In this connection, mention must be made of the scholarly article by the American physician, A. F. A. King who, in 1883, brought together an all but conclusive mass of argument in support of his belief that malaria was caused by mosquitoes. At about the same time, Finley, of Havana, was forcefully presenting his view that the mosquito played the chief rôle in the spread of yellow fever. To enter more fully into the general historical discussion is beyond the scope of this book. We shall have occasion to make more explicit references in considering various insect-borne diseases. Enough has been said here to emphasize that the recognition of insects as factors in the spread of disease was long presaged, and that there were not wanting keen thinkers who, with a background of present-day conceptions of the nature of disease, might have been in the front rank of investigators along these lines. THE WAYS IN WHICH ARTHROPODS MAY AFFECT THE HEALTH OF MAN When we consider the ways in which insects and their allies may affect the health of man, we find that we may treat them under three main groups: A. They may be directly poisonous. Such, for example, are the scorpions, certain spiders and mites, some of the predaceous bugs, and stinging insects. Even such forms as the mosquito deserve some consideration from this viewpoint. B. They may be parasitic, living more or less permanently on or in the body and deriving their sustenance from it. Of the parasitic arthropods we may distinguish, first, the _true parasites_, those which have adopted and become confirmed in the parasitic habit. Such are the itch mites, the lice, fleas, and the majority of the forms to be considered as parasitic. In addition to these, we may distinguish a group of _accidental_, or _facultative parasites_, species which are normally free-living, feeding on 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 larvæ, or maggots, 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 transmitters 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.