How a dragonflys brain is designed to kill DIY Neuroscience a TED series
Translator: Joseph Geni
Reviewer: Joanna Pietrulewicz
Greg Gage: If I asked you
to think of a ferocious killer animal,
you’d probably think of a lion,
and for all the wonderful
predatory skills that a lion has,
it still only has about a 20 percent
success rate at catching a meal.
Now, one of the most successful hunters
in the entire animal kingdom
is surprising:
the dragonfly.
Now, dragonflies are killer flies,
and when they see a smaller fly,
they have about a 97 percent
chance of catching it for a meal.
And this is in mid-flight.
But how can such
a small insect be so precise?
In this episode, we’re going to see
how the dragonfly’s brain is highly
specialized to be a deadly killer.
[DIY Neuroscience]
So what makes the dragonfly
one of the most successful predators
in the animal kingdom?
One, it’s the eyes.
It has near 360-degree vision.
Two, the wings.
With individual control of its wings,
the dragonfly can move
precisely in any direction.
But the real secret
to the dragonfly’s success
is how its brain coordinates
this complex information
between the eyes and the wings
and turns hunting into a simple reflex.
To study this, Jaimie’s been
spending a lot of time
socializing with dragonflies.
What do you need to do your experiments?
Jaimie Spahr: First of all,
you need dragonflies.
Oliver: I have a mesh cage
to catch the dragonflies.
JS: The more I worked with them,
the more terrified I got of them.
They’re actually very scary,
especially under a microscope.
They have really sharp mandibles,
are generally pretty aggressive,
which I guess also helps them
to be really good predators.
GG: In order to learn what’s going on
inside the dragonfly’s brain
when it sees a prey,
we’re going to eavesdrop in
on a conversation
between the eyes and the wings,
and to do that, we need
to anesthetize the dragonfly on ice
and make sure we protect its wings
so that we can release it afterwards.
Now, the dragonfly’s brain is made up
of specialized cells called neurons
and these neurons
are what allow the dragonfly
to see and move so quickly.
The individual neurons form circuits
by connecting to each other
via long, tiny threads called axons
and the neurons communicate
over these axons using electricity.
In the dragonfly, we’re going to place
little metal wires, or electrodes,
along the axon tracks,
and this is what’s really cool.
In the dragonfly, there’s only 16 neurons;
that’s eight per eye
that tell the wings
exactly where the target is.
We’ve placed the electrodes
so that we can record from these neurons
that connect the eyes to the wings.
Whenever a message is being passed
from the eye to the wing,
our electrode intercepts that conversation
in the form of an electrical current,
and it amplifies it.
Now, we can both hear it and see it
in the form of a spike,
which we also call an action potential.
Now let’s listen in.
Right now, we have the dragonfly
flipped upside down,
so he’s looking down towards the ground.
We’re going to take a prey,
or what we sometimes call a target.
In this case, the target’s
going to be a fake fly.
We’re going to move it
into the dragonfly’s sights.
(Buzzing)
Oh!
Oh, look at that.
Look at that, but it’s only
in one direction.
Oh, yes!
You don’t see any spikes
when I go forward,
but they’re all when I come back.
In our experiments,
we were able to see
that the neurons of the dragonfly
fired when we moved the target
in one direction but not the other.
Now, why is that?
Remember when I said that the dragonfly
had near 360-degree vision.
Well, there’s a section
of the eye called the fovea
and this is the part
that has the sharpest visual acuity,
and you can think of it as its crosshairs.
Remember when I told you the dragonfly had
individual precise control of its wings?
When a dragonfly sees its prey,
it trains its crosshairs on it
and along its axons
it sends messages only to the neurons
that control the parts of the wings
that are needed
to keep that dragonfly on target.
So if the prey is
on the left of the dragonfly,
only the neurons that are tugging
the wings to the left are fired.
And if the prey moves
to the right of the dragonfly,
those same neurons are not needed,
so they’re going to remain quiet.
And the dragonfly speeds toward the prey
at a fixed angle that’s communicated
by this crosshairs to the wings,
and then boom, dinner.
Now, all this happens in a split second,
and it’s effortless for the dragonfly.
It’s almost like a reflex.
And this whole incredibly efficient
process is called fixation.
But there’s one more
story to this process.
We saw how the neurons
respond to movements,
but how does the dragonfly know
that something really is prey?
This is where size matters.
Let’s show the dragonfly a series of dots.
Oh, yeah!
JS: Yeah, it prefers that one.
GG: Out of all the sizes,
we found that the dragonfly responded
to smaller targets over larger ones.
In other words, the dragonfly
was programmed to go after smaller flies
versus something much larger, like a bird.
And as soon as it recognizes
something as prey,
that poor little fly
only has seconds to live.
Today we got to see
how the dragonfly’s brain works
to make it a very efficient killer.
And let’s be thankful
that we didn’t live 300 million years ago
when dragonflies were the size of cats.