124 rupj:rt w. jack. 



later observations. In November 1914, the flies in the vlei were " Uke a swarm of 

 bees " around the individual who penetrated their haunts, and the ease with which 

 pupae were collected during that visit has already been recorded. In August 1916, 

 fly was extremely abundant, and hving pupae could be found (some 30 were collected 

 in a few hours), though the latter were not as abundant as in November 1914.* In 

 late August 1919, the \\Titer spent ten days at the vlei, for the purpose of chopping 

 down the evergreen trees as an experiment. Hardly half-a-dozen fly were seen in 

 the course of the day, where formerly they swarmed, and search for hving pupae 

 was entirely unsuccessful, notwithstanding that much more time was available than 

 on previous visits. Only a few empty cases were found in the usual breeding places, 

 and from these the flies had apparently emerged normally, there being no indications 

 of parasitism. 



In seeking a probable explanation of the decrease it was impossible to ignore 

 the marked influence that the heavy rainfall of the two previous wet seasons had 

 had on the country. The wet season of 1917-18 was the heaviest recorded since 

 1890-1, and was of a pecuhar nature, the skies remaining overcast for weeks together, 

 so that the maize crop suffered severely from excessive wet and lack of sunshine ; 

 conditions wxre thus ideal for maximum penetration. The following season's 

 rainfall was also above the average. The first diiTerence noted in the appearance 

 of the country was the fact that the gusu (Brachystegia) forest bordering one side 

 of the vlei had not yet lost its fohage, although quite leafless in August 1916 (see 

 photograph, Bull. Ent. Res. x, p. 90, pi. ii, fig. 2). The same remark apphed 

 in a lesser degree to the thorn thicket (isi-nanga). The mopani, on the other hand, 

 was practically leafless, except half-a-dozen trees in a wet situation on the edge of 

 the vlei itself. These w^ere in full leaf, having apparently grown out again since 

 the fall. The gusu, it may be remarked, showed no young foliage and the leaves 

 were falhng sufficiently rapidly to show an appreciable diminution of shade during 

 the ten days of the writer's stay. Careful inspection of the environment revealed 

 a considerable extent of green grass to the south-west and west of the vlei, which 

 was not in evidence during previous visits. Further, game was remarkably scarce 

 in comparison with previous experience, and such as occurred was not feeding at the 

 vlei itself, but on the green grass mentioned away from the vlei. With the exception 

 of two small herds of impala, one kudu cow, one wart-hog, a rhinoceros, a wild 

 dog and several duiker, no game was actually seen, in spite of daily excursions 

 in all directions. Fresh spoor of a herd of zebra and another of sable was found 

 in the vicinity. A hon drank one night at the vlei, as also did a sohtary buflalo bull. 

 This sounds a rather formidable hst, but it represents the results of ten days explora- 

 tion of the country within a radius of about eight miles of the vlei. A pack of 

 wild dogs was apparently hunting the neighbourhood, and this may account to some 

 extent for the scarcity of antelope, but the main point ascertained was that the few 

 antelope present in the vicinity were not visiting the vlei for the purpose of feeding 

 as is usually the case. They apparently found more attractive fare in the stretches 

 of green grass away from the vlei (see sketch-map). 



*In connection with the comparative smallness of the numbers quoted here and 

 elsewhere it should be noted that no trained natives ^ere available and that the pupae and 

 flies were practically all collected by the writer personally. 



