892 
INSECTS AND THEIR RELATION 
the tsetse-fly of Zululand carried a disease, and that this 
disease was due to a trypanosome very similar to that found 
by Evans, in India, fourteen years before. 
Early in the present century, sleeping sickness of man — a 
disease considered to have been existent for many years in 
West Africa — occurred in Uganda, and within a very short 
time assumed most alarming proportions. 
Investigations showed that a trypanosome, first seen by 
Dutton in the Gambia in 1902, was the cause, and subsequent 
work proved that transmission was being effected by one of 
the tsetse-flies — Glossina jpalpalis. 
Until 1909, however, proof was wanting that Gl. palpalis 
conveyed in any way but mechanical. It was realised that 
simple mechanical transmission alone did not explain the 
rapidity of spread of the disease, and its continued existence 
in places whence all known sources of infection had been 
removed ; but attempts to demonstrate a cycle within the fly 
had failed. In that year Kleine, working on Lake Tanganyika, 
discovered the secret, which has since been confirmed from 
every part of Africa, and has been shown to exist for nearly 
all well-known species of trypanosome occurring in tsetse-fly 
countries, and with all the common species of Glossina. 
The tsetse-fly is bom clean ; and should it feed only upon 
blood in which no trypanosomes occur, it will remain clean 
and harmless, despite the severity of its bite, and the avidity 
with which it will follow an animal in search of a meal. But 
let that fly once feed upon a beast — whether wild or domestic — 
or a man, in whose blood trypanosomes exist, a developmental 
cycle promptly commences, and, being completed about a 
fortnight later, that fly is capable of infecting a susceptible 
animal at each succeeding bite, practically speaking, for so 
long as it lives. 
The developmental cycle can be traced within the body 
of the fly ; and it is interesting to note that even such a low 
animal as a trypanosome can select the special part of a fly’s 
body in which the development can proceed. A group of 
trypanosomes, of which Tr. gambiense — the cause of sleeping 
sickness — may be taken as a type, develop in the intestinal 
tract, and finally invade the salivary glands : they do not 
