The True Flies 



carried the bacillus of anthrax from diseased cattle and by its bite 

 inserted it into the circulatory system of human beings. The car- 

 riage 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 facili- 

 tated by little midges of the genus Hippelates. An English army 

 surgeon has ascertained that the tsetse-fly of Africa carries patho- 

 genic 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 re- 

 sponsible 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 ex- 

 cited and an enormous literature has sprung up within the last few 

 years concerning this line of investigation. This interest has be- 

 come 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 larvse issue from the body 

 of the female. Such flies then become practically viviparous, or 

 "iarviparous." 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 " pupi- 

 parous." 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. 



81 



