TRANSMISSION OF DISEASES BY INSECTS 251 



worm, or nematode, known as Filaria bancrofti, which occurs in the 

 blood of man and of several of the lower animals as a slender larva 

 (microfilaria) about one-quarter of a millimeter in length. At night these 

 larvae swarm in the peripheral circulation, from which they are taken 

 into the alimentary canal of a blood-sucking mosquito (chiefly Culex 

 fatigans). In the mid-intestine of the mosquito the larva escapes from 

 its sheath and penetrates into muscular tissue, where it grows and devel- 

 ops for two or three weeks, after which it goes to some other part of the 

 mosquito's body, often to the base of the proboscis, whence the larvae 

 are carried into the blood of some vertebrate host, there to develop to 

 sexual maturity. 



The larvae are often common in human blood without seeming to 

 injure the host in any way, but the adults (three or four inches long and 

 often found in groups) and ova that have escaped from the parent female 

 sometimes obstruct the lymphatic canals and cause enormous swellings 

 of feet, legs, arms or other parts of the human body; this condition being 

 known as elephantiasis. 



OTHER DISEASES 



Cholera is undoubtedly transmitted by flies. As long ago as 1899 

 Dr. Nuttall wrote: "The body of evidence as to the role of flies in the 

 diffusion of cholera is, I believe, absolutely convincing." 



Dysentery is probably carried by flies, as Dr. Orton and others have 

 inferred from their experiments. 



Spillman and Haushalter, as well as several others, examined flies 

 that had fed on tubercular sputum and found in the intestinal contents 

 and in the dejections of these flies the. bacilli of tuberculosis. 



Dr. F. T. Lord summarizes his important investigations on this 

 subject as follows : 



"i. Flies may ingest tubercular sputum and excrete tubercle bacilli, 

 the virulence of which may last for at least fifteen days. 



"2. The danger of human infection from the tubercular fly-specks is 

 by the ingestion of the specks on food. Spontaneous liberation of tuber- 

 cle bacilli from fly-specks is unlikely. If mechanically disturbed, infec- 

 . tion of the surrounding air may occur." 



If it is true that tuberculosis can be transmitted by means of food, as 

 recent experiments with some of the lower animals seem to indicate, the 

 house fly is evidently a factor that must be reckoned with in the fight 

 against this disease. 



There is conclusive evidence that Egyptian ophthalmia is transmitted 



