Flies

(Order Diptera.)All the true flies, that is, those insects which are called flies and have but two wings, belong to the order Diptera. They are the only insects which possess but two wings, with the exception of the males of the scale insects, and a very few May flies (genera Cloeon and Coenis). Some insects in other orders have one pair of wings so greatly aborted that they appear two-winged as in the genus Psectra, one of the Lacewing flies. The wings are membranous and usually transparent and bear no scales, except in the mosquito family. The hind wings are represented only by two knobbed projections called haltered, or poisers. The metamorphosis is very complete, the larvae being always footless and usually apparently headless maggots and the pupae either some- what resembling those of butterflies and moths, with comparatively free legs and wings, or they are enclosed in the larval skin. Their mouth-parts are formed for sucking. The true flies comprise an enormous number of species. The most numerous of all of the orders of insects are the Coleoptera, or beetles, the Hymenoptera, which we have just discussed, and the Diptera, and for superiority in point of numbers the precedence must probably be given to the Diptera. About forty thousand species are known and it is estimated that the number yet to be described will bring this number fully up to three hundred and fifty thousand, against three hundred thousand which we have estimated for the Hymenoptera.Not only have the true flies a superiority in point of numbers, but entomologists are concluding that they probably stand at the head of the insect system in point of evolution, that is to say, they are the most highly specialized of insects. While they do not possess the apparent specialization in the way of intelligence and in other respects seen with the bees, wasps and ants, the very completeness of their transformations and the highly specialized organization of the adults of several families support this view.The order is not a popular one among entomologists and collectors. Aside from the fact that observations upon their life history are by no means as interesting as some of those which we mentioned in the preceding order, they have none of the beauty which attracts students and collectors to butterflies and moths and they have not the definiteness of structure characteristic of the beetles and they are much more difficult to preserve in collections in perfect condition. The hard-bodied, easily collected, and readily pinned beetles seem much more attractive.But the Diptera in many respects possess a peculiar interest and their study is of enormous importance from many points of view. Even in point of beauty, many of the families possess species of striking color and graceful shape ; and, everywhere abundant as they are, they are easy objects to collect. It is true that with some of the delicate species, especially the mosquitoes and crane flies, it is almost impossible to preserve specimens in good condition.Still, with many of the groups they keep well when simply killed and pinned and preserve their colors much better than do the dragon flies, for example. Very many species, and in fact entire groups, are harmful to man through damage to growing crops and to livestock. One of the most famous crop enemies in the world, the so-called Hessian fly, is a dipterous infect, and most of the insect parasites of livestock belong to this order. As late as 1884, Dr. S. W. Williston, then of Yale University, an authority upon this order, wrote : "As a whole, the order is a beneficial one to the human economy. While we may resent the troublesome mosquito's and the impertinent house-fly's molestations, and while the black fly and the horse-fly may cause the death of many horses and cattle, yet the larger number are purely parasitic in their habits, either in the larval or adult states, upon other and usually injurious insects. Many others, too, act as beneficial scavengers of unwholesome matters, which would otherwise often bring disease and death." Since Williston wrote these lines, a whole class of baneful work accomplished by flies has been discovered. That is their agency in the spread of disease.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 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.It is surprising how little of an intimate and exact nature is known concerning the life history of most flies. It is true that maggots are not attractive creatures, but the mode of life is so variable in the different groups of flies and the transformations are so remarkable that a very great interest attaches to many of these life histories.