4 
Introduction 
decaying 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 larvas, 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 role. 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 undergone 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 caninwn. The cysticercoid stage 
