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True Flies — Part 2

As early as 1864, Leidy attributed the spread of gangrene in hospitals during the Civil war to the agency of the house-fly, and the terrible disease known as malignant pustule was afterward discovered to be caused by the bite of one of the gad-flies which carried the bacillus of anthrax from diseased cattle and by its bite inserted it into the circulatory system of human beings. The carriage of the purulent ophthalmia of the Egyptians by the house-fly was later demonstrated, and the spread of the disease known as "pink-eye" in the South has been shown by Hubbard to be facilitated by little midges of the genus Hippelates. An English army surgeon has ascertained that the tsetse-fly of Africa carries pathogenic germs from diseased cattle and by its bite transfers them to the blood of healthy cattle, and late investigations have shown that certain flies, and especially the common house-fly, are responsible not only for the spread of Asiatic cholera but of the everywhere prevalent and dreaded disease known as typhoid fever. A vital stimulus to this line of investigation has been given by the discovery that certain mosquitoes are responsible for the spread of malarial fevers and a very great interest has been excited and an enormous literature has sprung up within the last few years concerning this line of investigation. This interest has become even more intensified by the experimental proof obtained by the United States Army Yellow Fever Commission of the agency of certain mosquitoes in the spread of yellow fever. The whole subject of the agency of insects in the transmission of disease is one of the most prominent subjects of medical investigation at the present time and nearly all of the insects concerned in this work belong to this order Diptera; so that, in spite of the benefits to humanity which the parasitic species bring by their destruction of injurious insects and in spite of the beneficial function which many Diptera exercise as scavengers, this incident of the lives of many of them, added to the ravages of many more on crops and domestic animals, makes the order a distinctly and markedly injurious one.

Many strange features in life history occur with the flies. With some no eggs are laid and living larvae issue from the body of the female. Such flies then become practically viviparous, or "larviparous." With others, although these are few in number, the development within the body of the female goes even farther and when the insect emerges from the body of its mother it is already in the pupal condition. Such forms are called "pupiparous." We have mentioned the wings of the Diptera, but in some forms there are no wings. Such species, and they are also few in number, are usually parasites, and the loss of wings is one of the degradational features consequent upon the parasitic life. See the bedbug among the Heteroptera, the true lice (Anoplura) and the bird lice (Mallophaga). With those species which lay eggs the larval development is usually rapid; and with some forms, particularly those which are true scavengers and feeders upon carrion or upon excrement, it becomes very rapid. The possibilities for enormous multiplication are apparently greater in this order than in any other group of insects. It is estimated that the progeny of a single house-fly, if undisturbed, would in the course of a single summer reach high into the billions in numbers, while an almost equally rapid multiplication takes place with some of the mosquitoes.

There is great variation in habits in the group. Most flies prefer the sunshine and are most numerous in the middle of sunny days. A few, however, such as the mosquitoes, fly at night. These, however, are the great exception. Very many flies frequent flowers, and thus exercise a beneficial function in the cross-fertilization of plants. Many species--comprising, in fact, whole families--are aquatic or sub-aquatic in their early stages, and some possess the faculty of living under what appear to be most disadvantageous conditions. Some of the flies of the peculiar family Ephydridae, for example, live in the strongly alkaline lakes of the far West where almost nothing else can live.