Stomoxys calcitrans may be separated from the house-fly by the use of the key on p. 145. It may be more fully characterized as follows:
The eyes of the male are separated by a distance equal to one-fourth of the diameter of the head, in the female by one-third. The frontal stripe is black, the cheeks and margins of the orbits silvery-white. The antennæ are black, the arista feathered on the upper side only. The proboscis is black, slender, fitted for piercing and projects forward in front of the head. The thorax is grayish, marked by four conspicuous, more or less complete black longitudinal stripes; the scutellum is paler; the macrochætæ are black. The abdomen is gray, dorsally with three brown spots on the second and third segments and a median spot on the fourth. These spots are more pronounced in the female. The legs are black, the pulvilli distinct. The wings are hyaline, the vein M1+2 less sharply curved than in the house-fly, the apical cell being thus more widely open (cf. [fig. 110]). Length 7 mm.
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.