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If you live on the east coast
of the United States,

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you've spent the last
17 years of your life

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walking, eating and sleeping

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above a dormant army of insects.

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These are the cicadas.

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Every 17 years,

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billions of them emerge
from the ground to do three things:

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molt, mate and die.

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There are 15 different broods
of cicadas out there,

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grouped by when they'll emerge
from the ground.

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Some of these broods
are on a 13-year cycle,

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others are on a 17-year clock.

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Either way, the cicadas live underground
for most of their lives,

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feeding on the juices of plant roots.

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When it's time to emerge,

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the adults begin to burrow
their way out of the ground

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and up to the surface,

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where they'll live for just a few weeks.

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During these weeks, though,

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everybody will know
the cicadas have arrived.

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There will be billions of them.

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And they're loud.

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Male cicadas band together
to call for female mates,

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and their collective chorus
can reach up to 100 decibels --

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as loud as a chain saw.

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In fact, if you happen to be using
a chain saw or a lawn mower,

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male cicadas will flock to you,

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thinking that you're one of them.

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Now, like most things in nature,

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the cicadas don't arrive without a posse.

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There are all sort of awesome
and gross predators and parasites

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that come along with the buzzing bugs.

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Take the fungus Massospora for example.

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This little white fungus buries itself
in the cicada's abdomen

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and eats the bug alive,

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leaving behind its spores.

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When those spores rupture,

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they burst out of the still-alive cicada,

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turning the bug into a flying
saltshaker of death,

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raining spores down upon
its unsuspecting cicada neighbors.

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But while we know pretty precisely

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when the cicadas will arrive
and fade away,

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we're still not totally certain of why.

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There are certain advantages

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to having your entire species
emerge at once, of course.

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The sheer number of cicadas
coming out of the ground

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is so overwhelming to predators,
it is essentially guaranteed

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that a few bugs will survive
and reproduce.

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And since cicadas emerge
every 13 or 17 years,

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longer than the lifespan
of many of their predators,

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the animals that eat them don't learn
to depend on their availability.

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But why 13 and 17 years,

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instead of 16 or 18 or 12?

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Well, that part no one really knows.

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It's possible the number
just happened by chance,

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or, perhaps, cicadas
really love prime numbers.

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Eventually, the cicadas
will mate and slowly die off,

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their call fading into the distance.

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The eggs they lay
will begin the cycle again,

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their cicada babies burrowing
into the earth, feeding on plant juice,

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and waiting for their turn
to darken the skies

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and fill the air with their songs.

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In 17 years, they'll be ready.

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Will you?