THE TSETSE PROBLEM IN NORTH MOSSURISE. 345 



side, were identical, yet brevipalpis had deserted the cleared piece (v. Expt. 1, p. 373). 

 This, with a failure to take brevipalpis in some high, dark secondary wooding on the 

 Sitatonga base, excepting at the occasional thickets, suggests that extra shade 

 may not be the only advantage gained from thickets. Protection from drying 

 winds, additional protection from the eyes of enemies (such as the thickets very 

 definitely afford) and some advantage to the pupae are the three that occur to me, 

 but I am convinced that shade is the chief consideration. 



Glossina morsitans was found in a very different kind of country. It was very 

 obviously far less dependent on good shade than G. brevipalpis, and I failed to find 

 even stray individuals in the densest forest types — but the stray individuals taken 

 were in any case very few. It occurs both on the granite-gneiss and the basalt 

 of the lowlands, the latter usually with very poor bush indeed (Combretmn, et-/., 

 PI. x, fig. 2) and the former with savannah forest of a poorer type than that of the 

 sedimentary area, though in each of these two it is Brachystegia. On the basalt and 

 on most of the gneiss this fly occurred during my visits only as very rare individuals, 

 and on the basalt and away from vleis only males were taken. The granite-gneiss, 

 however, carries (as I have related) very numerous vleis, many of them with 

 permanent water, and in spite of much painstaking search elsewhere, it was only at 

 these vleis (and then only at some of them) that morsitans was found in great 

 numbers together and its breeding going on with some vigour in the dry season. 



Here the male flies occurred in the usual bands, containing in some cases at least 

 a few hundred flies at a time on the short or shortish grass and sedge. They were 

 sometimes out in the sunlight amongst or beyond the s?attered chidsgwati shrubs 

 and stunted Parinariums that separate the open vleis from the Brachystegia bush 

 surrounding them, sometimes a little way in the Brachystegia bush itself, never very 

 far from the vleis. Some of these vleis with well populated fly-pockets consisted 

 of mere glades in the bush, in which there was now no definitely exposed water and 

 in some of which, had I not dug, I would hardly have expected water ; but bigger 

 vleis were part of the general series to which they belonged, and even in the glades 

 experimental digging always showed moisture near the surface and actual water 

 not many feet below. Some places would be moist on the surface, especially in 

 the early mornings, when dew also was present in such glades generally. The 

 flies were found in rather special association with a low, heavily-headed sedge 

 (Fuirena), with which I found the local natives had also learned to associate them. 

 This remained green after the grasses generally wxre dry and, with one or two 

 low-growing associates, covered considerable areas at the edges of certain vleis. 

 Almost the only low-growing plant that was transpiring at the time of my visit, 

 and doubtless cooler than the dried-up species, it possibly offered a greater certainty 

 of the deposition of dew. 



The stray flies, of which I took fewer of morsitans than of pallidipes, appeared 

 (as I shall indicate for pallidipes) to be finding their way to the vleis and rivers 

 by the end of July, with leaf-fall already producing a visible effect in dry areas 

 and at the actual vlei-edges, which latter, however, were also coming back into 

 leaf. The natives described to me the increase of the fly in the hot weather, its 

 spread in the early rains and its abundance then at places where, even wuth game 

 temporarily absent, one now found few or none. In general it would be difficidt 



