A. House-fly.
B. A grown-up mosquito, two larvæ, and a pupa.
"Well," said the guide, "the fly is such a little acrobat it can crawl up the steepest and most slippery wall and walk upside down or right side up with the greatest ease. Perhaps some day you can make a fly keep still long enough so that you can look at its foot. At the end of the foot are two little round pads thickly covered with downy hair. On each side are two sharp claws and many stiff, clinging hairs. With this flattened foot it can go wherever it wishes.
"But this same little foot is the chief reason why a fly should never be allowed in the house, for flies crawl into all sorts of dirty places, and the fine hairs catch and hold the dirt. When the fly lights on us or on the table, some of the pieces of dirt are shaken off."
"But they are so hard to catch," said Betty; "it takes Lizzie forever and forever to get them out of the dining-room in the morning."
"I know why they are hard to catch," added Jack, "for I've looked at a dead fly. They have such big eyes, like lighthouses, they can see all around."
"Yes," said Ben Gile; "there is no such thing as creeping up on a fly unawares. Flies are dirty creatures," continued the old man, "and the time is not very far distant when people will make war on them just as they do on mosquitoes. Mrs. Fly lays her eggs in unclean places, and as many as a hundred eggs at a time. These eggs hatch out quickly. It takes only twenty-one days to make a chicken out of an egg, but to make a baby fly it takes only a few hours, and ugly babies they are—little white maggots, or worms, that live and feed and grow rapidly in dirty places. Within six days the maggot becomes a tiny, dark-brown pupa, and after five days the pupa hatches out into a grown-up fly."
A dozen little girls at the party made up their minds promptly that after this evening they, at least, would make war on flies.
"And aren't flies of any use?" asked Betty.
"There is one little fly, Mrs. Tachina-Fly, who is of some use. She is a cousin of the house-fly. She is of use because she chooses a queer place to lay her eggs—on the back of a young caterpillar. After these caterpillars grow and shut themselves up into a cocoon to change into a butterfly the little fly eggs hatch out into maggots. Of course they are hungry—all babies are; and finding the nice, fat caterpillar in the round house, like dutiful babies they eat what is set before them until the fat, tender caterpillar is eaten up. After they are satisfied they lie still in their brown skins and change into grown-up tachina-flies, and at last out come a lot of busy, buzzing, bothersome flies. It is rather hard on the caterpillar. But when we think what harmful, greedy things most caterpillars are, perhaps it is good that there are tachina-flies to eat them. Is it time for supper yet?"