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II. The mechanism of the infection.

(a.) In suctorial insects.

(b.) In non-suctorial insects.

III. The relations between insect species and animal species.

IV. The relation between infected clothing and suctorial insects.

V. The infection of food, etc., by insects.

I. The factors necessary for infection of insects.

If an insect comes into contact with micro-organisms of an infectious nature, the latter may be deposited either on the surface of the body of the insect, e.g., the feet, wings, etc., or in the alimentary canal of the insect.

(a.) Deposition of germs on the surface of the insect.

The

It must be admitted that such a deposition is an every day occurrence. bodies and appendages of insects are covered with bacteria of all kinds, the nature of the latter being dependent upon the surroundings. Under ordinary conditions and in the absence of infectious diseases, the occurrence of such micro-organisms on the body surfaces of flies, etc., is of no great practical importance.

It is only when we have to deal with diseases known to be caused by patho- genic bacteria and to which germs, insects may have access, that the question be. comes one of great epidemiological importance.

Quite a number of researches have been published dealing with the presence of pathogenic bacteria on the body surface of insects.

Cholera vibrios have been found by SIMMONDS on flies (Centralb f. Bakt, Bd. IV), anthrax bacilli by HEIM on various insects (Compt. rend. No. 3, 1894). and PROUST and YERSIN have made similar observations (Bullet. de l'Acad. de Médicine, 1894).

During the epidemic of plague in 1903, I had the opportunity of examining a large number of flies. At the Public Mortuary flies were a great pest during the summer months. A similar condition of affairs was found in the Kennedy Town Infectious Diseases Hospital. The flies from both places were frequently examined bacteriologically for plague bacilli. From the attached table, it is seen that at least 75% of such examined showed the presence of plague bacilli. The presence of plague bacilli on the bodies of the flies did not appear to affect their life. There was no increased death-rate amongst flies during the epidemic of plague.

The bacteriological examination of these insects was conducted in the follow- ing way.

The flies on being caught were soaked in sterile normal saline or in bouillon. From this plate cultivations were made. The results were usually verified by animal experiment. Again, in other cases the flies were broken up in similar solutions and the routine methods proceeded with. The results obtained after washing the body surface of the fly were only occasionally positive. Those obtained from the crushed insect were highly satisfactory. One must remember that it is possible to obtain a negative result from the washings of the body surface, but a positive result from the contents of the alimentary canal and vice

versa.

Further, it is also probable that flies may convey plague bacilli on their backs so to speak for some considerable distance.

Small pieces of sugar, previously tested for the presence of plague bacilli, introduced into sterilised test tubes containing plague infected flies, have been found to contain plague bacilli when tested experimentally.

This is a most important observation from a domestic point of view, especially during plague epidemics.

I cannot confirm the statements of NUTTALL (Centralb. f. Bakt. Bd. 22) and YERSIN (Annal. Pasteur. Tome 8, 1894) that flies die soon when infected with plague bacilli.

As has been already mentioned, I have never seen an increased death rate amongst these insects during plague seasons.