The most generally accepted classification divides the insects into nineteen orders; as the Coleoptera, containing the beetles; the Lepidoptera, containing the butterflies and moths; the Hymenoptera containing the bees, ants and wasps, etc. Four or five of these orders will be of more or less interest to us.
The order Diptera, or two-winged flies, is the most important because to this belong the mosquitoes which transmit malaria and yellow fever, and the house-fly that has come into prominence since it has been found to be such an important factor in the distribution of typhoid and other diseases.
The order Diptera is divided into sixty or more families, many of which contain species of considerable economic importance. For our present consideration the flies may be divided into two groups or sections: those with their mouth-parts fitted for piercing such as the mosquito and horse-fly, and those with sucking mouth-parts such as the house-fly, blow-fly and others.
Some of the species belonging to the first group are among the most troublesome pests not only of man but of our domestic animals as well. Next to the mosquitoes the horse-flies (Fig. 22) are perhaps the best known of these. There are several species known under various names, such as gad-fly, breeze-fly, etc. They are very serious pests of horses and cattle, sometimes also attacking man. Their strong, sharp, piercing stylets enable them to pierce through the toughest skin of animals and through the thin clothing of man. The bite is very severe and irritating, and as the flies sometimes occur in great numbers the annoyance that they cause is often very great indeed. It has often been claimed that these flies as well as the stable-fly and others carry the anthrax bacillus on their proboscis from one animal to another, and although this may not have been definitely proven the evidence is strong enough to make a very good case against the accused. It is interesting to note in this connection that anthrax, a very common disease among the domestic animals and one which may attack man also, was the first disease to be shown to be of bacterial origin. It was only about thirty-five years ago that the investigations of Koch and Pasteur demonstrated that the presence of this particular germ (_Bacillus anthracis_) was the cause of the disease, and it was early recognized that such biting flies may be important factors in the spread of the disease.