202 
When our expedition readied the Congo, sleeping sickness was 
endemic, and the existing conditions were much the same as in 
Lganda. We therefore determined to repeat the work done there, 
with the primary object of ascertaining the longest period after feeding 
on an infected animal at which a tsetse fly, by its bite, is capable of 
transmitting trypanosomes to a healthy animal. It must be 
remembered that, like Bruce, we worked with adult flies caught in the 
bush and that our experimental animals were only partially protected 
from the chance bites of insects. 
The conditions of our experiments were therefore imperfect. The 
flies used should have been raised in the laboratory from the lame 
of known parents. Wild flies, caught when adult in an area where 
trypanosomiasis is endemic, may have fed when free on infected 
animals; it is only in flies bred in the laboratory that the dates of all 
infecting feeds can be accurately known. It is possible that the 
piogeny of infected tsetse flies are capable, or are alone capable, 3 of 
transmitting tire trypanosomes; these points can only be decided by 
experiments made with laboratory-bred flies.* 
\\ e do not think that the danger of chance infection of laboratory 
animals, only/ paitially protected from the bites of insects, is so great 
as might be imagined. None of our large stock of rabbits, guinea- 
pigs, rats and monkeys were ever found to have been so infected during 
33 months spent on the Gambia and on the Congo. 
In the Congo two main series of experiments were made with 
tsetse fires, one at Leopoldville and the other at Kasongo. A small 
nei series of experiments (Experiments 128, 128a, 129 and part 
... ' Vas ^ onc u *th flies caught along the Congo between Leopold¬ 
ville and Coquilhatville.f 
^ Leopoldville it was very difficult to obtain monkeys and we 
1 , °| 1Ce , l ° USe £ u ’ nea -pigs and rats in some of the experiments, 
gi tiey are, as a rule, rather less susceptible to Trypanoswa 
gam b tense. 4 * 
difficult to breed^theiu" °.jf S °! the bionomics of captive tsetse-flies (5), 11 15 
flies used in our experiment* ; Yu 1 a dozen of the pupae obtained f rorn th ® 
flies. ' p nents ln the Congo developed in the laboratory into adult 
distribution of^leeffin^Sifkllit a F ea traversed by the Expedition and showing the 
Liverpool School ^oFt .ropicaHVfed ici Free State, see Memoir XVIII of the 
