OCTOBER
2008

 

By
Tom Turpin
 
Professor of
Entomology
Purdue University

 

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Check out these books by Tom Turpin:

Flies in the face of fashion

What's Buggin You Now?

 

 

 

10-09-2008

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Some Insect Eyes Not for Vision


Eyes are organs that give the power of vision to animals. An important function, for sure. But in humans and other animals, eyes are used for more than vision! Eyes are used to convey messages. We admonish folks to make eye contact when speaking in order to improve communication. On the other hand, we avoid eye contact when we don't want to send an incorrect message to a stranger.

In nature, two basic eye designs exist: eyes with single lenses and eyes with many lenses. Eyes with single lenses are found in mammals, including humans. Single-lens eyes are also found in birds, reptiles, salamanders, amphibians and fish.

Eyes with single lenses allow the animal to see a distinct image. That is unless there is a physical problem in the structure of the eyeball. Problems like a misshapen eyeball or lens can result in blurred image. We humans correct such problems with artificial lenses or surgical procedures. Other animals without correct vision just have to live with it. That is unless the animal is lucky enough to be the pet of a human willing to pay a veterinarian to correct the problem.

The second type of eye found in nature has many lenses. Such eyes are found in insects. Scientists assume insects do not see clear images through their eyes. So the 1972 popular song, “I Can See Clearly Now,” written and recorded by Johnny Nash, has no meaning for insects.

Even if they can't see clearly, insects do have the ability to see in many directions at the same time. That is because of the number of images being captured through the many lenses. Because of this biological attribute, it is very hard to swat flies. Flies and other insects can't see exactly who or what you are but they can detect movement from many directions. And that allows flies a good chance to take flight in time to avoid death by swatter!

A number of species of insects also possess something that resembles eyes with a single lens like those found on other animals. These are not real eyes. These are fake eyes, structures called eyespots. Eyespots don't function for vision but, none-the-less, are valuable to insects. So how can a fake eye be of importance to an insect that possesses one? Eyespots can be used to convey a message. That message is a deceitful one.

The fake eye is used to startle predators and help keep the insect from becoming a meal. It works this way. Many animals that use insects as food items are themselves food for other animals, such as hawks and snakes. So if an insect suddenly displays coloration that resembles the eye a larger predator, the insect predator might think it has gone from the hunter to the hunted. Under these conditions, the insect predator might not stay around to determine the truth of the matter, and the insect lives for another day.

Most of the time eyespots on insects are hidden from view when the insect is at rest. When the moth is at rest, the eyespots are not visible because the back wings are hidden by the front wings. If disturbed, such as being pecked at by a bird, the moth will expose the back wings with the eyespots. The bird is normally startled at the sight of the "eyes" and exhibits a hasty retreat.

Eyespots are found not only on moths but on a few butterflies and some praying mantids. Mantids are predators on other insects. But as it turns out, mantids can be food for other animals and also need predator protection. Some caterpillars also have eye spots. On a caterpillar, the eyes, when exposed, produce the illusion of a snake.

The most remarkable part of eyespots on insects is how real the “eyes” look. In some insects, you can see the pupil and iris of the eye and even the light reflection from the lens. Hey, even in insects, if a message sent from an eye, even a fake eye, is to be of value, it must be convincing!

 

Writer: Tom Turpin
Editor: Olivia Maddox