[Illustration: FIG. 22--Horse-fly (_Tabanus punctifer_).]
[Illustration: FIG. 23--Stable-fly (_Stomoxys calcitrans_).]
[Illustration: FIG. 24--A Black-fly (_Simulium sp._). (From Kellogg's Amer. Insects.)]
[Illustration: FIG. 25--Screw-worm fly (_Chrysomyia macellaria_).]
[Illustration: FIG. 26--Blow-fly (_Calliphora vomitoria_).]
The stable-fly (Fig. 23) (_Stomoxys calcitrans_) which looks very much like the house-fly and, as will be noted later, frequently enters houses, is often an important pest of horses and cattle. Its blood-sucking habit makes it quite possible that it too may be concerned in carrying anthrax and other diseases.
In a later chapter it will be shown how the tsetse-fly, which is somewhat like the stable-fly, is responsible for the spread of the disease known as the sleeping sickness. This disease is caused by a Protozoan parasite, a trypanosome, which is transmitted from one host to another by the tsetse-fly.
In Southern Asia and in parts of Africa there is a very serious disease of horses known as surra which is caused by a similar parasite (_Trypanosoma evansi_). This parasite attacks horses, mules, camels, elephants, buffaloes and dogs, and has been recently imported into the Philippines. It is supposed that flies belonging to the same genus as the horse-fly (_Tabanus_ and others), and the stable-fly (_Stomoxys_) and the horn-fly (_Hæmatobia_) are responsible for the spread of the disease.
Nagana is one of the most serious diseases of domestic animals in Central and Southern Africa. In some sections it is almost impossible to keep any kind of imported animals on account of this disease which is caused by a parasite (_Trypanosoma brucei_) similar to the one causing surra. This parasite is to be found in several different kinds of native animals which seem to be practically immune but are always a source of danger when other animals are introduced. Two or three species of tsetse-flies are responsible for the transmission of this disease.
Another group of flies much smaller but more numerous and much more insistent are the black-flies or buffalo-gnats (Fig. 24). For more than a century these little flies have been recognized as among the most serious pests of stock, particularly in the south where, besides the actual loss by death of many animals yearly, the annoyance is so great as to sometimes make it impossible to work in the field. Human beings are often attacked, and as the bite is poisonous and very painful great suffering may result and cases of deaths from such bites have been reported.