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Cicadas have begun to swarm along the East Coast, following 17 years living underground and sucking on tree roots. But the mechanisms and evolutionary reasons behind the timing of the insects’ emergence remain mysterious, according to Nature.

The cicadas currently climbing up out of the ground in the northeastern and mid-Atlantic coast are part of Brood II, one of 12 broods that emerge on a 17-year cycle. Three other broods further south emerge every 13 years.

Scientists believe that the mass emergences are meant to overwhelm predators—they may be able to get some good mouthfuls of cicadas, but they won’t be able to decimate the population. Most of the emerging cicadas manage to mate and lay their eggs before their lives rapidly come to an end. The cicadas may emerge at prime intervals because it is less likely that predators will synchronize their cycles to the insects....