270 SCIENCE IN AFRICA 



principle and at lower cost, have been the subject of experiment 

 by the Tanganyika Tsetse Research Department for some years. 

 This department has also devised screens on which the flies are 

 caught by hand, and which, carried about through the concentra- 

 tion grounds of the flies, are more effective than any trap, and like 

 the traps attract great numbers of female flies. 



It has been surmised from laboratory experiments that the 

 females are apt to be so pestered by males whenever they appear 

 that abortion and sterility may result. Hence it has been suggested 

 that only females should be caught and killed. An experiment to 

 test this hypothesis was prepared in Tanganyika: females were to 

 be caught and killed, all males to be liberated, and also large 

 numbers of males were to be imported. The latest observations, 

 however, suggest that the hypothesis is unfounded, besides which 

 the skill of the females in hiding away would probably defeat the 

 experiment. 



Two members of the tsetse department, T. A. M. Nash and 

 C. H. N.Jackson, have shown that most species of tsetse flies have 

 perm.anent or seasonal foci of concentration where environmental 

 conditions are most suitable for them, and that G. morsitans has 

 also foci that are merely its feeding-grounds. During the rainy 

 seasons the flies spread into the surrounding country from these 

 foci, to which they are driven back by harder conditions in the dry 

 seasons. Attacks directed against these centres will, therefore, pro- 

 duce the maximum results. It has been shown that trapping by 

 itself, however intensively concentrated, can never effect complete 

 extermination even in the case of G. pallidipes and palpalis. 



Another recent discovery of importance is that of the effects of 

 densification of vegetation. Experiments in Tanganyika have shown 

 that, if a patch of tsetse-infected bush is protected from fires for 

 several seasons, the growth becomes so dense as to be highly un- 

 favourable to certain species of fly. Thus in one block of four 

 square miles which was protected from fire for three years the 

 numbers of G. swynnertoni were reduced by nearly 70 per cent., 

 although the game in the area increased slightly in the same period. 

 At the same time in a second block, where grass burning proceeded 

 normally, the flies increased by over 300 per cent. 



The above brief outline of the practical results achieved by the 



