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 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 fifteen 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 will begin to burrow their way

out of the ground

and up to the surface,

where they will 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 salt shaker 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 that 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?