Cicadas: The dormant army beneath your feet - Rose Eveleth

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Every 13 or 17 years, billions of cicadas emerge from the ground to molt, mate and die. Adult cicadas only live a few weeks above ground, but you'd be hard pressed to ignore them -- they are extremely loud! Rose Eveleth explains everything you need to know about these noisy insects, admitting that there's still some things we just don't understand.

Lesson by Rose Eveleth, animation by Eli Enigenburg.

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

you've spent the last
17 years of your life

walking, eating and sleeping

above a dormant army of insects.

These are the cicadas.

Every 17 years,

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

molt, mate and die.

There are 15 different broods
of cicadas out there,

grouped by when they'll emerge
from the ground.

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

others are on a 17-year clock.

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

feeding on the juices of plant roots.

When it's time to emerge,

the adults begin to burrow
their way out of the ground

and up to the surface,

where they'll live for just a few weeks.

During these weeks, though,

everybody will know
the cicadas have arrived.

There will be billions of them.

And they're loud.

Male cicadas band together
to call for female mates,

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

as loud as a chain saw.

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

male cicadas will flock to you,

thinking that you're one of them.

Now, like most things in nature,

the cicadas don't arrive without a posse.

There are all sort of awesome
and gross predators and parasites

that come along with the buzzing bugs.

Take the fungus Massospora for example.

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

and eats the bug alive,

leaving behind its spores.

When those spores rupture,

they burst out of the still-alive cicada,

turning the bug into a flying
saltshaker of death,

raining spores down upon
its unsuspecting cicada neighbors.

But while we know pretty precisely

when the cicadas will arrive
and fade away,

we're still not totally certain of why.

There are certain advantages

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

The sheer number of cicadas
coming out of the ground

is so overwhelming to predators,
it is essentially guaranteed

that a few bugs will survive
and reproduce.

And since cicadas emerge
every 13 or 17 years,

longer than the lifespan
of many of their predators,

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

But why 13 and 17 years,

instead of 16 or 18 or 12?

Well, that part no one really knows.

It's possible the number
just happened by chance,

or, perhaps, cicadas
really love prime numbers.

Eventually, the cicadas
will mate and slowly die off,

their call fading into the distance.

The eggs they lay
will begin the cycle again,

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

and waiting for their turn
to darken the skies

and fill the air with their songs.

In 17 years, they'll be ready.

Will you?