causing Disease in Man in Nyasaland. 517 



The study of the development of this trypanosome in G. morsitans has 

 therefore been rendered difficult by the small number of laboratory-bred 

 tsetse flies which could be obtained. Over and above that, flies bred from 

 captive flies are not so strong and healthy as those hatched out from wild 

 pupee. 



An attempt was made to use wild flies by feeding batches of about 20 on 

 healthy animals and picking out those cages which did not give rise to 

 infection. But this is at best a roundabout and clumsy method, as it can 

 never be certain, although every care is taken, that only clean flies are 

 being dealt with. 



The Development of the Trypanosome causing Disease in Man in 

 Nyasaland in G. morsitans. 



Eleven experiments were carried out with laboratory-bred flies. Three 

 were positive and eight negative. 



Five experiments were also carried out with wild flies, as no laboratory- 

 bred flies were available. All were positive. 



Tables I and II show these 16 experiments : the number of flies used ; the 

 number of infected flies found on dissection ; and the number of days which 

 elapsed before the flies became infective. As each fly died it was dissected 

 and the result noted. As will be seen from Table I, several infected flies 

 were found in the negative experiments. This probably means that the 

 flies were only infected, not infective. The number of days before a fly 

 becomes infective is arrived at by deducting seven days from the number of 

 days which elapsed between the first infected feed of the flies and the 



Table I. — Laboratory-bred Flies. 



Date. 



Expt. 



No. of 

 flies 

 used. 



Experiment 

 positive or 

 negative. 



No. of infected 

 flies found. 



No. of days 

 before flies 

 became infective. 



Temperature 

 at which, flies 

 kept. 



1912. 















May 22 



563 



18 













June 13 



668 



22 





1 







July 15 



879 



32 





7 







„ 29 



1003 



28 



+ 



2 



31 



84° F. (29° C.) 



Aug. 17 



1072 



27 





3 





Oct 23 



1494 



22 





3 







Nov. 6 



1560 



19 













Dec. 13 



1686 



24 





2 







» 23 



1710 



30 













„ 30 



1723 



35 



+ 



3 



14 



84° F. (29° C.) 



1913. 















Aug. 31 



2405 



30 



+ 



4 



23 



84° F. (29° 0.) 



