430 THE BIOLOGY OF INSECTS 



200,000." The researches of Bruce, Castellani, and their 

 colleagues demonstrated Trypanosoma gambiense (Fig. 84, e) 

 as the causal organism of the disease, and Glossina palpalis 

 as its alternative insect-host. Within the digestive tract 

 of the tsetse, the trypanosome undergoes a distinct phase 

 of its life-cycle with a result that after a few days of feeding 

 on trypanosome-infected blood, a tsetse becomes non- 

 infective and remains so for several weeks, until this develop- 

 mental phase is completed ; then members of a fresh brood 

 of the Trypanosoma are ready to pass into the blood of 

 human beings whom the fly may bite, and when it has thus 

 " acquired the trypanosome the tsetse can infect for the 

 rest of its life." Yet other creatures enter into this dangerous 

 association, for it has been proved experimentally that 

 monkeys can be infected with the Trypanosoma, and one 

 of the " harnessed antelopes " {Tragelaphus spekei), as well 

 as other kinds of African '* big game," may serve as " natural 

 reservoirs " for tlie deadly parasitic Protozoa. 



There are many features of interest in the connection 

 between the peoples of Africa, the tsetse-flies, and their 

 parasites. Europeans are rarely afflicted with sleeping- 

 sickness, because being more completely clothed than the 

 Africans they are less exposed to the attacks of Glossina. 

 The deadly incidence of the outbreak in Uganda is believed 

 to have been due to the introduction of the fatal strain of 

 Trypanosoma into a new district where the population, 

 entirely unused to it, had acquired no degree of immunity, 

 while the alternative insect-host was a fairly abundant 

 member of the fauna. The introduction of the tr}^panosome 

 seems to have been brought about by an increasing volume 

 of traffic, commercial and military, during the last thirty 

 years from the West Coast up the Congo basin to the great 

 lakes of Eastern Central Africa. Like other species of 

 Glossina, the female G. palpalis gives birth to a full-grown 

 maggot which has the habit of burrowing into loose dry soil 

 in shady places for immediate pupation. As a result of 

 this habit, it is found that the tsetse pupae abound on the 

 shores of the lakes, including islands, w^here forest or bush 



