potentially reducing competition for resources, such as suitable sites for laying eggs.

Another theory is that prime-numbered cycles help the cicadas avoid predators. Suppose the cicadas had a 12-year life cycle, and there were predators with two, three or four-year life cycles. The cicadas’ emergence would repeatedly coincide with peaks in predator populations. Emerging every 13th or 17th year makes it harder for predators with shorter, non-prime life cycles to synchronise with them.

There are also different broods of periodical cicadas. These are regional populations with their own emergence schedules. Brood I, for example, has a 17-year cycle and last emerged in 2012, whilst Brood XXII has a 13-year cycle and last emerged in 2014. 

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The last great cicada double act happened in 2024, when billions of the 17-year Brood XIII and billions of the 13-year Brood XIX emerged together across the Midwest and southeastern United States. Forget Amazon Prime Day, this was Cicada Prime Day! Before this, the last time these two broods emerged in the same year was 1803, when Thomas Jefferson was president. The next time will be 2245. 

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So, if you missed Cicada Prime Day, I’m afraid you’ll have to wait a while. 219 years, to be precise. 

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