May says she wishes that locust would keep quiet. It makes her warmer
than ever to hear him carrying on so this hot day.
John says it is the weather that is warm, not the song of the locust.
And yet, locusts generally sing during the hottest part of the summer,
so that we have learned to associate them with warm weather.
Since we must listen to its shrill out-cry, I wish we could also see
it.
Ah, that is a wish soon gratified! Here comes one out of John's pocket.
[Illustration]
John says it is _not_ a locust.
Ah, yes, the shorthorned grasshoppers are the real locusts, and this
fellow has somehow got the name.
But it is not a locust.
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.
[Illustration]
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.
[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 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.
[Illustration]
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.
[Illustration]
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 larvæ 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.
[Illustration]
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.
If we look, we will also find other larvæ shells. Down in the grass are
the cast-off coats of the grasshoppers and the crickets.
All we need do is to look, and we shall be sure to find them--like
unsubstantial ghosts of the active little wearers.
No doubt you all have heard of the seventeen-year locusts. They, too,
are cicadas, and they look very much like this one, only it takes the
young ones seventeen years to complete their growth.
Think of living in the ground and sucking the juices out of the earth
and of tree roots for seventeen years!
[Illustration]
How would you like to do it?
But no doubt the cicada is quite happy living in this way.
At the end of seventeen years the cicadas come up out of the earth in
great swarms.
They cast their skins for the last time. The queer little shells are
seen everywhere, and the air resounds with the songs of the freed
prisoners.
In the South it takes only thirteen years for these cicadas to develop.
[Illustration]
I once went up the side of a beautiful mountain in North Carolina, where
was such a mighty host of cicadas in the trees that I could not hear my
companion speak, and a little way off the noise sounded like a torrent
of rushing water.