TRYPANOSOMES IN ''SETSE-FLIES AND OTHER DIPTERA. 217 
other writers, he does not seem to realise the essential dis- 
tinction between multiplication and development. We can, 
perhaps, see in this attitude towards the problem the predis- 
position of an accomplished bacteriologist unaccustomed to 
think zoologically, if I may use such an expression. He 
expresses the belief that the trypanosomes found in biting 
insects are harmless parasites of the fly (p. 406) “derived from 
plant juices, stagnant waters, etc.” (p. 404). In speculating 
about things unknown it is surely safer to reason from estab- 
lished data than from unfounded hypothesis. At the present 
time true trypanosomes are only known to occur in the blood 
of vertebrates, and in the stomachs of insects which suck the 
blood of vertebrates; hence, it is reasonable to assume that 
the insects in question obtain their trypanosomes from the 
vertebrates. When trypanosomes have been found in plant 
juices or stagnant waters it will be time enough to speculate 
on the possibility of blood-sucking insects obtaining them 
from such sources. 
Another remarkable fact, to which I would draw special 
attention, 1s that trypanosomes, taken up into the digestive 
tract of the fly, do not infect susceptible hosts if artificially 
inoculated into them.. Bruce ([4], p. 5) first discovered 
this curious fact, and could only infect with T. brucii from 
the stomachs of tsetses if inoculated not more than half 
an hour! after the fly had infected itself; all inoculations at 
later periods gave negative results. This has been confirmed 
by many subsequent investigators of trypanosome develop- 
ment, for example, by Prowazek (86), Koch ([19], p. 14), 
Bouet ([2], p. 474), Gray and Tulloch (17), and ourselves 
(pp. 227, 234). I agree with the explanation, first suggested 
I believe by Manson,? of this fact, that the trypanosomes 
1 Since infection by the bite of the fly can be obtained up to forty-eight 
hours, the trypanosomes inoculated by the tsetse must be those which remain 
in the proboscis and do not pass into the digestive tract. Bruce (4) observed 
trypanosomes of nagana in the proboscis as late as forty-six hours after feeding, 
though rarely. This agrees with the experimental results. 
2 [ am not able to give the exact reference. 
