370 C. F. M. SWYNNERTON. r 



concentration in country relatively devoid of the fly. Secondly, my native expedition 

 reported coming across several such concentrations north of the Lusitu, with the 

 same male crowds at the edges of similar vleis, the same ease in finding puparia and 

 the same relative dearth of tsetses in the similar country between. They brought 

 me both morsitans and pallidipes and puparia of the former. Thirdly, the Masando 

 vleis and some vleis east of the Mapapa (see Map) were both stated by the local 

 natives to be the sites of concentrations, how great I do not know, and to have been 

 so in the present season. I had myself found a concentration at the Masando in 

 1900, but in 1918 we found the flies on the grass of the vlei-edges only in parties 

 of three or four — as at Kanyezi's, when a male crowd had been dispersed. Bufl^alos 

 and elands were present in small parties. We were led to the more easterly vleis 

 by a native (subsequently in my service and apparently reliable), who stated that 

 he had seen the flies there in great numbers only a week or two before. I had no 

 cattle with me and only two flies were taken, but a large herd of bufl alos had arrived 

 meantime. These two sets of vleis and Kanyezi's are all within a very few miles of 

 each other. 



Leaf-fall Concentrations. 



When I left Kanyezi's on 1st Augsut it seemed likely on the whole, though 

 uncertain, that this scattered, unconcentrated fly was even somewhat sparser than 

 it had been in June. The few that the cattle picked up on that particular day were 

 at vleis or glades — not that this was unusual. Natives on the Umvuazi River had 

 reported an increase in the flies about their villages, and one way in which the insects 

 might travel thither appeared to be illustrated by the fact that the flies that were 

 attaching themselves to us left us (one of them after following for 5| miles), not in the 

 rather barer bush, but on our reaching the somewhat shadier wooding beside streams. 

 The natives stated that this tendency to collect at the streams becomes more marked 

 rather later, but not (Kanyezi said — and I think we saw) at the expense of the vlei 

 population, which also increases. 



The natives attributed the streamward movement of the fly in the later dry 

 season to the fact that the general drying up of the country forces the game thither. 

 " The int-hesi then congregates there to feed on the game." While this explanation 

 is not quite accurate, it is certainly to be judged from what I have said above that game, 

 or natives, either going to the streams or merely travelling across them, will tend 

 to deposit there at this time any flies that they may have picked up either in the 

 drying-up country generally or at vleis, etc. These leaf-fall (and soil-moisture ?) 

 concentrations, whatever they may amount to here, are evidently to be dis- 

 tinguished from the more permanent breeding centres, although these also will 

 doubtless receive accessions from outside at this time. 



Primary Concentrations. 



That the concentration of the fly in the last-named centres is not purely the result 

 of dry season concentrations there of the game seemed certain. There was no 

 concentration whatever of the game at Kanyezi's vleis, where I stayed a fortnight in 

 all and at which fly was plentiful and breeding, nor had there been since the rains. 

 There was even less of a concentration at the Masando in June 1900, when I found 



