350 C. F. M. SWYNNERTON. 



In tlie bare glades and vleis of the Brachystegia bush, of the granite-gneiss not only 

 is there a continuous evaporation in the dry season from a considerable under- 

 ground water supply, but the nightly cooling is such as to produce regular dews — 

 as it does also, to a lesser extent, even in the drier open spaces in the woodland. 

 If, then, the sucking of moisture from wetted grasses which appeared to take place 

 in my tubes should also take place in nature, we should have a probable partial 

 explanation of the attachment of morsitans (and in the same area pallidipes) to the 

 v^icinity of vleis and glades. In one instance a large fly-cluster was observed to 

 travel quite a mile and a half in the course of a few days entirely along a connected 

 or but slightly disconnected series of small glades in the bush. 



In view of such evidence as is alreadyavailable (page 341) I rather strongly suspect 

 that tsetses will be found to partake fairly frequently of water in one form or another, 

 especially the species that do not rely on heavy shade, in which dew is also less. 

 It is conceivable that the slight occasional snacks of blood taken by male tsetses 

 when travelling are as often for moisture as for food. 



At any rate the point with regard to water-drinking deserves investigation, 

 seeing that the practical application of a discovery that tsetses regularly drink 

 dew might lie in the possibility of poisoning at least the male flies at their dry 

 season concentrations — as one of several contributory measures for the flies' destruc- 

 tion there. 



Rainfall probably affects the fly here mainly in so far as it influences the nature 

 of the bush, the period of leaf -fall and (for morsitans and pallidipes) the permanence 

 of the moisture in the vleis, etc. I found brevipalpis and pallidipes both east and 

 west of the Sitatongas and on the British border, though we have here, undoubtedly 

 three different rainfalls. The confinement of morsitans to the smaller rainfall area 

 east of the Sitatongas mil be referred to below. 



Up to 10th September (the end of the investigation), no matter how apparently 

 suitable the wooding ixiight be, I found no brevipalpis and only once a pallidipes in 

 the immediate neighbourhood of river banks west of the Sitatongas ; I refer to such 

 rivers as the Buzi, not mere overshaded streams like the Inyamarimu. Yet on get- 

 ting a bit back from the river, fly (mainly brevipalpis) was found generally and in 

 quantity and thence up the hills to the limit of the suitable bush. There seemed 

 to be a definite avoidance of the big rivers — possibly a matter of the greater winter 

 cold in their vicinity. With austeni it may be diflerent. 



Elevation. 



The eflect of elevation was tested for brevipalpis and morsitans. Forty pupae 

 of the latter fly were sent, twenty each, to two friends living in the mountains at 

 about 3,400 (G. D. Otterson) and 3,900 feet (J. W. Scott), respectively. Each 

 batch was placed in a little sand and leaf-mould (taken from the places I found 

 them in) in a wide-mouthed bottle covered with gauze. Each bottle was placed 

 imder a slightly raised log in semi-shade, this reproducing the natural conditions, 

 and the temperature was taken daily. I kept a control set in a similar bottle in 

 the lowlands. 



