Here’s what compound eyes really do — and why flies see you in slow motion. A few centuries ago, scientists believed insects saw thousands of tiny, repeated images — like a kaleidoscope of candle flames. But that’s not how compound eyes work.
TRANSCRIPT
If you close your eyes and try to think about how an insect sees the world, you might picture something like this.
Hollywood has used it as a shorthand for “bug vision” for decades, but it’s an idea that goes back centuries.
On a spring day in 1694, Antonie van Leeuwenhoek – the father of microbiology – used a magnifying lens to look at a candle through the dissected eye of a dragonfly.
But instead of seeing 1 candle flame, he saw hundreds of tiny flames, repeated over and over.
But spoiler alert — this is not how insects see.
Hi, I’m Niba, and today we’re going to explore how insects really see the world. We’ll make our own camera to demonstrate how our eyes work, figure out what’s going on inside the insect brain, and try to understand how insects detect motion and experience color.
Welcome to Big Ideas, a new show from the team behind Deep Look. While Deep Look zooms in on one small animal, Big Ideas zooms out, answering the big questions about how animals survive.
Okay, let’s get up close and personal with the compound eye. All adult insects with vision have them.
And since insects make up oh, somewhere around 75 to 80 percent of all known animal species on Earth, the compound eye has the distinction of being the most common type of eye in the entire animal kingdom.
But it’s not just insects. Other species have compound eyes, too.
Some crustaceans, like the mantis shrimp, have them. And so do some segmented worms, like the fan worm, that have their compound eyes positioned on a pair of specialized tentacles.
Tentacles that can see.
Now take an even closer look at the insect compound eye.
There’s a collection of hundreds — sometimes thousands — of individual eye units.
One unit is called an ommatidium, which means “tiny eye” in Greek. Two or more are ommatidia. And each one has its own separate lens.
This is different from our eyes. We have two camera eyes, and each eye has only a single lens.
Let’s break this down even further.
Here, light gets focused through this lens onto the back of our eye — the retina — where a bunch of cells called photoreceptors take this light information, turn it into an electrical signal, and send it to our brain to build a picture of the world.
We can see how this works with an old-fashioned device called a camera obscura. Early models were big — an actual room or a tent.
Light comes in through a tiny hole, which then projects an upside-down image of the outside world onto the wall. It was kind of like having a photograph before cameras existed.
Over time, more portable models were invented.
We’ve made our own portable camera obscura, but DIY-style, using cardboard, tracing paper, and a glass lens.
You can totally make this at home. The lens here is like the lens in our eye, focusing the light onto tracing paper in the back — just like how light hits our retina.
And when I look into it — oh wow — it kind of looks like eight-millimeter film. I can push the lens closer or farther away until I get the focus just right.
What’s projected here is an upside-down image.
But when light hits our retina, the photoreceptors send that visual info to our brain, which then interprets the image correctly as right-side up.
Now it’s tempting to think that what’s happening in our camera eye might be what’s happening in a compound eye.
Take the image that each ommatidium creates, stack them side by side, top to bottom, and you might expect to see what old van Leeuwenhoek saw over 300 years ago.
So why then does a fly, with something like 6,000 ommatidia, not see 6,000 repeated images?
Because each ommatidium is only receiving light from a tiny segment of the overall picture — but not the entire picture.
Check out this diagram of a cross-section of an ommatidium.
See this long column here?
It’s deep and narrow, so only a sliver of light — containing the visual information from a really small section of the overall image — can pass through the lens at the top and make its way down to the photoreceptors.
What that means is that each ommatidium can only send a tiny piece of visual information to the insect’s brain, where they’re all stitched together to create a full and complete image.
So an insect with hundreds of ommatidia might be seeing something like this.
Scientists like to think of it like pixels on a TV screen, where each ommatidium is a single pixel representing a small portion of the overall picture.
Add more pixels and you can get a wider field of view. Like the praying mantis, for example. With around 9,000 ommatidia packed onto globular eyes, it has extreme widescreen vision.
You can also add more pixels, but make them smaller, which gives you better resolution — a crisper image.
This is basically what the dragonfly has done. It has around 30,000 ommatidia packed together as close as possible.
It’s a big reason why it has some of the sharpest vision of all the insects.
And it’s no accident that ommatidia can pack together so tightly. It’s in their shape — they’re hexagonal, meaning they have six sides.
That’s the most efficient shape to cover a surface and not waste space.
But there’s a wrinkle to all of this. Cause despite thousands of teeny-tiny hexagons, the dragonfly’s compound eye still ends up producing a low-quality image.
From our perspective, it would look kind of pixelated.
Turns out that even one of the best compound eyes on the planet can’t compete with our camera eye when it comes to creating crystal-clear images.
That’s because we have more room on our retina to densely pack photoreceptors, meaning that we’re sending much more visual information to our brains.
The human eye has millions of photoreceptors. A single ommatidium has around eight, but really they function together like one photoreceptor unit.
So even the mighty dragonfly — with 30,000 ommatidia — really only has 30,000 bits of visual information getting to its brain, compared to millions for us.
But hold on a sec. If insect vision is so low-quality, then how do they clock a predator sneaking up to make them their next meal?
How do they successfully identify prey, so they don’t starve? What advantage does the compound eye give them?
Basically, insects’ eyes are really good at detecting motion.
A lot of them see faster than us, so their eyes and brains process visual information much quicker than we do.
What that means is that for them, the world is moving in slow motion compared to how we perceive it.
So if we’re looking at a stopwatch, we experience the second hand move like this… but for a fly, it would look something like this.
The second hand is moving almost four times slower. That means they have almost four times as much process time to detect and respond to movement, and their brain is stitching this info together almost four times faster than we can.
This is why it’s so hard to get that pesky fly with a fly swatter. Try coming at them head-on as quickly as you can, and they’ll still see you — because to them, you’re moving too slowly.
Seeing in slow motion doesn’t just let insects avoid being prey. It also makes them fantastic predators. It’s a big reason why the dragonfly is considered to be the most efficient predator in the entire animal kingdom.
It can calculate exactly where its prey will be to land the perfect strike.
And it’s not just motion that the compound eye does differently. It’s also color, which adds a whole new layer to how a species can avoid predators, find food, and make a real go of it in this crazy world of ours.
But first, let’s understand how we see colors.
The photoreceptors in our camera eyes are trichromatic, so we can see the three colors of red, green, and blue.
We combine these three colors together in different ways to see the full spectrum of colors that we call visible light.
Quick science-class moment here: light is electromagnetic radiation composed of different wavelengths.
We can’t see a lot of these wavelengths, but what we can see falls in this range — violet, the shortest wavelength at around 380 nanometers, all the way up to red, the longest wavelength at around 700 nanometers.
Other light wavelengths that are shorter or longer simply don’t get picked up by our eyes.
But many insects can detect shorter wavelengths, which include light that we can’t see — like ultraviolet light.
Many flowers have UV reflective patterns that are invisible to our human eye but attract bees.
For these aerial insects, these patterns are like airport runways, with little landing zones that point them toward the parts of the plant with nectar and pollen.
The dragonfly takes color perception and turns it up to eleven — literally. One new study showed that dragonflies can combine eleven, or maybe even more, different color wavelengths.
This brings us to the biggest question of them all.
Can we ever truly know what the subjective experience of seeing is like for an insect?
We can’t even be certain of what other people see. Color, for example, only exists in our brain.
It’s a perception created by our brain’s interpretation of light wavelengths reflected from objects — meaning color itself isn’t a property of the object, but rather a construct of our mind.
So maybe — just maybe — the blue that you see might not look exactly like the blue that I see.
And so the same reasoning might apply to how an insect’s brain is experiencing vision.
We can rule things out. We can make really good, educated guesses. But we can’t be 100 percent certain.
Want to see animals using their amazing eyesight to survive?
Watch Deep Look’s episode about mantis shrimp — they see wavelengths of light that are invisible to other animals.
And peregrine falcons keep track of their next meal as they fly towards it at high speeds.
See you there.
