Flies as Carriers of tion of the habits of house-flies and their rela tives will show how prone they are to feed upon excrementitious matter, and to be attracted to any decaying or purulent substance. When this is the product of wounds, sores or diseased bodies it is likely to contain the germs of dis ease; and these may be sucked into the blood or cling to the body of the insect. If, then, the fly alights upon a human being or a susceptible animal, and punctures the skin with its sucking proboscis (for ordinary flies do not properly speaking), it is likely to leave in the puncture some of the germs it has fed upon, and so infect the person with the disease to which they give rise. Wounds may thus be inoculated with and certain dis eases, Frouped as may arise from taking into the stomach, in eating spoiled vege tables, minute flies (Anthomyia) breeding in them. Even vegetable diseases may be so trans mitted, as is the case with the °scab° of pota toes, .which, according to Hopkins, is spread from plant to plant by the visits of a fungus gnat (Epidopus scabiei).
That this theoretical transmission of disease actually occurred has been demonstrated since the latter part of the 19th century by observa tion and experiment. It was first ascertained of certain mosquitoes, whose responsibility for much if not all of the malaria from which men suffer was shown. This may be found more fully treated under MosQurrozs, where the agency of these small ubiquitous flies in spread ing other diseases is also shown. Dr. Joseph Leidy attributed the spread of gangrene in the hospitals at Washington during the Civil War to the flies; a few years later it was shown that gad-flies that had settled upon the dung of cattle afflicted with anthrax would communicate the disease to healthy kine. In 1888 an Italian in vestigator showed that flies fed upon a bacillus culture would drop virulent bacilli in their ordure, capable of infecting new cultures; further experiments in India showed that flies fed with a culture of the bacilli of bubonic plague and of Asiatic cholera survived and con veyed the plague to man. An English physician asserted that typhoid fever was spread by in sects; and this was abundantly proved during the prevalence of that disease in the military camps during the Spanish-American War of 1898. Flies which have access to the excreta of patients and afterward alight upon food so infect the food that whoever eats it is in danger of falling ill with the fever. Howard demon strated that the common house-fly was the prin cipal agent in this transmission. Subsequent investigations showed that the danger of the propagation by flies (and other household in sects) was equally great in diphtheria and yel low fever. In another series of cases, as among
mosquitoes, the fly acts as intermediary host for disease germs which develop in its blood to the point where they are virulent when introduced into the circulation of man or beast. Such is the case with the African tsetse fly (q.v.), which transmits a greatly dreaded cattle-disease. The purulent ophthalmia, known as and particularly prevalent in the South, is facilitated by minute gnats of the genus Hippelates.
Dr. L. O. Howard, of the United States Department of Agriculture, has given special attention to this matter, and has found that no less than 77 species of flies frequent human excrement and are therefore liable to obtain and carry disease germs. As the innumerable in sects themselves are beyond reach, the measures for protection must be preventive. Dr. Howard says that in order to avoid epidemics of typhoid fever it is necessary to abolish the box privy, prevalent in rural and village districts, and sub stitute earth-closets, where water-closets can not be installed; to place stable manure in recep tacles and treat it with chloride of lime to destroy the maggots, throwing a shovelful over each day's addition. Pantries, dining-rooms and kitchens should be carefully screened to keep out flies; and especial pains taken in summer to keep flies out of sick-rooms. Detailed instruc tions and the reasons for them are given by Howard in his pamphlet, 'How Insects Affect Health in Rural Districts,) issued as Farmers' Bulletin 155, by the United States Department of Agriculture.
The classification of the Diptera has proved difficult. An early division was based on the structure of the antenna: Nemocera— those having the antenna thread like and with 6 to 36 joints ; and Brachycera, those with the antenna three-jointed and bristle like. A later and more widely accepted sub division was based upon the way the splits on emergence of the fly; in the.Orthora pha, the pupa escape from the larval skin through a T-shaped orifice; in the Cycloropha the pupa escape through a circular opening at the anterior end. American entomologists, fob lowing the special investigations of D.' W. Coquillet, now usually divide the order into two suborders based upon the character of the mouth-parts, and characterized as follows: Sub-order Proboscidea.----:Antenne con spicuous, inserted at upper end of the face, sometimes many-jointed, proboscis usually.fur nished with terminal lips, body rather soft and brittle, legs approximated, wings usually present and frequently furnished with a chscal. cell; adults oviparous or larviparous, never- in all their stages living externally on mammals, birds or honey:-bees.