d.

You think cicada has a very broad back, Nell?

So it has, and a broad head.

[Illustration]

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.

[Illustration]

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.

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 lose