_Musca domestica_ commonly lays its eggs upon horse manure. This substance seems to be its favorite larval food. It will oviposit on cow manure, but we have not been able to rear it in this substance. It will also breed in human excrement, and from this habit it becomes very dangerous to the health of human beings, carrying, as it does, the germs of intestinal diseases such as typhoid fever and cholera from excreta to food supplies. It will also lay its eggs upon other decaying vegetable and animal material, but of the flies that infest dwelling houses, both in cities and on farms, a vast proportion comes from horse manure. [Illustration: Fig. 4.--One of the blue-bottle flies (_Phormia terrænovæ_): Adult, enlarged. ( Author's Illustration.)] It often happens, however, that this fly is very abundant in localities where there is little or no horse manure, and in such cases it will be found breeding in other manure or in slops or fermenting vegetable material, such as spent hops, or bran, or ensilage. At Salem, Mass., Packard states that he reared a generation in 14 days in horse manure. The duration of the egg state was 24 hours, the larval state from 5 to 7 days, and the pupal state from 5 to 7 days. At Washington the writer has found in midsummer that each female lays at one time about 120 eggs, which hatch in 8 hours, the larval period lasting 5 days and the pupal 5 days, making the total time for the development of the generation 10 days. This was at the end of June. The periods of development vary with the climate and with the season, and the insect hibernates in the puparium condition in manure or at the surface of the ground under a manure heap. It also hibernates in houses as adult, hiding in crevices. The Washington observations indicate that the larvæ molt twice, and that there are thus three distinct larval stages. The periods of development were found to be about as follows: Egg from deposition to hatching, one-third of a day; hatching of larva to first molt, 1 day; first to second molt, 1 day; second molt to pupation, 3 days; pupation to issuing of adult, 5 days; total life round, approximately 10 days. There is thus abundance of time for the development of 12 or 13 generations in the climate of Washington every summer. [Illustration: Fig. 5.--The green-bottle fly (_Lucilia cæsar_): Adult, enlarged. (Author's Illustration.)]