CHAPTER III 



PARASITIC ARTHROPODA AFFECTING MAN 



The relation of insects to man as simple parasites has long been 

 studied, and until very recent years the bulk of the literature of medi- 

 cal entomology referred to this phase of the subject. This is now 

 completely overshadowed by the fact that so many of these parasitic 

 forms are more than simple parasites, they are transmitters of other 

 microscopic parasites which are pathogenic to man. Yet the impor- 

 tance of insects as parasites still remains and must be considered in a 

 discussion of the relation of insects to the health of man. In taking 

 up the subject we shall first consider some general features of the 

 phenomenon of animal parasitism. 



Parasitism is an adaptation which has originated very often among 

 living organisms and in widely separated groups. It would seem 

 simple to define what is meant by a "parasite" but, in reality, the 

 term is not easily limited. It is often stated that a parasite is "An 

 organism which lives at the expense of another," but this definition 

 is applicable to a predatory species or, in its broadest sense, to all 

 organisms. For our purpose we may say with Braun: "A parasite 

 is an organism which, for the purpose of obtaining food, takes up its 

 abode, temporarily or permanently, on or within another living 

 organism". 



Thus, parasitism is a phase of the broad biological phenomenon of 

 symbiosis, or living together of organisms. It is distinguished from 

 mutualism, or symbiosis in the narrow sense, by the fact that only one 

 party to the arrangement obtains any advantage, while the other is 

 to a greater or less extent injured. 



Of parasites we may distinguish on the basis of their location on or 

 in the host, ecto-parasites , which live outside of the body; and endo- 

 parasites, which live within the body. On account of their method 

 of breathing the parasitic arthropods belong almost exclusively to the 

 first of these groups. 



On the basis of relation to their host, we find temporary parasites, 

 those which seek the host only occasionally, to obtain food ; and the 

 stationary or permanent parasites which, at least during certain stages, 

 do not leave their host. 



Facultative parasites are forms which are not normally parasitic, 

 but which, when accidentally ingested, or otherwise brought into the 



