e. Eyes microscopically hairy; each abdominal segment with two spots. Larvæ in dung. Myiospila meditabunda
ee. Eyes bare; abdomen gray and brown marbled. Muscina
f. With black legs and palpi. M. assimilis
ff. With legs more or less yellowish; palpi yellow. Larvæ in decaying vegetable substances, dung, etc. M. stabulans
It is almost universally believed that the adults of Musca domestica hibernate, remaining dormant throughout the winter in attics, around chimneys, and in sheltered but cold situations. This belief has been challenged by Skinner (1913), who maintains that all the adult flies die off during the fall and early winter and that the species is carried over in the pupal stage, and in no other way. The cluster-fly, Pollenia rudis, undoubtedly does hibernate in attics and similar situations and is often mistaken for the house-fly. In so far as concerns Musca domestica, the important question as to hibernation in the adult stage is an open one. Many observations by one of the writers (Johannsen) tend to confirm Dr. Skinner's conclusion, in so far as it applies to conditions in the latitude of New York State. Opposed, is the fact that various experimenters, notably Hewitt (1910) and Jepson (1909) wholly failed to carry pupæ through the winter.
The house-fly breeds by preference in horse manure. Indeed, Dr. Howard, whose extensive studies of the species especially qualify him for expressing an opinion on the subject, has estimated that under ordinary city and town conditions, more than ninety per cent of the flies present in houses have come from horse stables or their vicinity. They are not limited to such localities, by any means, for it has been found that they would develop in almost any fermenting organic substance. Thus, they have been bred from pig, chicken, and cow manure, dirty waste paper, decaying vegetation, decaying meat, slaughter-house refuse, sawdust-sweepings, and many other sources. A fact which makes them especially dangerous as disease-carriers is that they breed readily in human excrement.
The eggs are pure white, elongate ovoid, somewhat broader at the anterior end. They measure about one millimeter (1-25 inch) in length. They are deposited in small, irregular clusters, one hundred and twenty to one hundred and fifty from a single fly. A female may deposit as many as four batches in her life time. The eggs hatch in from eight to twenty-four hours.
The newly hatched larva, or maggot ([fig. 108]), measures about two millimeters (1-12 inch) in length. It is pointed at the head end and blunt at the opposite end, where the spiracular openings are borne. It grows rapidly, molts three times and reaches maturity in from six to seven days, under favorable conditions.