[♩INTRO]
So you’re walking along, minding your own
business,
when you notice something out of the corner
of your eye and look up.
That’s when you see thousands upon thousands
of spiders
on long silk balloons falling from the sky
all at once.
You’ve just witnessed one of the most incredible,and
terrifying,
natural phenomena on the planet: spider rains.
For a long time, scientists assumed that,
like kites,
ballooning spiders can fly because their silken
threads
generate enough lift to ride currents of air.
But according to a study published in Current
Biology this week
by researchers at the University of Bristol
in the UK,
they don’t actually need a breeze at all.
Turns out, spiders can fly using the electricity
in our atmosphere.
Spider ballooning was first documented by
an English naturalist
in the 17th century, and ever since, scientists
have been
trying to figure out exactly what they’re
doing and why they’re doing it.
A lot of the time, the ballooners are baby
spiders
looking for a place of their own to settle
down.
They can reach altitudes of almost 5 kilometers
and fly for hundreds of kilometers.
Talk about putting some space between you
and your parents.
But instead of loading up their Volvos and
moving to Montana,
to take off, the spiders find somewhere high
up,
then stand tall, raise their rears,
and emit thin, meter-long silk threads in
the shape of a sail.
When they let go, they’re pulled into the
air with surprising speed,
even on calm days.
And that speed is one of the things that has
never quite added up
with the idea that these spiders ride the
wind.
Biologists have seen spiders ballooning when
winds are almost imperceptible,
or even when it’s raining.
And the wind hypothesis doesn’t explain
how the spiders eject their silk
so forcefully without the help of their legs,
or how the strands maintain a fan-like shape
without tangling.
So the team from the University of Bristol
decided to test something
no one else had: whether the spiders can ride
electricity.
The idea that electrostatic forces provide
the necessary lift
has been around for centuries, but no one
ever really looked at it.
Then, in 2013, a physicist from the University
of Hawaii
worked out some of the theoretical details.
He released his paper as a preprint that was
never officially published,
but the authors of the new study thought it
was worth investigating.
The whole thing hinges around the fact that
no matter what the weather is,
there’s a difference in electric charge
between the ground and the sky that creates
an electric field.
So if the spiders’ silk picked up some static
charge,
those threads could be pushed by the electric
field.
Since like charges repel one another, the
charge of the ground,
or whatever the spider is standing on, would
propel the silk out and up.
And enough pushing could fling the spider
into the sky.
But since the 2013 paper was purely theoretical,
the new study’s authors decided to put it
to the test.
They took ballooning spiders and placed them
on a small cardboard pedestal
in a special chamber designed to have no electric
field or air movement.
Then they induced electric fields of different
magnitudes,
and watched what the spiders did.
Even in the complete absence of wind, the
spiders began
to get into that rump-raising position that
sets them up for ballooning.
And with a strong enough field, they started
to spin silk, and even flew.
Once airborne, the researchers could make
the spiders rise or fall
just by turning the electric field on or off.
An earlier study, published last month in
PLOS Biology,
noted that these spiders seem to test the
wind with their legs
before they start to spin their silk sails.
And this week’s study found that the hairs
on the spiders’ legs
moved in response to changes in electric fields,
too.
But those hair movements were different from
the way they moved
in response to wind, which means the spiders
might be feeling around
for both of those things.
Riding electricity could explain some of the
weirder aspects of their flight
like how they take off on seemingly windless
days or in the rain.
But most of the time, air isn’t completely
still,
so the spiders probably use a combination
of electricity and wind to fly.
There are still some parts of this left to
figure out, though
like how the spiders’ silk becomes charged
in the first place,
or whether they can control their flight to
decide where to land.
Learning more about how spiders fly can help
biologists predict
when they’re going to do it,
and get a better understanding of their ecological
needs.
And it might also make it easier to predict
those rare episodes of spider rain.
Because I don’t know about you, but if ten
thousand spiders
are going to land in my neighborhood,
I’d would prefer to know that that’s going
to happen before it happens.
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[♩OUTRO]
