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CHAPTER XVII. 

 INSECT " PORTERS " OF DISEASE. 



THOUGH the direct effects of the bites of insects or the 

 ravages of their larvae are sometimes serious, the diptera 

 are more important as carriers of disease. In the case 

 of certain protozoa and helminths they act as definitive 

 intermediate hosts in which a part of the life of these 

 organisms has necessarily to be spent, instances of which 

 were considered when dealing with these parasites ; 

 or, in a purely mechanical manner, they convey and 

 distribute germs of many diseases. When acting as 

 mechanical carriers they are justly described as "porters" 

 and the germs may belong to any class of pathogenic 

 organisms, protozoa, helminths, or bacteria. 



For an insect to be an efficient and important " porter " 

 it must be (i) a fairly common species ; (2) it must fre- 

 quent places where, either in search of food or for ovi- 

 position, products containing germs of disease are present ; 

 and (3) it must have access to articles of food destined for 

 human consumption, or to a surface or portion of the 

 body capable of being directly infected by such germs. 



The common house-fly, Musca doniestica, fulfils all 

 these conditions to a greater extent than any other insect, 

 though a similar role to a minor extent may be carried 

 out by other flies, by ants, and other insects. 



M . domestica is found all the world over and at all times 

 when suitable conditions of temperature are present. It 

 feeds on sugar syrups, fruit, as well as faeces, and 

 animal and vegetable refuse. It deposits its eggs on any 



