Part 54

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.

Our cicada has one organ that is very interesting; it is the little apparatus by which it sings.

Turn it over, Ned, and all of you look at the two thin plates lying against the abdomen just below the thorax.

Those membranes are like two little kettle drums, and they are its song organs.

There are other membranes beneath them, and large muscles within the body to move the membranes.

The membranes being set in rapid vibration we get the shrill cry of the locust.

Only the male has the kettle drums. In the female these organs are rudimentary, and she is dumb.

Cicada, you are a pretty little thing with your clear, glasslike wings and your black body with red and green trimming. See its mouth lying in that little groove under its head. It is a tube, and sharp. The cicada sticks it into a leaf or young twig to suck out the juice.

Nell wants to know if the young cicadas are like the old ones. Indeed, they would be cunning little things if they were, and--yes, they would look very much like flies.

But the young cicadas are queer babies, indeed. They do not look very much like their parents, although they have a head, a thorax, and an abdomen.

The female cicada makes a slit in the bark of the tree twig with her ovipositor and lays the eggs there. As soon as they hatch out, the tiny cicadas drop down to the ground and burrow into the earth.

You would not know that they are cicadas, they are such queer-looking little things. But they have strong, sucking mouth parts with which they pierce holes in the roots of trees and suck out the juices.

Of course these larvae grow and moult and continue to do so until they have moulted a good many times and grown quite large.

They stay down under the ground two years.

At the end of that time they crawl up to the surface of the earth in the early summer.

They climb trees, or weeds, or fence posts, and then the skin splits down the back for the last time, and out comes a full-grown cicada with bright glassy wings.

The wings of the larva do not grow at each moult like the wings of the grasshopper.

The larva never gets beyond short little wing pads. See John's eyes twinkling! I believe--yes, he has! He has brought us the cast-off skin of a cicada to look at.

Why, John, you are like a good fairy to us to-day, giving us just the things we want just when we want them.

Now, see this little shell. See the front legs, like strong paws to dig with. And see its little glassy eyes, and its little wing pads!

It is a perfect cast of the cicada larva.

Yes, May, this little cast is made of chitin, and it will last a long time. Chitin is a very indestructible substance; even fire will not destroy it, but in course of time the moisture and the acids in the earth destroy it, so that at last the millions of cicada shells and grasshopper cast-off skins, which are also of chitin, and cricket moults, and all the other little cast-aside chitinous overcoats of the insects, return again to the earth and the air whence they came. The minerals and gases that compose them let go of each other, as it were, and the chitin is no longer chitin.

Amy says she has seen these little cicada shells hundreds of times but did not know what they were.

Yes, we are sure to find them almost every summer.