W. S. Patton and C. Strickland 
320 
not seen, in spite of the fact, as the authors tell us, that they had fed on 
blood containing male and female forms of T. brucei. In another series 
of 96 freshly caught flies fed on the cattle, 10*4 °/o became subsequently 
infected with T. brucei, and it will be observed that this percentage 
corresponds to that of freshly caught flies, though, if they were feeding 
on suitable material containing male and female forms of T. brucei, it is 
only natural to expect more would become infected. We would not 
however have expected this, as we regard these flagellates of G. fusca as 
natural parasites. Keysselitz and Mayer also found that 11'2 °/ 0 of 
freshly caught flies subsequently fed on healthy rabbits became infected, 
they do not explain how this took place. As their results are in direct 
opposition to Stuhlmann’s observations on flies ( G. fusca) bred from 
pupae when 80—90°/ o became infected, Keysselitz and Mayer explain this 
discrepancy by the fact that only 10°/o of Stuhlmann’s flies subsequently 
retained the infection. In our opinion the fact that in the majority of 
the flies the parasites tend to disappear from the alimentary tract, the 
longer the flies are kept, clearly suggests that the parasites have passed 
out of the digestive tube in their migration to the ovaries. 
Keysselitz and Mayer conclude that G. fusca is only capable of being 
infected once in the course of its life, that is to say it is only once 
capable of offering conditions suitable for the further development of 
T. brucei, and that is when it takes its first feed of blood (from special 
animals) which contain male and female forms of T. brucei. All the 
trypanosomes which may be subsequently ingested die out, and this 
explains why other observers have failed to study the further develop¬ 
ment of T. brucei in freshly caught Glossina fusca. Why subsequently 
ingested male and female trypanosomes do not develop we must confess 
we do not understand. 
Keysselitz and Mayer examined the “ Geschlechtsproducte ,,:i of four 
female fusca, and it would thus appear they at least suspect the 
possibility of the flagellates of the fly being transmitted hereditarily. 
Throughout Keysselitz and Mayer’s paper we can find no reference to 
the possibility of these flagellates of G. fusca being anything more than 
harmless parasites of the fly and which have no connection with 
T. brucei. Although it is clear the only possible way they could be 
transmitted from fly to fly is by hereditary infection,—no encysted 
stages being found,—neither Stuhlmann nor Keysselitz and Mayer 
has carried out exhaustive feeding experiments to disprove this. 
1 By “ Geschlechtsproducte,” literally “sexual products,” the authors doubtless mean 
the ova and the developing embryos. 
21—2 
