Why 8 Eyes Are Better Than 2 (…If You're a Spider 🕷) (1)
- Oh, hello, look at you.
Oh, aren't you a deadly little hunter?
So fuzzy, so cute.
No, no!
Zoom in you fools.
That's better.
Oh, look at those eyes.
Oh, you such a cutie.
Who's your cute little spider?
That's right you are, yeah!
Hey, smart people, Joe here.
Behold my children.
I mean, my pets.
I used to hate spiders okay, but jumping spiders,
these are the ones that made me change my mind.
Like I legitimately love spiders now.
It's because of these guys,
and it's the eyes, that's what did it for me.
They're so curious.
They look up at you, they follow you.
They're genuinely so stinkin' cute.
But beyond their cuteness,
jumping spiders possess one of the most advanced
and highly tuned visual systems in the animal kingdom.
I remember a few years ago I read an article
that said Jumping spiders might be able
to see the moon in detail.
I was like, are you kidding me?
Nothing else that's this small
can see anywhere near that kind of resolution.
So how do they do it?
I made this video to find the answer to that question.
We are gonna get up close and personal with these guys
so that we can see some of the really cool things
that they can do.
And how their one-of-a kind visual system
makes that possible.
And I visited a lab
to see how real scientists study these things.
(light music)
Okay but first things first, what is a jumping spider?
Spiders come in a lot of shapes and forms,
but about one in eight known spider species
is a jumping spider,
making them the biggest family of spiders on earth.
There are more jumping spider species
than species of mammals.
They're officially known as salticidae or salticids.
That comes from the Latin word to jump.
And jumpers come in a lot of shapes in sizes too.
Unless you live in Antarctica
there's probably a jumping spider near you right now.
You should be honored.
So this is kind of a weird thing to study.
- Usually when I tell people that I study spiders
I get sort of a very shocked reaction.
First, people didn't even really know
that there are people that do spend a lot of their time
watching spiders and seeing how they behave.
But I think especially in the case of jumping spiders,
because of their large charismatic eyes,
a lot of people start to become really intrigued
and ask questions and you know what do they see?
- What is it like to be a jumping spider?
What's your day like?
What's your environment like?
What do you do?
- So the sensory sort of world of jumping spiders
is very different than ours.
Well a jumping spider detects
a lot of cues in its environment through vibrations.
They're also tasting things with their feet.
Their visual world is also very different
because us and many other vertebrates
we have two eyes that look around our world,
but their eyes are set up very very differently.
- Okay, most spiders have eight eyes.
It's like the second thing you learn
after spiders have eight legs.
But most types of spiders
don't actually rely on vision as their number one sense.
They detect vibrations in their surroundings
using tiny hairs,
or they smell chemicals in their environment.
But vision is number one for jumping spiders,
it's what makes them such deadly hunters.
And we're about to put that to the test
with this little cricket.
Circle of life guys.
The idea is that this spider is going to eat this cricket
and we're gonna get the pounce in slow motion.
All you gotta do okay is just go eat that okay, go eat that.
It's right there in front of you, just take it.
Please jump, please jump, please jump.
Oh, I hate you so much.
Please back up.
Oh my god, oh my god, go.
Yes, yes, yes, yes, yes. (dramatic music)
Oh my gosh, that took like a whole day.
(Joe laughs)
When your hunting strategy
involves flinging yourself at your prey,
you need eyes with really good depth perception,
sensitivity, and detail.
Now just like a camera, there's usually a trade off.
More sensitivity typically means less detail and vice versa.
- Basically the more light
that a single cell or single photo receptor can take in
it reduces a resolution.
So if you sort of think of it
like a TV screen with the number of pixels you have,
if you have really large pixels,
each individual one could be bright
but less pixels make up the whole image.
There's constantly a trade off
between how many photons you can take in
versus how small details you can resolve
in your environment.
- If you want more sensitivity
you can make bigger sensory cells to sample a lot of light
but then you can't fit as many in your eye
so you lose detail.
If you want more detail you make your sensors smaller
so you have more of 'em, but then they're less sensitive.
Different animals have evolved different kinds of eyes
to solve this trade off.
Insects obviously have very tiny peepers,
but they solved the detail problem using compound eyes.
Every facet on a compound eye
acts kinda like its own eyeball.
So the more they pack in their head, the more detail.
Each one only samples a super tiny area,
but they add up to give a kind of mosaic view of the world.
Dragonflies have pretty much
the best compound eyes out there
with around 30,000 of those little units per eye.
To get that level of detail though,
their head is like all eyes.
They've run out of eye real estate, that's their limit.
For a compound eye
to see as much detail as the human eye can,
it'd have to be the size of a basketball
which would make it pretty hard to fly.
Humans and other animals
went for a totally different solution.
Round eyes with a lens up front
that focuses light on a big layer of cells at the back.
Packing lots of those cells back there
means a crisp detailed picture.
We've got hundreds of millions of those light sensing cells
in our eyes.
And some birds of prey have like 10 times more than we do.
Things with backbones and skulls can support huge eyes,
but jumping spiders are not huge.
Most of them can fit on like a pencil eraser.
You can't fit big eyes or millions of light sensing cells
in this guy.
Yeah when it comes to detail and sensitivity
they have the best eyes of anything
without a backbone on land.
But they don't have compound eyes
and they don't have eyes like humans.
So the big question is how do they do it?
It all starts with those two big adorable eyes up front.
They're not actually round,
they're tubes, kinda like telescopes.
Lens on the front to focus light onto a retina in the back.
- The front two eyes, they're called the principle eyes.
But with jumping spiders,
although their eyes are big relative to their body size,
they're still very, very, very small.
- To get high detail out of those tiny eyes
these guys have a really cool trick up their eight sleeves.
They can move those telescope eyes inside their heads.
- Each eye tube has its own set of muscles,
and what they can do is they can independently
move each of them.
- Jumping spider eyes are only big enough
to fit a few hundred light sensing cells,
but as each little telescope eye moves
it samples little bits of the larger image,
almost like shining a flashlight
at different parts of a picture
until you can make out the whole thing.
So they get amazing vision in a fraction of the space
that around eye would require.
Telescopes for eyes.
Are you kidding me?
But that telescope scanning trick is only part of the story.
Wonder if these guys would chase a laser pointer
like a cat does?
There's only one way to find out.
Oh, what'd you see?
See this over here?
What's this?
What's this?
Oh, you see it?
Over here.
Over here.
Over here.
Look what's this?
Oh, what's this?
Now most jumping spiders only have
two types of color sensing cells
in their big old cute main eyes.
Cells sensitive mainly to green light
and cells sensitive mainly to ultraviolet light.
But weird physics happens to light
when you're at really small scales.
And jumping spider eyes are actually built
to correct for that.
See different wavelengths or colors of light,
they get bent by a lens at slightly different angles.
It leads into what's known as chromatic aberration.
Now this is not that big of a deal in a big eye like yours,
but in an itsy bitsy spider eye
it means different colors of light are coming into focus
at totally different distances from the lens.
- And when we find
when you look inside the jumping spider retina
you'll find photo receptors
that are more sensitive
to UV or ultraviolet wavelengths closer,
and then deeper or potentially longer wavelength.
Photo receptors that are sensitive to certain colors
are actually at different depths.
So all colors can be in focus at the same time.
- Like that's ridiculously cool.
They can even use those stacked cells
to see how out of focus different parts of an image are
in different layers.
And they can use that to calculate depth perception.
We need two eyes to get depth perception,
but jumping spiders can judge distance with just one eye.
Did you see what happened when I shined that laser pointer
behind the spider?
It always turns to point its two big telescope eyes
at whatever is interesting,
and that is what those other six eyes are for.
These other eyes
are a jumping spider's motion detection system.
- [Alex] The motion detecting eyes called secondary eyes,
they have a nearly 360 degree view
around the hemisphere of the spider.
- Those secondary eyes are much more simple
than the spiders to telescope eyes.
They don't see color, they're super low resolution,
but they are great at sensing motion.
- So you can't sneak up on a jumping spider.
- [Joe] It just automatically is like motion, turn body.
- Turn body.
- Wow.
Whenever those six other eyes detect motion,
the spider instinctually turns its big eyes
to face it and get a more detailed picture.
As soon as I learned about this,
something really big just clicked in my brain,
because these eight tiny spider eyes
do the same thing that we do with two eyes.
We use the center of the light sensing part of our eye
to make out detail and color.
On the other hand, most of our peripheral vision
is really low detail but really sensitive to motion.
Jumping spider eyes do the same thing.
Those telescope eyes make out detail and color,
but all those secondary eyes,
well they sense motion without much detail.
They've divided all that work up into eight parts
instead of two like us.
All eight eyes combined,
jumping spider eyes see better than dragonflies,
as good as pigeons,
and actually about one fifth as good as us.
All with peepers that are no bigger than a pinhead.
But how do we know any of this anyway?
I mean it's not like you can give a spider an eye test.
Well, actually yes, you can.
Show me where you keep your babies.
This is like the Ritz Carlton of spider habitats.
- Oh yeah.
Show them anything you want. - You didn't tell me
spiders are just watching TV all the time in here.
Can we go see it?
I wanna give a spider and eye exam.
- Yeah let's do it.
- [Joe] To get a spider ready for their eye exam
