TRYPANOSOMES IN TSETSE-FLIES AND OTHER DIPTERA. 207 
infection with trypanosomes, without having been fed 
previously upon infected animals. Thus, in one case a monkey ~ 
became infected when 216 freshly-caught flies had been fed 
on it in-batches over a period of fifteen days (loc. cit., 
p- 62) ; in a second case 894 flies produced an infection after 
twenty-eight days (loc. cit., p. 62) ; and in a third case, 759 
flies infected after twenty-three days (loc. cit., p. 63). To 
these three cases may be added two experiments recorded 
by Greig and Gray (18); in the first 980 flies produced 
an infection after forty-six days (loc. cit., p. 106); im 
the second, 2299 flies were fed on a monkey over a long 
period without infecting it. Finally, reference may be made 
to the result obtained by us (29, p. 245), in which a batch 
of 134 flies fed on a monkey (as a control experiment) pro- 
duced an infection in it. If we add all these results together 
we find that a total of 5282 tsetse-flies, freshly caught in the 
neighbourhood of Entebbe, infected five animals; but if we 
take separately the experiments of Bruce, Nabarro, and 
Greig (6), made at a time when remedial measures had 
not been undertaken at Entebbe, a much higher average 
is the result, since we find that 1869 freshly-caught 
flies produced three infections, an average of one infection 
for 6283 flies, which is a number very little higher than that 
of the flies which were operative in the eight-hour transmission 
experiments. There was, therefore, the possibility that the 
infections produced in the transmission experiments tabulated 
above were not really produced by transmission from the 
infected to the healthy subject, but by flies which had been 
in an infective condition when first caught, before being used 
for the experiments. It is noteworthy also how much more 
quickly, in all the successful experiments, the infection was 
produced with the freshly-caught flies, so that it would 
appear to be almost more dangerous to be bitten by free 
flies in a sleeping sickness locality than by those artificially 
infected in the laboratory. 
A further point to note in the experiments tabulated above 
is the relatively large number of flies required to produce 
VOL. 92, PART 2,—NEW SERIES. 16 
