3 o SOME MINUTE ANIMAL PARASITES 



To complete the life-story of the trypanosome as 

 far as it is known at present, the period passed by 

 it in the fly must be considered. Recent work on 

 this subject has been published by Sir D. Bruce 

 and his colleagues, and Miss Robertson. After de- 

 tailed examination of many infected flies, these 

 workers concluded that many of the Trypanosoma 

 gambiense remain largely unchanged except for multi- 

 plication during the period that they are within the 

 alimentary tract of the fly, or in other words, that 

 many of the parasites keep the trypanosome shape 

 and appearance in the fly. When the parasites reach 

 the salivary glands of the tsetse, they become some- 

 what stout or stumpy, and then present the appear- 

 ance of the stout trypanosomes found in the 

 circulating blood, and in this form they are inocu- 

 lated into the next victim who is bitten by the fly. 



A remarkable feature in the life-history of the 

 trypanosome in the fly is the fact that the trypano- 

 somes seem to disappear very soon in the flies, which 

 are not infective for more than a short time after 

 feeding. But about twenty days after the first feed 

 of infected blood the fly again becomes capable of 

 infecting a fresh host, and may remain infective even 

 for the rest of its life at any rate, up to one hundred 

 days. This remarkable fact of delayed infectivity of 

 the tsetse was first demonstrated by Kleine. During 

 this period the trypanosome is undergoing develop- 

 ment in the tsetse fly. 



However, the traveller in the tropics may be 

 assured somewhat by the fact that in Nature only 

 about i per cent, of the G.palpalis are infected with 



