​The 17-year cicada


​The 17-year cicada

No sound is so associated with sultry summer days in Southern France as a loud buzzing insect chorus. These cigales are not crickets, but cicadas. Whilte the cricket stridulates, chirping by striking their wings against each other, the cicada is a monotonous drum machine. On its abdomen, it has a membrane, the tymbal, that makes a deafening noise when vibrated.

The mediterranean cicadas know how to draw attention, but leave it to their American cousins to build up a hype. For sixteen years, the periodical cicadas are quiet as a mouse. As larvae, “nymphs”, they live hidden underground. But in the spring of the 17th year, they crawl by the millions out of the ground to find a place to moult, buzz, mate and lay eggs. For several weeks, whole forests are invaded by these winged aliens. And then, they go silent again.

It might seem like a biblical plague, but it’s a feast for many animals: birds and squirrels, spiders and turtles find these clumsy, stingless bugs an easy treat. This is actually a surival strategy of the cicada, called predator satiation. The insectivores do their best, but at some point their bellies are full. Enough cicadas will survive to start a new generation.

In different areas, the cicadas emerge in different years. These pockets are called broods, and scientists have recognized 15 different ones, including three with a 13 year life cycle. Last year, it was Kansas’ turn. This year, Brood V emerges in West Virginia, Ohio and surrounding states.

13, 17. ‘Prime numbers!’, the mathematician would exclamate. Correct, but why? The famous evolutionary biologist Stephen Jay Gould had an interesting answer. Predator populations also tend to undergo cycles. The emergence of a hypothetical 12-year cicada can coincide with the rise of predators with life cycles of 2, 3, 4 and 6 years. Eventually, the predators could adapt to this, and the effect of predator satiation disappears. Extinction is near! With a long, prime lifespan, the 13- and 17 year cicadas avoid these predator peaks.

Seventeen years, it’s even for humans hard to imagine. The currently emerging brood is the offspring of the cicadas of 1999. A time when Britney Spears dominated the charts and another bug, Y2K, was the main worry. A lot has happened since then. Imagine how different the world will be in 2033, when the new generation of Brood V emerges.