In and about our homes many bits of things drop, and might decay and mould. This would make the air foul. But the busy and greedy fly drinks up all the soft part of these things. So we see that what we call the dirty flies help to clean away much dirt. It is true too, that flies carry poison on their feet and trunks from place to place. In this way they do great harm. The fly serves for food for many birds, and fish and frogs, and some insects. Some of these things we use for our food. Others are full of beauty, or are of use to us, each in its own way. Thus, though the fly is often a trouble to us, we find it is not without its uses. Look at one of these little creatures through a glass that will magnify it. You will see that the poor insect has really much beauty. From what you have read in this lesson you must not think that all foul smells kill, nor that things that have no bad smell are always safe. There are some gases that have no odor at all, which yet are very deadly. LESSON XX. A SWARM OF FLIES. Have you heard people speak of swarms of flies? By a swarm of flies we mean a great number of flies rather near together. By a swarm of bees we mean a number of bees that live and work in one place. A swarm of bees divides the work of its hive. It has one queen bee. She is the mother and ruler of the rest. But flies have no home where they live in common. They have no work. They have no one mother or queen, for whom the rest work. Each mother fly drops her eggs where it seems best to her. Then she goes off. She leaves her children to grow as best they can. I have said that the fly likes best to place her eggs on a piece of fresh meat. These eggs soon turn to worms or grubs, and so spoil the meat. To keep the meat from the flies the cook puts a cover over it. The cover is often made of wire net. “Now,” says the cook, “I can keep away that dirty fly.” But Mrs. Fly says, “Oh, can you, Mrs. Cook? We will see about that.” So Mrs. Fly sits on top of the wire cover. She puts her little egg tube through one of the fine holes in the net. She drops egg after egg from the tube. The eggs fall right on the meat, just where Mrs. Fly wishes them to be. Then the cook cries out, “How _ever_ did that fly get to my meat?” Is it not strange that Mrs. Fly knows that her egg tube is the right size to go through the mesh of the wire net? How does she know that the eggs will fall on the meat? Flies do another queer thing. If many flies are in a room, and you begin to chase them to kill them, they hide. They creep into holes and cracks. They hide in curtains. They go behind pictures. After the hunt is over, out they come, one by one! Flies also know how to sham death, “play dead,” you would say. If you hit one and make it fall, it will lie very still, and seem to be dead. Then, after a little, it softly spreads out its legs and its wings. Then it shakes itself. A moment more, off it goes.