Written by Nancy Miorelli
Eyes are remarkable structures that have evolved independently at least three times. The most obvious advantage of eyes is that they help us understand the world around us by absorbing what is essentially data, which our brains then decode to tell us there’s a bus coming and to get out of the way.
Well, the thing about evolution is that it doesn’t have to work perfectly – just good enough, like your perfectly average C student. The overall effectiveness and subsequent modifications to the compound eye is summarized very well by Nilsson.
It is only a small exaggeration to say that evolution seems to be fighting a desperate battle to improve a basically disastrous design.
So, the benefit that the compound eye gives the fly is that the fly can see. It lets the fly know if something is coming towards it, where the fly is positioned in its environment, what’s there, and tells the fly that it’s moving in relation to other things.
But I’m assuming that that’s not *really* what the question is asking. So let’s get a couple things out of the way before we talk about what compound eyes do and some modifications that flies have made to them over the years.
- Evolution isn’t picky about how things get done. So light sensing organs pop up and stick around because organisms that have them usually get less dead than things that don’t.
- That is, of course, if the organism needs to see. Eyes are usually the first things to disappear if you live in caves. Or if you’re a fly maggot that invests its early days face first eating rotting corpses.
- It’s not just flies that have compound eyes. All insects that have eyes have compound eyes. Many insects cheat and have both simple light sensing receptors and compound eyes.
- There are about 150,000 described species of described true flies (Diptera) with an estimated total number of fly species to be around 240,000. So, this is going to be very generalized and does not at all encompass every organism.
So let’s talk about the compound eye and how insects and specifically flies have tailored it to fit specific needs.
What Is a Compound Eye?
The compound eye is nothing like the human eye. We have two eyeballs and in each one we have a lens that focuses the image on our retina. Cones help us see color and rods help us see in the dark. The optic nerve is the cable that runs from the eyeball – the data center – to our brain – the interpreter. It’s hard to talk about what the resolution of the human eye actually is for lots of reasons (whichVsauce explains below). Basically, the resolution is pretty good and, all things considered, doesn’t require that much physical space to produce.
The compound eye is made up of many “ommatidia”, the basic units that the insect eye is comprised of. Each one is kind of like an eyeball in the fact that it has a lens which focuses light and has pigments (opsins) for detecting color. One advantage of this system is that the image is projected in each ommatidum right side up, unlike our eyes. Insects with large, globular eyes practically have a 360 panoramic view of the world and don’t have to turn their heads to get a sense of their environment. This setup, for most insects, removes obvious blind spots and is part of the reason flies can see you coming and dodge your unsuccessful swatting attempts.
Flies Make a Bad System Better
Resolution of the compound eye is achieved in different ways.
- You can just add more units. That’s basically what the dragonfly did with all 30,000+ ommatidia it has. Some subterranean insects only have 20.
- You can make bigger units.
- You can modify your units. This has been done in a few groups and there are definitely some trade offs.
Flies that rely heavily on their vision, for the most part, have cashed in on all of the options. House flies (Musca domestica) have a moderate amount of ommatidia, but males have more ommatidia (~3,500) than females (~3,400) and bigger eyes, suggesting that vision plays an important role in mate determination. In fact, this pattern is readily seen in two other families of flies, the Flesh Flies (Sarcophagidae) and the Blow Flies (Calliophoridae). House flies were at the lower end of the spectrum with some Blow Files coming in closer to the ~5,500 ommatidia Honey Bees have. While this isn’t the 30,000 that Dragonflies have, each ommatidium of a House Fly, Flesh Fly, or Blow Fly is bigger than that of a dragonfly. Plus they still have a lot more than the 2,000 American Cockroaches have and the 800 that Drosophila have. So we’ll call it a nice middle ground.
All flies have a rhabdom in each ommatidium. This is basically what does the “seeing” in an insect. Light is focused through the lens and onto the rhabdom and photopigments are stimulated. In most insect eyes, each ommatidium acts like a single “pixel” that the insect can see. While it’s not actually a “pixel”, for simplicity it works as a pretty good analogy. True flies (the order Diptera) have their rhabdom split into seven parts. It’s a pretty complicated system, but basically flies can increase their resolution by a factor of 7 without increasing the size of their eye.
The take home point is that compound eyes are a bad design for resolution because to get more, you have to take up a lot of space. And space isn’t a commodity on a small animal like an insect, so insects have to get crafty.
Insects can’t focus on objects by changing the shape of their lens or the position of their lens, so they have to move closer or farther away to see things clearly. They sacrifice depth perception and the ability to focus to see a lot of stuff (wide-angle vision) and perceive contrast. Therefore, something called visual acuity is very important. Basically, the greater their visual acuity the more detail the insect can see on an object. Obviously, the more detail you can see, the better. Some insects are at the maximum limits of their resolution so they have some other nifty tricks up their sleeves. This is where sight lines come in.
Some insects need high visual acuity for mating or for catching prey. Therefore, certain areas of their eyes are allocated to see things with greater acuity while sacrificing the image quality of other parts of their eyes. Dragonflies have great visual acuity on the top and middle of their eyes. This helps them snatch their prey. Male flies, in addition to having bigger eyes, have more specialized acuity zones than their female counterparts. One Hover Fly in particular (Syritta pipiens) can see the female at a distance where she can’t see him. He literally stalks her. Flies aren’t known for being romantic.
Vision is the Art of Seeing What’s Invisible to Others ~Johnathon Swift
Humans are pretty good at seeing colors. We can see about 10 million different colors with three types of cones. Insects and crustaceans use opsins to detect light. Mantis shrimp have 16 opsins and were previously thought to hold the world record, being able to see trillions of colors, until some dragonflies butted in. Certain species of dragonflies were found to have over 30 opsins, but we are uncertain of their specific ability to distinguish colors. Using the mantis shrimp, the color differentiation capabilities of the compound eye is being reconsidered. The big question here is, “why is it important to see all these different colors?”. In both of these cases, these animals are fast predators and need to be able to distinguish edible things from not edible things. Quick color discretion is important, but maybe determining the difference between #75D1FF and #83D6FF isn’t. In fact, if these two weren’t labeled, and you were shown one at a time, could you tell the difference? Technically your eyes can, but your brain doesn’t, especially if you don’t have words to describe the different colors.
Okay so some compound eyes can (probably) distinguish more colors than us, but many insects are blind to red and orange light. However, insects can see UV light while we cannot. Their resolution may be worse than ours, but they can see harmful and damaging UV wavelengths. This is used by pollinators (many of which are pollinators like hover flies!) to navigate to flowers and for complex mating signals, but can also lead them to their death. Insects can also see polarized light, and many use it to navigate and for mating.
Insects that are completely nocturnal modified the structure of the compound eye. Normally, the inside of the ommatidium is lined with pigment cells. This prevents light seeping into the adjacent ommatida. Usually this is good because the more light that floods in makes your resolution shoot down the tubes. But night flying insects don’t have these pigment cells, so the light does flood the ommatidia which allows them to perceive more at night but at a lower resolution. Also, while the rhabdom usually sits right under the structures in one ommitidia in day flying insects, in night flying insects the rhabdom is disconnected and separated by a clear zone. This allows light from adjacent ommatidia to stimulate one rhabdom to make a better image.
Many little gnats and mosquitoes are crepuscular meaning they fly at dawn and at dusk but flies generally have eye modifications for frolicking around in the sunlight. Normally they’d be out of luck seeing once the sun dipped below the horizon, but flies have that 7 way split rhabdom. This helps them fly in low light conditions because the 7 parts of the rhabdom are separated and act in a similiar manner to the rhabdoms separated from the ommatidia by the clear zone. Specifically for gnats and mosquitoes, it gives them an extra 15 or so minutes before dawn and after sunset. This doesn’t seem like much for us, but it gives them a small window to swarm, mate, and feed without being incredibly visible to predators.
Some Bizarre Modifications
Most often seen in dragonflies (but some flies have this as well), the top part of the eye is a darker color than the rest of the eye. This has been suggested that it acts like a pair of sunglasses and protects their eyes from direct sunlight. It’s also been suggested for dragonflies and owlflies, that this dark spot helps the insect see prey flying against the blue sky.
Borrowing Kanye West’s Sunglasses:
Lots of flies have crazy color patterns on their eyes. We’re not sure of the exact purpose. For some it may play a part in mating. Horse Flies (Tabanidae) probably use it as a convenient color filter. Horse flies are blood feeders and the things they attack are big, bulky, grass feeders that are usually surrounded by lots of vegetation. By having green eyes, Horse Flies effectively put on green glasses that make the background appear grey and their hosts stand out from the background.
Keep It, If It’s Sexy:
Sometimes evolution just doesn’t care if you can see and it’s only important how sexy you are. That’s what happened to the stalk eyed fly. The outrageous stalks that the males carry around actual hinder their flying capabilities but they can still see relatively well.
Compound eyes aren’t really the greatest because it’s hard to compensate for the low resolution. However, insects and flies specifically have some pretty nifty tricks to compensate. Compound eyes don’t really let insects see red or orange light, but insects can see well into the UV range and even use polarized light for navigation.
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