How insects see the world

Hyperaxion September 26, 2020 2:41 am

Understanding how insects see the world is the key to observing other worlds. Learn how vision works and how we use technology to see beyond the visible.

Humans have at the back of their eyes cells responsible for capturing the light of the world. These cells are of two different types known as rods and cones.

Rods and cones

The rods are responsible for perceiving luminous intensity, distinguishing the light from the dark, and various intermediates on a grayscale.

Cones, on the other hand, can differentiate one color from another.

Wave-length

What we understand as “color” is nothing more than a light wave of finite length.

For example, if a wave corresponding to the blue color is somehow stretched, its length changes and we start to perceive it as a different color, perhaps green.

On the other hand, if this wave of light is shrunk, its length decreases, and we start to perceive it as another color, like violet.

Cones

Our eyes have three types of cones, one specializing in blue, one in green, and one in red.

Together, they detect all colors in the visible light spectrum – which comprises all the wavelengths that a human can see.

The electromagnetic spectrum with the visible light spectrum highlighted.
The electromagnetic spectrum with the visible light spectrum highlighted. That small fraction is all that our eyes can see.

When a cone is hit by a light wave, it is triggered, and if that cone is specialized in red, it is more likely to be triggered in the presence of red, and less likely in the presence of other colors.

This graph shows how much light is absorbed by each cone at each wavelength:

The cone absorbs more light when the wavelength corresponds to its specialization.

Different evolutionary paths

While humans can only see the wavelengths of the visible light spectrum, the same is not true for other species.

Each species had an evolutionary path with different needs, which often resulted in different cones.

For example, in the case of the human species, at some point in our evolutionary history it became advantageous to know whether a fruit was ready to eat, not too green, not too ripe.

Take a look at this fruit:

A ripe mango.
A ripe mango. (Credit: Ancelin Bonnet / Pixabay).

If we take out the red cones, it’s difficult to know its ripening stage:

A... green mango?
A… green mango?

This problem, however, may not be relevant to other forms of life, such as insects.

How insects see the world

Instead of having to evaluate the appearance of a fruit, some of them actually needed to find flowers, such as the bee.

Bees have a different spectrum of vision than ours. Instead of red cones, their eyes have ultraviolet cones.

This means that they cannot perceive red, but they can see ultraviolet, and this has consequences.

The following flower (Turnera diffusa), for example, appears to have a single solid color to us:

Turnera diffusa, also known as damiana.
Turnera diffusa, also known as damiana. (Credit: DEZALB / Pixabay).

However, the bees’ eyes can see beyond, and reveal patterns hidden from us.

For them, the flower petals form a target, which can assist them, while flying, to spot flowers and aim their landing.

But not all insects are interested in flowers, and for that reason, most of them actually have only two cones, one to see yellow and the other to see blue and a bit of ultraviolet.

From our human point of view, you could say that they are color blind.

However, from the perspective of a common fly, we are color blind, because while we only have three different types of cones in our eyes, they have five.

And among insects, the record goes even further.

An Australasian butterfly, the common bluebottle (Graphium sarpedon), has 15 cones.

There are two ultraviolet cones, one violet, two blue, five green, one yellow, one orange, and three red.

The puzzling thing is that there is no apparent advantage to having more than 5 cones.

So why does the common bluebottle have so many of them? Short answer: to look without seeing.

Compound eyes

The compound eyes of a fly.
The compound eyes of a fly. (Credit: skeeze / Pixabay).

Unlike humans, insects have compound eyes, structures composed of numerous small receptors called ommatidia.

Each compound eye can have from five ommatidia to tens of thousands of them in some species.

And within the same compound eye, ommatidia from different regions can have different cones.

In the case of the common bluebottle, which has 15 cones, four of them are only found in ommatidia at the top of the compound eye, and eight only at the bottom.

Only three cones can be found throughout the compound eye.

Only these three are actually used for the sight of these animals.

The other 12 do not participate in the process of composing the images seen by these insects.

Interestingly, the images seen are not just a product of the cones, but mainly, of the brain.

Vision and the nervous system

When some of our cones capture light and fire a signal, that information is processed by the brain, which then forms the images we see on a daily basis.

However, there are cones in insects that send signals to a different path in their nervous system.

For example, when a green cone is not used for vision, it just alerts whether green is present or not, letting the insect know that there is “green” in a given direction without it actually seeing the color, which is the case of the common water fleas.

Cones with specialized functions

Common water fleas need to know which direction has the most “green”, because that’s probably where there are more algae for them to eat.

Their ultraviolet cones, on the other hand, serve only to alert them when there is too much ultraviolet radiation, which can damage their DNA.

In other words, they are cones that are not actually used to see the world, but to trigger specific behaviors.

Insects have specialized cones to find food, partners for breeding, and even cones for orientation, as a kind of compass.

Therefore, insects that have more cones, do not necessarily see more colors than we do, just different colors.

A universe not made for us

And sometimes, this is how we should look at the Universe.

Something we know for sure is that the Universe does not care about humanity, it is not friendly to us.

Living in other worlds is a very difficult task, and sometimes even seeing them can be challenging.

The view of the planet Saturn, only with the versatility of our eyes, is very limited.

The planet Saturn in the visible light spectrum.
The planet Saturn in the visible light spectrum. (Credit: NASA and E. Karkoschka / University of Arizona).

But if we use technology capable of detecting ultraviolet light to see it in a way more similar to that of insects, we get a different image.

The planet Saturn in the ultraviolet light spectrum. This is how an insect would see Saturn.
The planet Saturn in the ultraviolet light spectrum. This is how an insect would see Saturn. (Credit: NASA and E. Karkoschka / University of Arizona).

What once seemed somewhat homogeneous, now reveals valuable new information.

With ultraviolet, we can see differences in the composition of atmospheric gases in the southern and northern hemispheres of Saturn.

And comparing the two images allows us to understand this planet even better.

Both images, side by side.
Both images, side by side. (Credit: NASA and E. Karkoschka / University of Arizona).

In fact, scientific and technological advances allow us to detect not only ultraviolet, but also x-rays, microwaves, gamma rays, and radio waves.

Together, they help us to see farther than before, painting the sky with colors beyond our comprehension. They allow us to look and fall in love with the grandeur of the Cosmos, just as a small insect looks at a flower.

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