A LTHOUGH flies are of use, they also do evil to men in many ways. It is well to look at things on all sides.
The fly you have been reading about is the common house-fly. That fly, with its noise, dirt, and spoiling of food by laying eggs in it, is bad enough. But yet the house-fly makes the least trouble of any of its kind.
There are many kinds of flies. To the family of flies belong gnats, midges, mosquitoes, and the big daddy-long-legs with wings.
You know well how some of these things sting, you say "bite," you. Mr. Daddy-long-legs hurts the grass lands with his grubs, which spoil grass roots and the shoots of plants.
There is a fly called a "gall-fly" because it bites trees, and lays eggs in twigs. Then upon the twigs grow over the eggs round balls called "galls," and these injure the trees.
There is also the "bot-fly," which lays its eggs on the hide of the horse. The egg causes the skin of the horse to itch. He licks the place, and the egg goes into his stomach.
The egg of the bot-fly is apt to make the horse sick. The grub eats holes in the stomach of the horse. That makes the horse sick. The farmer will say that his horse is sick with "bots."
In Africa flies kill horses and oxen by biting them. The bite poisons the cattle and causes fever.
Farmers will tell you of a very bad fly that spoils wheat and other grain. It is called the "Hessian" fly.
Flies, as they flit from place to place, sometimes carry with them the poison of disease, as of sores and ulcers. Thus they spread these troubles among people.
But while I tell you of that, I must not fail to say that flies, as they go to flowers for honey, carry the dust of the flowers from one to another. This helps new flowers to grow.
There is a large and handsome bright green fly, very fine to look at, which bites horses and worries them. It is called the "horse-fly." In some lands a small sand-fly causes sore eyes.
Flies have been on the earth about as long as men have, or a little longer, and there are some dead flies worth a great deal of money.
How is that? These are flies in amber. Amber is clear, hard, and bright yellow. It is used for jewelry. Sometimes we see a perfect fly, held in a clear, light mass of amber.
How did it come there? The amber was once a soft gum and the fly lit on it. It stuck fast, and the amber flowed over it and grew hard, and so buried the fly in a clear, golden tomb. A piece of amber with a fly in it will bring a high price.
The "Spanish fly" is a large blue-green beetle. It is very handsome, and is most useful when it is dead. It is used in medicine. It makes blisters on the skin.
Do you say, "Oh, blisters are very bad!" Yes, they cause pain. But even pain can be of use in this world. The blister, though it pains us, is of use. It cures what might be a worse pain.
This Spanish fly is not a fly at all. It is a beetle which has been given a fly's name. It is put here at the end of the lessons on flies, because in the next lessons you are to read about beetles.