In 2013, cicadas emerged from the ground after not being seen since 1996. Scientists have no idea how they were knew when it was the right time to leave the ground
The adolescents emerge from their dark little homes, after a 17-year slumber. They arrive noisy, in a frenzied search for a partner. No, they’re not teenagers on the way to their school prom. They’re cicadas.Periodical cicadas are, for the most part, quiet and solitary, spending their days burrowing underground. They’re long-lived for insects and don’t mature until their 17th year. In the summer of this year, however, something happens. En masse, they rise from the ground as a plague of lust, in search of mates.
Two or three weeks later, they die, leaving behind the products of their
furtive love-making. The hatchlings worm their way into the soil to
start the cycle again. The question is: how do they do it? If you were
stuck in the dark for a few days, you’d probably have trouble telling
the time. These guys don’t. In 2013, they hadn’t been seen since 1996.
But that summer, billions of Magicicada septendecim appeared, bang on
schedule, to make cicada babies across the eastern U.S.
It gets weirder. While 17-year cicadas hit the north-east states, the more south-easterly states are struck by an invasion that takes place every 13 years. The connection between 13 and 17? They’re prime numbers. This has led scientists to speculate that the cicadas’ life cycles evolved to avoid coinciding with those of their predators. Both 13 and 17 have only two factors (one and themselves). So, cicadas stayed out of sync with foes that had two- or three-year life cycles. In fact, they avoided every predator with a life cycle longer than one year and shorter than their own. A neat trick, but it doesn’t explain how they keep track of the time.