The relation of insects to man as simple parasites has long been studied, and until very recent years the bulk of the literature of medical 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 importance 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 body, are able to exist for a greater or less period of time in their unusual environment. These are generally called in the medical literature "pseudoparasites" but the term is an unfortunate one. We shall now take up the different groups of arthropods, discussing the more important of the parasitic forms attacking man. The systematic relationship of these forms, and key for determining important species will be found in Chapter XII. ACARINA OR MITES The ACARINA, or _mites_, form a fairly natural group of arachnids, characterized, in general, by a sac-like, unsegmented body which is generally fused with the cephalothorax. The mouth-parts have been united to form a beak or rostrum.