This fly is widely distributed, being found the world over. It was probably introduced into the United States, but has spread to all parts of the country. Bishopp (1913) regards it as of much more importance as a pest of domestic animals in the grain belt than elsewhere in the United States. The life-history and habits of this species have assumed a new significance since it has been suggested that it may transmit the human diseases, infantile paralysis and pellagra. In this country, the most detailed study of the fly is that of Bishopp (1913) whose data regarding the life cycle are as follows: The eggs like those of the house-fly, are about one mm. in length. Under a magnifying glass they show a distinct furrow along one side. When placed on any moist substance they hatch in from one to three days after being deposited. The larva or maggots (fig. 110) have the typical shape and actions of most maggots of the Muscid group. They can be distinguished from those of the house-fly as the stigma-plates are smaller, much further apart, with the slits less sinuous. Development takes place fairly rapidly when the proper food conditions are available and the growth is completed within eleven to thirty or more days. The pupa (fig. 110), like that of related flies, undergoes its development within the contracted and hardened last larval skin, or puparium. This is elongate oval, slightly thicker towards the head end, and one-sixth to one-fourth of an inch in length. The pupal stage requires six to twenty days, or in cool weather considerably longer. The life-cycle of the stable-fly is therefore considerably longer than that of _Musca domestica_. Bishopp found that complete development might be undergone in nineteen days, but that the average period was somewhat longer, ranging from twenty-one to twenty-five days, where conditions are very favorable. The longest period which he observed was forty-three days, though his finding of full grown larvæ and pupæ in straw during the latter part of March, in Northern Texas, showed that development may require about three months, as he considered that these stages almost certainly developed from eggs deposited the previous December. The favorite breeding place, where available, seems to be straw or manure mixed with straw. It also breeds in great numbers in horse-manure, in company with _Musca domestica_. Newstead considers that in England the stable-fly hibernates in the pupal stage. Bishopp finds that in the southern part of the United States there is no true hibernation, as the adults have been found to emerge at various times during the winter. He believes that in the northern United States the winter is normally passed in the larval and pupal stages, and that the adults which have been observed in heated stables in the dead of winter were bred out in refuse within the warm barns and were not hibernating adults.