28 



Dr. H. L. Duke. Experiments with [Sept. 28 r 



cases a day's starvation before the experiment commenced insured vigorous^ 

 feeding of all the flies. Again, considerable time must often elapse before 

 an infected box of flies is obtained, a large number of boxes having to be- 

 rejected after some 30 days' observation, owing to the flies proving non- 

 infective. It was thus found necessary on several occasions to employ a. 

 positive box for several different experiments. Also, owing to the great, 

 sacrifice of experimental animals involved in testing the infectivity of each 

 box, it was sometimes found necessary to employ untested boxes, relying 

 on the ultimate dissection of the flies to prove their infectivity. Fortunately 

 two monkeys were available which proved highly infective to flies. 

 Thanks to a large number of experiments carried out by Miss Eobertson,. 

 in which these two animals were employed for the infecting feeds, it became* 

 evident that flies infected from either of these monkeys were invariably 

 infective by the 30th day of the experiment, and often several days earlier. 

 This fact, together with the observation derived from Miss Robertson's 

 experiments — that the infectivity of the fly coincided with the invasion of 

 the salivary glands by the flagellates — made it possible to employ untested 

 boxes as above described. All such boxes were kept until the 32nd day, 

 and then assumed to be infective. If on subsequent dissection no + flies 

 were found, then the box was ignored. In the following tables it is stated 

 whether or not a tested box was employed. It will be noted that in positive: 

 Experiment 757 only untested boxes were used. 



The following facts must also be considered as having an important 

 bearing on the present series of experiments : — 



1. The mere introduction of the proboscis of a positive fly into the skin 

 of its victim can produce infection. This was proved by Eraser and myself 

 by an experiment in which interrupted feeding was employed, and is in 

 agreement with the contention that the salivary gland flagellates are- 

 responsible for infection. Thus a fly need not have actually extracted any 

 blood to have infected the monkey. 



2. A single positive fly can infect a clean monkey on three consecutive 

 days. This experiment was reported by me to the Royal Society in April,. 

 1912. This applies to the occasional employment of the same box in 

 different experiments on consecutive days. 



3. The facts recorded in Section I relating to the persistence of the 

 salivary gland flagellates and the infecting power of a fly, even after the gut 

 has been cleared by arsenic feeding. 



The actual experiments may now be considered. The headings of the- 

 columns are self-explanatory. The variation in the number of positive flies 

 on the two days of an experiment is due to the necessity of apportioning 



