In Insects, the Eyes Have It!
Nothing is more important to adult insects than their eyes. Looking casually at insects, most humans are more likely to notice other appendages first—legs, wings and antennae, for instance. But to the insect, the eyes are just as important as these more obvious protrusions.
To be sure, legs allow insects to run, jump and cling. Their wings provide the power of flight. Antennae are wonderful devices for smelling. But the eyes play very important roles in many aspects of insect lives.
Some insects, those like lice and fleas that are parasitic, don't have eyes at all. A few insects have single-lens eyes, but most adult insects have compound eyes. In some insects, the number of lenses is in the thousands. In dragonflies, the number of lenses is more than 10,000.
Because of compound eyes, entomologists assume that insects don't “see” in the same way that humans do. A single lens in an eye allows a sharply defined image. The multiple lenses in compound eyes probably, at best, produce a blurred image.
But then, the goal of the insect eye is not to provide the clearest image but to keep in visual contact with a large area. That is the reason that eyes sometimes occupy a sizable portion of the insect head. The eyes also frequently bulge out to develop a wide field of vision.
In some insects, such as dragonflies and some flies, the eyes actually meet in the midline of the head. Through multiple lenses, these insect eyes function much as a radar system that continually rotates, scanning the horizon. And the purpose is much the same: to pick up anything within sight.
While insect eyes might not provide the clearest image, they do provide an excellent motion detector. That is the reason it is difficult to sneak up on an insect. Insects need such protection, since it seems the world is out to get them—primarily to make a meal out of them. This is the case in insect eaters, such as birds and frogs.
Humans sometimes just want to remove the insect from the face of the earth. We developed such things as fly swatters to aid us in the task. But you still have to be quick, since it is a sure bet that the fly sees the swatter coming and many times manages to escape.
Insects also use their eyes to judge distance. This is important to predaceous insects that need to catch other insects for their food. The praying mantid uses its eyes for that purpose. By looking at a potential insect meal, it is able to accurately assess the distance to the prey, based on the view from several different lenses. It, therefore, waits until the prey is within range before grabbing for it.
Insect eyes are also sensitive to UV light waves. Such sensitivity allows the insect to see things that we humans cannot. Honey bees detect the UV patterns on flowers. Night-flying insects navigate using UV light. That is the reason that UV traps are so effective in luring insects to their deathes in an electric grid.
Polarized light is also detected by insects. Honey bees navigate based on the polarized light from the sky. This is known as the light-compass reaction where the insect can detect the pattern of polarized light from a patch of blue sky.
So to an insect, “I can see clearly now” doesn't have much meaning. Insect eyes are more UV light and movement sensors. But the system seems to work, since insects don't have trouble finding their way or avoiding predators.