An old proverb says, “The eyes are windows to the soul.” But what if the eyes belong to a bug? Bugs may or may not have souls – whatever a soul actually is – but their amazing eyes are an endless source of fascination and beauty and endow insects and their allies with surprising abilities.
Beauty isn’t just in the eye of the beholder, it’s right there in the eye itself. We are familiar with the eyes of friends, spouses, children, and even strangers. Not only do we consider many people’s eyes beautiful, we often have emotional responses when we make eye contact. This also happens between different species. Consider the cat, and occasionally the dog, who stares deeply into your eyes.
But what about the eyes of the insects, crustaceans, and other arthropods that have been around in some form or another for at least a half a billion years? I believe that there is just as much beauty in their varied eyes as in those eyes we are more familiar with, though admittedly the connection we feel is usually less emotional.
Some arthropod eyes, like the compound eyes of the Atlantic Horseshoe Crab (Limulus polyphemus),(1) have remained virtually unchanged for most of those half a billion years, and despite the alien appearance of the their eyes,(2) they share a number of features common to our own.(3)
Each of the dark dots, or facets, on Limulus’s compound eye is the top of a tube called an ommatidium that focuses light onto light-detectors (receptors) at the tube’s base. Think of empty Pringle’s potato chip tubes with light coming in one end and hitting the receptor at the other end. An ommatidium is the tube with its optics and receptors, and it functions similarly to our own cornea and lens that focus light onto our retinas. Each tube looks at a tiny patch of Limulus’s environment and all the tubes combined view a large part of the world.
What beautiful bug-eyes you have!
Insects showed up on Earth more than 50 million years after horseshoe crabs and were no slouches in the compound eye department. Their descendents include deer flies with their stunning, colorful eyes.
The function of the metallic patterns is not completely understood but may be important in the flies’ color vision, serving as light filters. They may also play a display role between different fly sexes – who wouldn’t be attracted to these glamorous eyes!
Can you see me now?
A common misconception is that the fly’s view of the world is composed of many tiny images of the overall scene; I remember seeing similar representations of insect vision as a kid. In other words if a fly were looking at our dog Poppy, the fly was said to see something like the left hand image below:
This is incorrect. Recall that each tube-like ommatidium looks only at a tiny part of the image, kind of like a single, blurry pixel in a digital picture. When the complete image is “assembled” the fly sees a blurry version of Poppy, like the image on the right, and misses her overall beauty.
Nevertheless, flies have an excellent ability to detect motion—ever try to catch a fly?—and they have done quite well for millions of years without sharing our appreciation of aesthetics.
If you look closely you will see that the compound eyes of the fly, and many other insects as well, have 6 ommatidia (plural of ommatidium) arranged around a single, central one with this hexagonal pattern repeating across the eye.(4) Acuity, or sharpness of vision, is related to the total number of ommatidia in the compound eye – the more of the hexagonally-arrayed facets, the better the visual acuity.
For example, the deer fly has a few thousand ommatidia while the ant has only around 400 in each compound eye.
This ant would be even less able to appreciate Poppy’s beauty than the deer fly.
Nevertheless, ants have other keen senses and are exceptionally successful animals, despite their “poor” vision. Evolution is about being “good enough” to survive and reproduce and ant vision is “good enough” to thrive. With more than 20,000 species on every continent except Antarctica ants are poster children for survival.
What big eyes you have!
No discussion about compound eyes would be complete without mentioning the extraordinary eyes of dragonflies.(5) Dragonflies are among the most efficient of all predators with a success rate of 95%, and their vision plays an important role in their success. The 25% success rate of lions pales in comparison.
Each dragonfly eye may have up to 30,000 ommatidia, so their visual acuity is very sharp among insects, as expected for an efficient, high-speed, flying predator. They also have specialized regions of high acuity, and the transition between the low and high acuity zones is shown in the dragonfly photograph’s inset. The high resolution areas are located in the upper half of each compound eye and are characterized by larger facets, which help the dragonfly detect and target insects flying overhead.
To be a wasp is to know a wasp
Wasps all look alike to me, but perhaps I haven’t looked closely enough. On the other hand, some wasps can recognize fellow wasps. They will respond aggressively to wasps whose facial patterns have been modified with paint, but as the paint-modified wasps become more familiar to the other wasps, the aggression declines. That is, the unpainted wasps come to recognize the modified wasps.
It turns out that those paper wasps with this ability to recognize other wasps as being familiar have also evolved sharper vision—just like the dragonflies with large facets for sharp vision. Some of these wasps have larger facets than wasps that didn’t recognize individual, fellow wasps. Apparently, the ability to recognize each other and sharper vision evolved hand-in-hand in these social insects—sharp vision was needed to see who is who.
Yet sharp vision clearly isn’t enough to recognize fellow insects, for example, dragonflies don’t recognize individual dragonflies. Clearly, the face-recognizing wasps aren’t just another pretty face in the insect world—there is an “alien” intelligence behind those eyes that we are just beginning to understand.
The next time you see a bug I hope you look at it with new eyes, both yours and the bug’s. There is beauty on both sides of the gap, and appreciation of that will only serve to lessen the divide. So stare away and pay attention to the new emotions that may arise when you make eye contact.
1 All of the animals photographed here were alive or found dead with the exception of the deer fly that I swatted while it was biting me.
2 The horseshoe crab actually has a total of 10 eyes but I will only discuss its large, compound eyes.
3 The evolution and function of the many different types of eyes are beyond the scope of this essay. In-depth reviews are included in the references.
4 This is not true of all arthropods, for example lobsters, spiders and centipedes to name a few.
5 Dragonfly eyes are also more complicated than the simple Pringle’s tube” description. See the references for details.
6 Unlike the bright colors seen in the deer fly eyes, the mottled coloration of the wasp’s eyes is an artifact of death and is not present in the living wasp.