424 
Tsetse Flies and Trypanosomiasis 
IX. COMPARISON BETWEEN G. PALPALIS AND G. MORSITANS AS 
VECTORS 6f MAMMALIAN TRYPANOSOMES. 
Glossina palpalis feeds freely on reptiles. It shows distinct preference for 
certain reptiles (crocodiles and Varanus) over many mammals, and notably 
for reptiles over man. Reptiles da not, as far as we know, harbour in nature 
the species of trypanosomes pathogenic to mammals. G. morsitans, on the 
other hand, feeds most freely on game animals and is not known to attack 
reptiles nor freely to attack birds. It follows that, of a given number of flies 
of each species, a larger proportion of morsitans than of palpalis is likely to 
ingest mammalian trypanosomes. 
There is a considerable amount of evidence to show that, after ingestion 
by the fly, mammalian trypanosomes are more likely to complete their cyclical 
development and render the insect permanently infective if the fly continues 
to feed on mammalian, and particularly a ruminant, than if it feeds on reptile 
blood. It is to this extent more likely that mammalian trypanosomes will 
complete cyclical development in G. morsitans than in G. palpalis. Although 
G. palpalis shows a distinct preference for other hosts over man, G. morsitans 
appears to be as freely attached to cattle as to its most favoured hosts among 
the game animals. There is a proportionately greater likelihood that infective 
morsitans will attack oxen than that infective palpalis will attack man. 
Briefly, a smaller proportion of flies is likely to ingest mammalian try¬ 
panosomes, and of the flies ingesting them a smaller proportion is likely to be¬ 
come infective in the case of G. palpalis than of G. morsitans. Also, of the flies 
which do become infected, a smaller proportion is likely to transmit infection 
in the case of G. palpalis than of G. morsitans —to man and the domestic 
ruminants respectively. 
But despite the small proportion of infected G. palpalis, and despite the 
relative immunity of man to attacks by fly, there are yet enough flies infective 
and enough men attacked to make it almost incredible that some men are not 
attacked by infective flies each year. It seems certain that some natives have 
been attacked each year and that cases of trypanosomiasis have resulted with¬ 
out being detected, or else that the parasite is much less virulent to man in its 
present condition than it is believed to have been when sleeping sickness was 
epidemic. 
It is also difficult to believe that many cases of sleeping sickness could have 
developed without detection, but this does not apply to trypanosome fever 
which might very easily escape notice if it failed to develop into sleeping 
sickness. 
It is, therefore, a plausible hypothesis that, in its present state, the try¬ 
panosome (as a parasite of fly and game) is less virulent to man than formerly; 
and that, as transmitted to man by fly deriving it from game, it either fails 
to infect or causes only a mild and chronic form of disease. 
