In Insects, The Eyes Have It
Insects, like the proverbial school teacher, must have eyes in the backs of their heads. At least that's the way it seems when you try to squash a fly with a swatter.
No one really knows what the world looks like to an insect. Comedian Heywood Banks in his song, “Lookin' at the World Through Fly's Eyes,” doesn't shed much light on the subject. He concentrates on attitude rather than visual acuity.
Cartoonist Gary Larson, in his cartoon with the caption “The last thing a fly sees,” suggests that a fly's-eye view could be described as a repeating mosaic. In this case Larson has created a rather ominous mosaic at that — a many-faceted view of a woman with a poised swatter.
The idea that insect vision might resemble a mosaic is due to the compound structure of the insect eye. They are composed of many units. Unlike the single-lens eye common in other animals, insect eyes can have thousands of lenses. The common dragonfly has over 10,000 lenses per eye. It is no wonder that dragonflies have a head that appears to be all eyes.
Insects probably do not form a clear image with their eyes, but their eyes are good at detecting motion. This is important to insects that are about to be captured by some predator. It is also important to insects that are themselves predators. Predatory insects like dragonflies and praying mantids have eyes that are highly curved. This ensures overlap of their visual fields so they can accurately judge distance. Praying mantids seldom try to catch an insect that is beyond reach.
Insect eyes do not respond equally to all wavelengths of light. For instance, insects do not see well in the longer wavelengths. For this reason, insects do not perceive red lights. And yellow lights are less attractive to night-flying insects, so homeowners may use them on porches and other places where they do not want insects to appear.
On the other hand, insects are especially sensitive to shorter wavelengths, including ultraviolet light. Many flowers are attractive to insects, not because of the color pattern we see, but because of the UV light reflection. We humans have exploited the UV light attraction of insects to lure them to devices called light traps or bug zappers.
Insects also perceive polarized light. Honey bees are able to navigate using polarized light, thus they do not have to see the sun to know which way the hive is located.
Insects could be considered real cool dudes. They don't need to purchase UV or Polaroid glasses; they're already wearing them!