THE DANGER OF FLIES 725 



Lake Tanganyika ; Transvaal; Natal; Sokotra ; Madagascar; 

 St. Helena ; Madeira ; Nova Scotia ; Colorado ; Mexico ; St. 

 Lucia ; the West Indies ; Para, Brazil ; Monte Video, Uruguay ; 

 Argentine Republic ; Valparaiso, Chili ; Queensland ; New 

 Zealand." It is carried all over the world in ships and trains, 

 and seems to be equally at home in the low latitudes of 

 Finmark or in the humid heat of equatorial Brazil. 



The diseases which flies convey from man to man — which 

 rendered them by no means the least formidable of the plagues 

 of Egypt, and fully justified Beelzebub's title of the " Lord of 

 Flies " — are for the most part conveyed mechanically. The 

 proboscis acts as an inoculatory needle. No part of the life- 

 history of the disease-causing organism must necessarily be 

 carried on in the body of the fly ; it is conveyed mechanically 

 and without change from an infected to a healthy subject. The 

 mouth-parts can pick up the anthrax bacillus, and if the fly then 

 alight upon a wounded surface it will set up "wool-sorters' 

 disease." It, together with the flea, is accused of transmitting 

 the plague-bacillus, not only from man to man, but from rat to 

 man. Flies are active agents in disseminating cholera ; and any 

 one who has watched them clustering around the inflamed eyes 

 of the children in Egypt, or in Florida, will not readily acquit 

 them of being the active agents in the spread of inflammatory 

 ophthalmia or of " sore eye." 



It is worthy of note that after exhaustive experiments on the 

 Tsetse-fly (Glossma palpalis), which conveys that most fatal of 

 diseases, sleeping sickness, Prof. Minchin and his colleagues, 

 Mr. Gray and Mr. Tulloch, have come to the conclusion that the 

 Protozoon {Trypanosoma gambiense), which causes the disease, 

 does not — as might be expected — pass through certain stages of 

 its life-history in the fly, but is mechanically conveyed upon the 

 biting mouth-parts of the insect. The deadly parasite is, indeed, 

 so easily cleaned off these appendages that a single bite is 

 sufficient to wipe them off. A tsetse-fly which has bitten an 

 infected person will set up the disease in the next person (or 

 monkey) it bites ; but the insertion of the proboscis, quick and 

 instantaneous as it is, serves to clean it — to wipe off adhering 

 trypanosomes, and if it now bite a second person (or monkey), 

 it fails to infect them. This is a most important discovery, and 

 contrary to what we should have expected ; but our knowledge 

 of the history of the genus Trypanosoma is still too small to 



