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Cicadas return: How do they know when it's time to emerge? (+video)

17-year cicadas spend almost their whole lives burrowed under the ground, suckling on the roots of trees. How do they keep track of the time?


After 17 years, they're back! Cicadas are popping up between the Carolinas and Connecticut and there's nothing you can do about them. (May 6)
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Try this backyard science project: Find a tree, dig a hole near it, bury yourself underground, and wait exactly 17 years before you reemerge, surviving off the juices from your tree's roots in the meantime. You are not allowed to use any kind of clock. 

Pretty difficult, wasn't it? 

As everyone who lives anywhere near US Interstate 95 is finding out, billions of cicadas are popping out of the ground as if on cue, seeing the sky for the first time since 1996.

How do they keep track of the time when they are underground? Do they have some sort of countdown timer in their little arthropod brains? 

That's what most researchers thought until 2000. But then University of California, Davis, researcher Richard Karban advanced a novel idea: Cicadas rely on trees to keep time.

To test this hypothesis, Dr. Karban devised a clever experiment. By manipulating the amount of light that peach trees in a climate-controlled room in Davis, Calif., received, Karban manipulated the trees to blossom twice a year. Then he transplanted a population of 15-year-old cicadas that normally emerge every 17 years, and attached them to the roots of the double-blossoming trees.

Fooled by the accelerated trees, the cicadas emerged a year early.

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