Some think they are to see objects close at hand, while the compound eyes see more distant objects.
Others think the ocelli are only capable of distinguishing light from darkness.
Yet others think they are merely a "survival" of the eyes of the worms. You know, way back in time, before there were winged insects there were worms. In some way the insects are descended from the worms, and though they have got rid of many of their wormlike parts they still retain some of them, and probably among these are the ocelli.
When an animal of any kind keeps organs that belonged to its ancestors, but that are of no use to it, we say these organs are "survivals." They have not yet had time wholly to disappear.
Yes, John, the time may come when the ocelli will disappear from the insects. A good many insects have lost them already.
Indeed, you are right, May; they have lost them because they did not use them. When an animal ceases to use an organ in course of time, for lack of exercise, that organ dwindles away and disappears. It generally takes a very long time for this to happen.
Yes, Mabel, thousands or even millions of years may pass before an organ that has gone out of use entirely disappears. As generations succeed each other each generation loses a little power in that organ until, finally, there is no organ left.
John is puzzled to know just what is meant by an organ. It is some particular part of the creature. An arm is an organ, a stomach is an organ, an eye is an organ. The whole creature is made up of organs, and is called an organism.
Your whole body, John, is an organism, but your legs and arms are organs. Now, I think you understand.