Insect eyes are a wonder of natural engineering. Anyone can tell that bug eyes are special in a complex way with just one glance at their shape and construction. But when you get down into this marvel of natural engineering, the story gets even more interesting.
Most insects have what are called compound eyes. As you've likely noticed, compound eyes are a mosaic-like structure of closely-packed ommatidia. Let's all say that outloud together now, ommatida. According to the handy Biology Online, these are "the structural and functional units of vision" and they vary in number depending on the insect. Some insect species have as few as six while others, like dragonflies, have over 25,000!
So does that mean that an insect has way better vision than humans? Well, no. In explaining how butterfly eyes work, Professor Ron Rutowski tells us that, at least in the case of butterflies he has studied, they have far worse. If humans consider perfect vision to be 20/20, butterflies have more like 20/200 vision. They have to be much, much closer to an object to be able to see it clearly.
But that doesn't mean that insect eyes don't have amazing abilities.
As Biology Online points out, "As was originally suggested by Johannes Muller(1829) in his so called ‘mosaic theory’, each ommatidium receives the impression of a luminous area corresponding to its projection on the visual field; and it is the juxtaposition of all these little luminous areas, varying in the intensity and quality of the light composing them, which gives rise to total erect image perceived by the insect. Since insects cannot form a true (i.e. focused) image of the environment, their visual acuity is relatively poor compared to that of vertebrates. On the other hand, their ability to sense movement, by tracking objects from ommatidium to ommatidium, is superior to most other animals. Temporal resolution of flicker is as high as 200 images/second in some bees and flies (in humans, still images blur into constant motion at about 30 images/second). They can detect polarization patterns in sunlight, and discriminate wavelengths in a range from ultraviolet to yellow (but not red)."
And the special abilities don't end with superior motion tracking.
One research paper from 2010 shows that insect eyes actually have the ability to repel dirt, a discovery that could mean a biomimetic solution for technology like more efficient solar cells.
"[I]n contrast to the rest of the body, ommatidia of various insects remain clean, even in a heavy contaminated environment... We assume that this anti-adhesive phenomenon is due to a decrease in the real contact area between contaminating particles and the eye’s surface. Such a combination of three functions in one nanostructure can be interesting for the development of industrial multifunctional surfaces capable of enhancing light harvesting while reducing light reflection and adhesion."