[Illustration: Life history of house flies, showing from left to right the eggs, larvæ, pupæ, and adult flies. (Photograph, about natural size, by Overton.)]
The Fly a Disease Carrier.--We have already seen that mosquitoes of different species carry malaria and yellow fever. Another rather recent addition to the black list is the house fly or typhoid fly. We shall see later with what reason this name is given. The development of the typhoid fly is extremely rapid. A female may lay from one hundred to two hundred eggs. These are usually deposited in filth or manure. Dung heaps about stables, privy vaults, ash heaps, uncared-for garbage cans, and fermenting vegetable refuse form the best breeding places for flies. In warm weather, the eggs hatch a day or so after they are laid and become larvæ, called maggots. After about one week of active feeding, these wormlike maggots become quiet and go into the pupal stage, whence under favorable conditions they emerge within less than another week as adult flies. The adults breed at once, and in a short summer there may be over ten generations of flies. This accounts for the great number. Fortunately relatively few flies survive the winter. The membranous wings of the adult fly appear to be two in number, a second pair being reduced to tiny knobbed hairs called balancers. The head is freely movable, with large compound eyes. The mouth parts form a proboscis, which is tonguelike, the animal obtaining its food by lapping and sucking. The foot shows a wonderful adaptation for clinging to smooth surfaces. Two or three pads, each of which bears tubelike hairs that secrete a sticky fluid, are found on its under surface. It is by this means that the fly is able to walk upside down, and carry bacteria on its feet.
[Illustration: The foot of a fly, showing the hooks, hairs, and pads which collect and carry bacteria. The fly doesn't wipe his feet.]
[Illustration: Colonies of bacteria which have developed in a culture medium upon which a fly was allowed to walk.]
The Typhoid Fly a Pest.--The common fly is recognized as a pest the world over. Flies have long been known to spoil food through their filthy habits, but it is more recently that the very serious charge of spread of diseases, caused by bacteria, has been laid at their door. In a recent experiment two young men from the Connecticut Agricultural Station found that a single fly might carry on its feet anywhere from 500 to 6,600,000 bacteria, the average number being over 1,200,000. Not all of these germs are harmful, but they might easily include those of typhoid fever, tuberculosis, summer complaint, and possibly other diseases. A recent pamphlet published by the Merchants' Association in New York City shows that the rapid increase of flies during the summer months has a definite correlation with the increase in the number of cases of summer complaint. Observations in other cities seem to show the increase in number of typhoid cases in the early fall is due, in part at least, to the same cause. A terrible toll of disease and death may be laid at the door of the typhoid fly.