IN CONCLUSION, it should be noted that the medical and entomological literature of the past few years abounds in suggestions, and in unsupported direct statements that various other diseases are insect-borne. Knab (1912) has well said "Since the discovery that certain blood-sucking insects are the secondary hosts of pathogenic parasites, nearly every insect that sucks blood, whether habitually or occasionally, has been suspected or considered a possible transmitter of disease. No thought seems to have been given to the conditions and the characteristics of the individual species of blood-sucking insects, which make disease transmission possible." He points out that "in order to be a potential transmitter of human blood-parasites, an insect must be closely associated with man and normally have opportunity to suck his blood repeatedly. It is not sufficient that occasional specimens bite man, as, for example, is the case with forest mosquitoes. Although a person may be bitten by a large number of such mosquitoes, the chances that any of these mosquitoes survive to develop the parasites in question, (assuming such development to be possible), and then find opportunity to bite and infect another person, are altogether too remote. Applying this criterion, not only the majority of mosquitoes but many other blood-sucking insects, such as Tabanidæ and Simuliidæ, may be confidently eliminated. Moreover, these insects are mostly in evidence only during a brief season, so that we have an additional difficulty of a very long interval during which there could be no propagation of the disease in question." He makes an exception of tick-borne diseases, where the parasites are directly transmitted from the tick host to its offspring and where, for this reason, the insect remains a potential transmitter for a very long period. He also cites the trypanosome diseases as possible exceptions, since the causative organisms apparently thrive in a number of different vertebrate hosts and may be transmitted from cattle, or wild animals, to man. Knab's article should serve a valuable end in checking irresponsible theorizing on the subject of insect transmission of disease. Nevertheless, the principles which he laid down cannot be applied to the cases of accidental carriage of bacterial diseases, or to those of direct inoculation of pyogenic organisms, or of blood parasites such as the bacillus of anthrax, or of bubonic plague. Accumulated evidence has justified the conclusion that certain trypanosomes pathogenic to man are harbored by wild mammals, and so form an exception. Townsend believes that lizards constitute the natural reservoir of verruga; and it seems probable that field mice harbor the organism of tsutsugamushi disease. Such instances are likely to accumulate as our knowledge of the relation of arthropods to disease broadens. CHAPTER XII HOMINOXIOUS ARTHROPODS