122 RUPERT W. JACK. 



little molested by fly on one side of a vlei, whereas on crossing to the other, some two- 

 hundred yards or so away, the party was immediately assailed by great numbers. A 

 rather striking instance of the fly's Hmitations in this respect occurred in October 

 1919. The writer passed along a foot-path about 9 a.m. to examine a salt vlei near 

 the Shangani for fly. None was seen till this vlei was reached and an hour was spent 

 walking all round the margin, during which time only four tsetse were caught. 

 Keturning, the route taken aimed to cut the path at an acute angle, and when within 

 100 yards of the path on reaching a shady tree a number of tsetse suddenly attacked 

 the party and " bit " with extreme voracity. Yet about an hour earher the writer 

 with his natives had passed within a hundred yards without being perceived. It 

 cannot be proved definitely that the fly were there an hour earher, but there was no 

 spoor to indicate that any game had passed in the interim, and as the morning was 

 hot the fly would hardly have crossed the leafless mopani to this particular tree. The 

 waiter was extremely impressed by this particular instance, and as a matter of fact the 

 limitations in the fly's capacity for readily detecting a food supply has been a constant 

 source of surprise throughout investigations now extending over ten years. A move- 

 ment of fifty yards or so in a fly haunt frequently brings an accession in numbers of 

 hungry insects and in numerous cases when all the fhes seen were being collected, 

 this phenomenon has been particularly marked. If it be accepted as an axiom that 

 an animal or human being attracts all the hungry tsetse within a radius not exceeding 

 the fly's powers of perception, then the hmitations of the fly in this respect have been 

 proved again and again ; for fresh hungry flies have undoubtedly been constantly 

 encountered after a very short movement, whilst some two hundred yards or so has 

 been found sufficient to keep a party comparatively free from attack whilst the fly 

 swarmed that distance away. To mention another instance, in September 1913, 

 the writer halted for breakfast on the south-east side of Sipane Vlei, between the 

 Sengwa and Sassame Rivers in the Sebungwe district. A certain number of tsetse 

 had been picked up en route, and the boys fetching water brought in more, so that a 

 few bites were received. Some time after breakfast the writer took three natives to 

 the north-west side of the vlei; some 300 yards away, and found the fly in numbers, 

 some 87 being caught with one net in an hour. The wind was from the south-east 

 and had been blowing over the party towards the fly haunts for at least two hours ; 

 yet the bulk of the flies had not been attracted over this comparatively short distance,, 

 though they attacked the party in considerable numbers when their haunts were 

 entered. Further visits to this spot have been made since, and it has been found 

 that by camping on the south-east side of the vlei the attentions of fly could always 

 in a great measure be avoided. 



From the foregoing considerations it is beheved that the tsetse-fly is only capable 

 of readily detecting its hosts at a comparatively short distance, probably less than a 

 hundred yards. It might, however, be longer in the wet season, when scent pre- 

 sumably carries further than in the dry. 



If this hmitation be proved, the fact increases the difficulty of accounting for the 

 presence of fly in numbers in areas where large mammals are markedly scarce. It 

 apparently remains to be proved, however, that fly occurs in numbers in any tract of 

 country where large mammals are markedly scarce at all seasons of the year. If 

 such a phenomenon does exist, the writer admits frankly that it appears irreconcilable 

 with the theory of the vital association of the two forms of hfe. 



