4 Introduction 



deca3dng substances, but which when accidentally introduced into 

 the alimentary canal or other cavities of man, may exist there 

 for a greater or less period. For example, certain fly larvae, or mag- 

 gots, normally feeding in putrifying meat, have been known to occur 

 as accidental or facultative parasites in the stomach of man. 



C. Finally, and most important, arthropods may be trans- 

 mitters and disseminators of disease. In this capacity they may 

 function in one of three ways ; as simple carriers, as direct inoculators, 

 or as essential hosts of disease germs. 



As simple carriers, they may, in a wholly incidental manner, 

 transport from the diseased to the healthy, or from filth to food, 

 pathogenic germs which cling to their bodies or appendages. Such, 

 for instance, is the relation of the house-fly to the dissemination of 

 typhoid. 



As direct inoculators, biting or piercing species may take up from 

 a diseased man or animal, germs which, clinging to the mouth parts, 

 are inoculated directly into the blood of the insect's next victim. It 

 it thus that horse-flies may occasionally transmit anthrax. Similarly, 

 species of spiders and other forms which are ordinarily perfectly 

 harmless, may accidentally convey and inoculate pyogenic bacteria. 



It is as essential hosts of disease germs that arthropods play their 

 most important r61e. In such cases an essential part of the life cycle 

 of the pathogenic organism is undergone in the insect. In other 

 words, without the arthropod host the disease-producing organism 

 cannot complete its development. As illustrations may be cited the 

 relation of the Anopheles mosquito to the malarial parasite, and the 

 relation of the cattle tick to Texas fever. 



A little consideration will show that this is the most important of 

 the group. Typhoid fever is carried by water or by contaminated 

 milk, and in various other ways, as well as by the house-fly. Kill all 

 the house-flies and typhoid would still exist. On the other hand, 

 malaria is carried only by the mosquito, because an essential part of 

 the development of the malarial parasite is imdergone in this insect. 

 Exterminate all of the mosquitoes of certain species and the dis- 

 semination of human malaria is absolutely prevented. 



Once an arthropod becomes an essential host for a given parasite 

 it may disseminate infection in three different ways : 



I. By infecting man or animals who ingest it. It is thus, for 

 example, that man, dog, or cat, becomes infected with the double- 

 pored dog tapeworm, Dipylidium caninum. The cysticercoid stage 



