Flies like manure because it is the best place they can find in which to lay their eggs. Each female fly lays about three hundred eggs. They do not hatch directly into flies, as hen's eggs hatch into chickens, but when the fly's eggs hatch you find maggots, and these maggots later hatch into flies.
The journey of the fly
Turn over the manure some spring morning, and you will see it full of white specks. These specks are maggots that will hatch into flies. Watch the flies as they leave the manure pile and see where they go. If there is a dead dog or cat or a filthy garbage can near, they will fly to it. Then they will go into the water-closet and crawl over the filth there. Their next trip will probably bring them to the kitchen, where they will crawl over the food. From here they will go to the cuspidor and take a drink of water, and will get their feet covered with the dirt that is in the cuspidor. Next they will try a walk over the nipple of the baby's bottle, or they will light on your face, or get into the butter or milk.
After the fly has been in dirty places, he "washes" his face and hands, that is, he rubs his feet together and then rubs them over his head. Did you ever see a fly wash himself with water? No, you never did.
Fig. 43. Where a fly has walked; each little spot represents a growth of germs left by the fly.
After a fly has made his journey, you would suppose that his feet would be covered with dirt and germs, and so they are. Not only does he carry germs on his feet and body but he also eats dirty and diseased things. Moreover, fly specks contain the germs of disease, and the fly is not at all particular about where he puts his specks.
Proof that the fly is a germ carrier
If you let a fly walk over a culture plate, there will be a growth of germs wherever his feet touch. A culture plate is simply a glass plate covered with gelatine or something else in which these germs like to grow, and where they can easily be seen. Each germ will multiply so fast that there will soon be a spot of them large enough to be seen readily with the naked eye. In the picture showing a culture plate over which a fly has walked (Fig. 43), the little specks are not single germs, but each speck represents a growth containing many thousands of germs.
