"In 1895 the writer became interested in the study of the . Breeding-cage experiments with some detail later on in this paper early convinced him that is the favorite food of this species. Even in the presence of kitchen garbage, , and , flies in confinement oviposited exclusively on horse manure. In the absence of the latter substance but in the presence of the others, he noted egg-laying on decaying fruit and on cow dung but the resultant larvæ failed to develop. He considered himself warranted in the statement that probably 95 percent of the flies found in cities come from the piles of horse manure everywhere so prevalent, especially in the vicinity of stables."