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And what is most important, YERSIN (Annal. Pasteur, T. 13, 1899), WILM (Hyg. Rundschau, 1897), ABEL (Cent. f. Bakt. Bd. 21), HANKIN (Cent. f. Bakt. Bd. 22), SIMOND (Annal. Pasteur, 1898), NUTTALL (Cent. f. Bakt. Bd. 23) and others have shown the presence of plague bacilli in the intestines of flies, fleas, bugs, mosquitoes, ants and moths.

These results, some of which I have verified, certainly show the frequent presence of plague bacilli and other micro-organisms in the intestines of insects. Given, however, this fact, an important point to consider is what are their chances of spreading the disease for which they are responsible.

According to HANKIN (Cent. f. Bakt. Bd. 22) plague bacilli remain virulent. for some considerable time in the intestines of ants. In the intestines of flics they remain virulent for 48 hours or more. In the intestines of bugs and fleas plague bacilli die rapidly. Similar results have been published by NUTTALL, SIMOND, and others who showed in addition, that in some cases, such micro-organisms have their virulence altered by such a passage through the alimentary canal of an insect. In the case of flies, plague bacilli pass through the intestine uninjured. In the case of bugs and fleas, the virulence of plague bacilli would appear to be lowered by such a passage.

With so little time at my disposal this part of the research had not received that amount of attention which it merits. The conclusions drawn by different scientists in regard to the subject are so varied that at present one must accept the results with reserve. So far as we know, some micro-organisms pass through the intestines of insects uninjured, others have their virulence diminished, while, according to one investigator Cao (Ufficiale sanitario, 1898) some non- pathogenic bacteria gain virulence on passing through thr alimentary canal of certain insects, e. g., the Periplaneta orientalis. The latter observation, however, is an isolated one. Again it must be borne in mind, that different insects will react to different bacteria and infections in different ways.

In regard to the question of the vitality of micro-organisms on or in the body of insects, we must always take into account the effects of drying and sunlight on the one hand, and the action of chemical and bacterial products in the alimentary canal on the other hand.

II. The Mechanism of the Infection.-That diseases are communicated to man through the agency of insects is a fact established beyond dispute. The investi- gations of the last decade have revealed to us the important role played by insects in the dissemination of certain diseases. The mosquito and its relation to malaria and filiariasis and other diseases may be instanced, and year by year there is being added to our knowledge an accumulation of facts bearing upon the insectiverous spread of disease. So far these investigations are limited to diseases which are occa- sioned by parasites of a certain degree of organisation, parasites which pass through a definite cycle of changes during their development, either in the body of the host. or intermediate host. These changes are complicated and show that in these parasites we have to deal with an organism which, in its mode of development and conditions of life, is something very different from germs of the type of cucci, bacteria, or vibrios. The latter are organisms of an altogether different type from the plasmodium malaria. Convincing observations are wanting at present to prove the direct transference by an insect of any coccus, bacterium or vibrio which is the causal agent of a definite specific infectious disease, to the tissues of man. In the case of malaria or filiariasis there exists a special mechanism through which the parasite gains the human body but no such process is indicated in regard to the dissemination of pathogenic microbes by insects. It would rather appear from the standpoint of our present knowledge that the direct inoculation of the human subject with pathogenic parasites by means of an insect, is limited to a class of organism considerably removed and higher in the scale of development than those simple cocci, bacteria, or vibrios which are the causal agents of so many infectious diseases. Taking up the question of the mechanism of infection at this point, we find numerous observations in literature in regard to the direct connection be- tween insects and disease. The majority of the examples cited in literature are isolated observations, and their importance from a modern epidemiological point of view is extremely doubtful. For instance, gnats were held responsible for the occurrence of abscesses, bugs for relapsing fever, mosquitoes for leprosy, and ants, emmets and other insects for plague.