The flesh-fly instead of laying eggs deposits the living larvæ upon meat wherever it is accessible, and as these develop with astonishing rapidity they are able to consume large quantities of flesh in a remarkably short time. In this way they may be of some importance as scavengers, but it is better to get rid of the waste in other ways than to leave it for a breeding-place for flies that are capable of causing so much damage and suffering. Not infrequently the larvæ of certain flies are to be found in the alimentary canal where as a rule they do no particular damage. Altogether the larvæ of over twenty different species of flies have been found in or expelled from the human intestinal canal. In Europe, the majority of these larvæ belong to a fly which looks very much like the house-fly except that it is somewhat smaller and so is often known as "the little house-fly" (Fig. 29) (_Homalomyia canicularis_). The same species is very common in the United States, frequently occurring in houses. Under certain conditions it may even be more abundant than the house-fly. It is believed that the larvæ in the intestinal canal come from eggs that have been deposited on the victim while using an outdoor privy where the flies are often very abundant. Instances are also on record where these larvæ have been discharged from the urethra. Another fly (_Ochromyia anthropophaga_) occurring in the Congo region has a blood-sucking larvæ which is known as the Congo floor-maggot. The fly which is itself not troublesome deposits its eggs in the cracks and crevices of the mud floors of the huts. The larvæ which hatch from these crawl out at night and suck the blood of the victim that may be sleeping on the floor or on a low bed. BOT-FLIES Another group of flies known as the bot-flies (Fig. 30) have their mouth-parts rudimentary or entirely wanting so of course they themselves cannot bite or pierce an animal. Nevertheless they are the source of an endless amount of trouble to stockmen and sometimes even attack man. Although these flies cannot bite, the presence of even a single individual may be enough to annoy a horse almost to the end of endurance. Horses seem to have an instinctive fear of them and will do all in their power to get rid of the annoying pests. The eggs of the house bot-fly are laid on the hair of the legs or some other part of the body. The horse licks them off and they hatch and develop in the alimentary canal of their host. Sometimes the walls of the stomach may be almost covered with them thus of course seriously interfering with the functions of this organ. When full grown the larvæ pass from the host and complete their transformation in the ground. [Illustration: FIG. 27--Blue-bottle fly (_Lucilia sericata_).] [Illustration: FIG. 28--Flesh-fly (_Sarcophaga sp_).] [Illustration: FIG. 29--"The little house-fly" (_Homalomyia canicularis_).] [Illustration: FIG. 30--Horse bot-fly (_Gastrophilus equi_).]