The flies which enter dwellings in Iowa develop largely in the manure of farm animals and in refuse from the home. They are important from several standpoints. Some species are known to carry upon their bodies the germs of certain human diseases; furthermore, they are able to consume disease organisms with their food and subsequently to discharge these organisms in a living condition. Germ-laden flies may then carry the organisms to human food or place them upon the lips of sleeping children. A number of species are known to spread intestinal parasites of man; others carry important parasitic and disease-producing organisms of farm animals. Several species are pests of farm animals, which they annoy by biting or by crawling over their skin. Occasionally the larvae of certain flies enter wounds or the digestive and urinary tracts of man, producing a diseased condition known as myiasis.

Aside from spreading human and animal diseases, flies are destructive pests in the home. Whenever abundant, they befoul windows and woodwork and sometimes permanently damage fixtures, wall paper, pictures and hangings. In addition, the larvae of a number of species live in various foodstuffs and so render them unfit for human consumption.