Well, gills in fishes and in such insects as have gills, and in crabs and lobsters and other creatures that live in the water, are parts that often look like fringes or flat plates.
The gills of fishes have a great many blood vessels running through them. The walls of these blood vessels are very thin, and the oxygen from the air that is in the water pa.s.ses into the blood that is in the gills, and then this blood, all full of oxygen, circulates through the fish"s body.
You see in fishes the blood vessels come into the gills and get the oxygen.
In insects it is different. There are air tubes running like tiny pipes all through the gills and into the body of the insect. The oxygen of the air that is in the water pa.s.ses out through the walls of these tubes into the blood of the insect.
Yes, John, in fishes the blood comes to the air, in insects the air goes to the blood. The air pa.s.ses into the air tubes of the insects, and thus is carried all through their bodies.
The blood takes the oxygen out of the air.
Without oxygen in the blood no animal could live.
Now let us go back to our May flies. They remain in the larval state a year, and some species remain two years. Think of living in the mud for two long years!
In the mud they creep about, eating, eating, eating. Then some summer day they leave the mud and swim to the surface of the water.
Pop! they are gone.
They were so quick about it we could not see what happened.
The larval skin burst open and forth leaped the May fly, like a winged fairy from a prison cell.
They do not come out slowly and wait for their wings to dry like the dragon fly.
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They spring out all of a sudden and fly away, leaving their cast-off skin in the pond.
Unless their motions were quick they might be snapped up by the fish that are so fond of them.
But though they seem to emerge thus quickly into perfect winged May flies, they are not quite done with infancy. They are still wrapped about by a very delicate skin that they have to get rid of. So they fly to a bush near the water and stay a little while until this skin splits and comes off, and they are free.
In spite of their quick motions when they spring from the water, many of the May flies fall back into it and are caught by the fish.
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It is said that the trout become fat and good-flavored when the May flies emerge, they eat so many of them. And what the fish do not catch the birds try to. Swallows and other insect-loving birds have a glorious feast when the May flies come out. For a season they live in the midst of more delicacies than they can possibly use.
Fish like the May fly larvae, too, which is probably the reason the larvae have learned to live in the mud, out of reach.
Fishermen dig up the larvae for bait, so you see the May flies have a hard time to get safely through the world.
But in spite of difficulties a great many of them live, and some summer day out they come trooping.
They spring all at once from the surface of the water as by magic, hundreds and thousands, yes, millions of them. They fill the air, they cover everything.
The great naturalist Swammerdam, who was the first to make a thorough study of the May flies, thus tells us how they appeared in France one year:--
"I then saw a sight beyond all expectation. The ephemerae filled the air like the snowflakes in a dense snowstorm.
"The steps were covered to a depth of two, three, or even four inches. A tract of water five or six feet across was completely hidden, and as the floating insects slowly drifted away, others took their places. Several times I was obliged to retreat to the top of the stairs from the annoyance caused by the ephemerae, which dashed in my face, and got into my eyes, mouth, and nose."
These swarms of May flies appear only from three to five days at a time.
Wherever there are streams there are May flies, and the ca.n.a.ls of Holland make good breeding places for them; no wonder, then, the Dutch, who you know live in Holland, have a saying, "As thick as May flies."
Although so many of the May flies perish at once, mult.i.tudes of them drop their eggs into the water to renew the race of May flies.
Is it not wonderful that after so long a period of creeping about in the mud as larvae, these graceful and beautiful little creatures have but a few hours in which to dance joyously about in the upper air on wings of gossamer? Some, indeed, live less than an hour, and some, that come out in the evening, finish their dance of life and perish before sunrise, without ever having seen the beautiful daylight.
Yes, strange little beings are they.
They do us no harm and we should not kill them.
Let them live their short lives and be happy.
THE STONE FLY FOLK
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John has been fishing.
What do you think he caught?
Nary fish, my dears, but a goodly number of stone flies, which he has brought to show us.
Yes, Mollie, they do remind us a very little of our May flies, only, of course, they are many times larger.
It is rather a clumsy creature in spite of its large wings, and John says he had no trouble whatever in catching it.
See, it has four wings, and the hind ones are the larger.
Yes, May, they fold up in plaits, like the sticks of a fan.
See its long antennae and its compound eyes. Its eyes are not so large as are those of the dragon fly. It does not spend its time pursuing other insects, but is more like the May fly after it gets its wings.
Yes, Ned, it lives longer than the May fly, but it does not live very long, and it eats little.
It is a pretty little gray thing as it rests on the side of John"s box, with its wings folded like a gossamer cloak over its body.
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It lays its eggs in the water, and out of them hatch little six-legged larvae that are not troubled by want of appet.i.te. If the winged stone fly does not eat, its larva does; it is like the other larvae we know, always devouring something.