What Colors Are Flies Attracted to and How Do They See the World?

Illustration of rainbow colored fly

There’s a surprising amount of beauty to be found in the insect world. People are often so quick to assume that all bugs are inherently creepy that they fail to notice some of the more beautiful aspects of this incredibly diverse class of organisms. One can find an immense amount of beauty in the wings of a butterfly or the often psychedelically colored shells of some species of beetle. However, there’s one species of insect in particular which people usually hold up as the definition of an ugly bug.

The humble housefly just doesn’t get much respect for its artistic flair. It’s true that a fly doesn’t have much visual appeal to the human eye. But what if the fly had a noble artistic spirit in its little body? Do flies wander about in search of beautiful vistas? And are there any colors which catch a fly’s attention?

All the Better to See a Meal With

Macro photo of eyes of noon fly

One might imagine that a fly can see some amazing things simply from the shape of its head. A fly’s head is dominated by two huge compound eyes. The size of a fly’s eyes are obviously quite different from what you typically finds in mammals. However, at first glance a fly’s eyes seem fairly in line with our own mammalian assumptions. A fly appears to have its two eyes prominently on its head and over its mouth. However, appearances can be deceptive.

In addition to the two main eyes, a fly will also boast additional simple eyes called ocelli. Different species of fly will have different amounts of ocelli. However, on average, most flies will have two main eyes and three ocelli. Things get even stranger when we start to look into the function of those five eyes. The true artistry of a fly’s life becomes more apparent as we look further into how those eyes function.

Compound Eyes Compound the Difficulty of Imagining a Fly’s Worldview

Sharp and detailed fly compound eye surface at extreme magnification

Consider what you see when you look outside on a semi-cloudy day. You’ll typically see a complex interplay of different visual phenomena. The sun and the clouds will both produce some elaborate shadows on the ground. Flying insects and birds might briefly occlude your view of the outdoors. And everything is filled with a complex tapestry of color. Now consider that you’re able to see all that with just two eyes. What’s more, your eyes are highly tied together to form a singular image. Look right with one eye and the other will follow. This limits the human range of vision when compared with animals whose eyes are set further apart.

Now consider the five eyes of the fly and how it looks out at the world. We put special emphasis on light, movement and shadow when imagining a landscape. This is because humans need to invest some special, albeit unconscious, work into distinguishing these aspects of the environment. Not so for the fly.

A fly uses its simple eyes for many of those tasks. When it notices movement, it’s primarily initiated through the simple eye. The same goes for moments when the lighting in its environment changes. The simple eyes are, as the name implies, fairly simple. They don’t see color at all. However, they are able to handle some of the most basic and important aspects of vision. You can almost think of them as visual life support. The simple eyes are there to handle some of the most fundamental tasks. However, it’s in the fly’s two compound eyes that we see some truly breathtaking feats.

Closeup portrait of a green fly

A fly’s two large eyes are composed of structures called ommatidium. You can think of every ommatidium as roughly analogous to a mammalian eye. This might give an impression that flies have a wide range of vision. That would be a dramatic understatement. In addition to the simple eyes a fly has 3,000 ommatidium in each compound eye. This brings the total areas of visual input to 6,000 complex structures capable of seeing color and three simple eyes.

Color Comes From Interplay Within the Fly’s Eye

All of this circles back to an earlier question. A fly looks quite drab to the human eye. But what kind of artistry can a fly see through its own eyes? The answer can be found by considering how ommatidium processes color.

Every fly species has a different distribution of something called screening pigment amid the ommatidium. Different screening pigment allows for different permutations of the larger color scheme. This isn’t an easy comparison to make when looking at mammalian eyes. However, you could imagine a familiar beach. Then consider how that beach would look with a filter applied over it.

Photo of beach with a retro color filter applied to it

A filter will put different aspects of the same picture into greater or lesser prominence. Or to put it another way, filters can give us a different artistic appreciation of the same scene. Imagine how amazing the world would look if you were able to see an instant filter next to everything your eyes took in. That’s somewhat analogous to how a fly’s compound eyes see the world.

Each ommatidium can have slightly different visual input depending on the screening pigment in its periphery. It should be noted that each individual ommatidium won’t produce a very sharp image. This is also a problem with the small size of a fly. There’s only so much an organism can take in when it’s so small. This is also how a fly benefits from having such a large portion of its head taken up by the compound eyes. It’s able to see over a very large area while still scaled down to a very tiny form factor.

Meanwhile the simple eyes can handle the most important tasks while leaving the compound eyes to more neurologically taxing operations. After all, the data from the compound eyes needs individual processing to come together as a greater whole. That’d be taxing for a human’s huge brain. And insects aren’t exactly known for their neurological complexity. Having a simple eye system in conjunction with the larger structure helps them interact with the environment at different levels of speed. This is also one of the benefits of a compound eye system. An individual ommatidium has some built in processing ability when environmental data comes in.

A Fly Lives in a World of Color While Maintaining a Few Favorites

Abstract crystal background with iridescent textured faceted gem

With all that in mind, consider just how amazing a fly’s visual world really is. We tend to think of them as the most drab inhabitants of the world. But what they see through their eyes is truly breathtaking to imagine.

A fly’s world is one of bountiful color and motion. Their visual acuity is further limited by a lower representation of color receptor cells. This causes some bleed between the colors around a fly. You can almost imagine them as living within an abstract watercolor. Except even that would be too limiting. Because a fly isn’t just looking at the world through the colors we’re familiar with. A fly is able to see ultraviolet light and differentiate between polarized and unpolarized light. Basically, a fly is continually drifting through a world of color. This helps to explain why they sometimes seem confused with seemingly simple tasks like getting out of an open window. They’re navigating through a very complex world. However, in the midst of all that color, they do have a few favorites.

The Superstars of a Fly’s Color Spectrum

The earlier discussion of a fly’s complexity shows why its color preferences are rather complex. A fly isn’t looking at or thinking about a single thing when it gazes out into the world. It’s taking in over 6,000 separate points of data from a wider spectrum of light than humans are capable of seeing. Again, consider just how small a fly’s brain is. Handling all of that information requires some prioritization.

Ray color index infographic with radio, microwave, infrared, visible light, ultraviolet, x-rays and gamma

A fly is looking for a lot of things in its environment. However, it has a special neurological cluster devoted to processing ultraviolet light. This can essentially blind it to other types of light. This is a spectrum of light which is only visible to humans who have aphakia. This form of color is primarily found in sunlight. This makes for a somewhat confusing answer to the question of what a fly’s favorite color is. A fly’s favorite color is essentially invisible to humans. It’s the color found in sunlight.

That said, much of the attraction to ultraviolet light is done through a separate pathway than normal color perception. A fly is drawn to ultraviolet light in a similar way to how it’s drawn to food. What about more subtle colors? Part of the reason why a fly will come indoors is that it also likes the color black. It associates black with safe hiding spaces which will allow it to properly camouflage itself. This is also part of why one of a fly’s favorite spaces is the interior of a home. A fly which can move between sunlight coming from a window and dark shadows is experiencing two of its favorite colors.

The last color preference is shades of blue. It’s thought that blue light is simply the best color for hitting as much of a fly’s complex visual system as possible. Blue light may simply be the most intense, and therefore interesting, to a fly. This means that a fly is most attracted to a mix of ultraviolet light and black shadows. However, in terms of raw color, it likes blue more than anything else. In fact, given a choice, flies will usually head toward areas colored blue if there isn’t any visual noise to distort its perception.

An Unappreciated Artist Adrift in an Ocean of Pigments

Phoenix painted in light and flame colors

All of this paints a very different picture of an often unappreciated species. Humans look at flies and see something unremarkable. However, every fly is a constant and dynamic painter. Humans primarily focus on shape. But a fly is obsessed with color. This humble insect is in many ways akin to a phoenix painted in light and flame. It’s worth taking a moment to appreciate a fly when it passes into our field of vision. The fly might not be beautiful in and of itself. But it’s constantly appreciating over 6,000 points of beauty with every turn of its head.