They deposit their eggs in stable manure or other decaying matter; the maggots, developing very rapidly in warm weather, form the usual hard parchment-like chrysalids from which the perfect flies emerge. Many interesting observations have been lately made on the habits of house flies and the danger of their spreading diseases by carrying germs or particles of putrid matter upon their feet, and thus contaminating food or transferring germs into open wounds; it was proved in the Spanish-American war that the swarms of flies had a great deal to do with the spread of fever in this manner. _Musca domestica_, the common house fly, is almost world-wide in its distribution, and is the chief species found inside the house. In the larval state it chiefly develops in stable manure. It measures about ¼ of an inch in length; is of a uniform black tint but is so thickly clothed with grey tomentum that it appears to be brown; the eyes are red; the thorax is clothed with stiff black bristles, and has four parallel bars down the centre of the dorsal surface. The freshly deposited eggs hatch within a day or two; the maggots develop within six days, and remain in the pupal state for only a few days in the summer; so that it is no wonder that they multiply with such marvellous rapidity, particularly when we discover that one house fly will lay over 1,000 eggs in the season. _Musca corvina_ is a smaller darker tinted species, showing only two parallel stripes down the thorax. It is a common bush species and a great pest in the bush all through the summer, swarming in countless thousands from the eastern coast into the interior. It has a wide range over Europe, North America, Ceylon, and the Malay Archipelago. _Stomoxys calcitrans_ is of a more brownish tint, with the abdomen more flattened, and it differs from the last two species in having a well developed biting mouth; this fly is a troublesome pest to horses, and will alight on one’s hand and bite quite sharply. It has a wide range from Europe across Asia to Ceylon, Java, and Australia.