Another group of bacteria, which float about in the air almost everywhere, are the yeasts, which we harness to our use for the very wholesome and healthful process of bread-making. Millions upon millions of bacteria of different sorts live and grow naturally in our stomachs and intestines; and while they are probably of no special advantage to us, yet at the same time the majority of them are practically, within reasonable limits--not to exceed a few billions or so--harmless. Insect Pests. One kind of "dirt" that should be avoided with special care is insects of all sorts. No one needs to be told to try to keep a house, or a room, clear of fleas, bed-bugs, or lice; indeed to have these creatures about is considered a mortal disgrace. Not only is their bite very unpleasant, but they may convey a variety of diseases, including plague and blood poisonings of various sorts. But there is another insect pest far commoner and far more dangerous than either fleas or bed-bugs, whose presence we should feel equally ashamed of; and that is the common house fly. This filthy little insect breeds in, and feeds upon, filth, manure, garbage, and dirt of all sorts, and then comes and crawls over our food, falls into our milk, wipes his feet on our sugar and cake, crawls over the baby's face, and makes a general nuisance of himself. Take almost any fly that you can catch, let him crawl over a culture plate of gelatin, put that gelatin away in a warm place, and you will find a perfect flower-garden of germs growing up all over it, following the pattern made by the tracks of his dirty feet. In this garden will be found not "silver bells and cockle shells and pretty maids all in a row," but a choice mixture of typhoid bacilli, pus germs, the germs of putrefaction, tubercle bacilli, and the little seeds which, if planted in our own bodies, would blossom as pneumonia or diphtheria. [Illustration: AN EDUCATIONAL FLY POSTER] The fly is an unmitigated nuisance and should be wiped out. No half-way measures should be considered. Fortunately, this is perfectly possible; for his presence is our own fault and nothing else, as he can lay his eggs and hatch only in piles of dirt and filth found about our own houses, barns, and outbuildings. He is not a wild insect but a domestic one and is practically never found more than a few hundred yards away from some house or barnyard. His favorite place for breeding is in piles of stable manure, especially horse manure; but neglected garbage cans, refuse heaps, piles of dirt and sweepings, decaying matter of all sorts, which are allowed to remain for more than ten days or two weeks at a time, will give him the breeding grounds that he needs. [Illustration: A BREEDING PLACE OF FLIES AND FILTH]