Part 53

It is also called the dog-day harvest fly, but it is not a fly, though it looks considerably like one.

Really, you know, it is a--bug!

Yes, it belongs to the bug order.

Its true name is cicada, and its shrill midsummer song has been famous from the beginning of time.

It looks like an enormous fly, but its mouth parts are the mouth parts of the bug, and in other respects it resembles the members of the bug order, when it is examined closely.

What glassy wings!

Let us spread them out carefully. Four of them it has.

The cicada, you see, has no wing covers. Nor are its upper wings, half wing cover, and half wing, like those of so many of the bugs.

No, all four of its wings are alike, and all four are flying wings.

When it is at rest, the inner wings slip out of sight under the outer ones, which fold down like a roof over its body.

See how beautifully the wings, are veined.

You think cicada has a very broad back, Nell?

So it has, and a broad head.

See its black eyes on the corners of its head!

How many facets have its eyes?

I wish I knew, but I do not. This, however, I can tell you. If you look on the top of its head between its compound eyes, with a magnifying glass, you will find it has three little eyes there.

These small eyes are simple, and are called ocelli.

Many insects have ocelli, indeed, some of the grasshoppers have these extra eyes on top of their head.

May says the grasshoppers are very astonishing insects.

You think you know all about them, and you are all the time finding out something new. You would not be apt to notice these little ocelli on the grasshopper's head, they are so small, and besides, some of the grasshoppers do not have them.

Yes, Mollie, it is the same with the crickets and katydids. Some species have ocelli, and some have not.

If you look full in the face of a cicada, you can see the three little round ocelli between the compound eyes.

They show very plainly with a magnifying glass.

Indeed, it is difficult to explain what the ocelli are for.

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.