368 C. r. M. S^\"S^NNEETON. 



experiments also. If I was right in believing that in the experiment on the Inya- 

 marimu (described fully on p. 373) the individuals of pallidipes that attacked 

 the cattle did so from outside the clearing, the small number that came to the cattle 

 compared with the considerable number of these flies outside suggested that about 

 30 yards may be the extreme limit of this fly's attack under favourable conditions. 



A live goat tied inside the net attracted little attention. The flies bit occasionally, 

 but allowed themselves to be driven off easily, and none fed to anything like 

 repletion, yet the two oxen, when brought close up to windward, always attracted 

 a great rush, even on the part of females that had been resting quietly on protective 

 bark surfaces and had not been in the least attracted by the goat. I should say 

 that I w^as myself much more readily attacked in the net than the goat. The 

 liver of a cow was also not specially attractive. 



XVI. — Natural Enemies. 



During my first week at the Kanyezi vlei I was camped 150 yards or rather 

 more from a considerable cluster of male flies. These were preyed on daily,* 

 sometimes continuously and for a long time, by half a dozen to eight birds belonging 

 to three species — Dicrurus afer (African drongo), Brady ortiis ater (mimetic fly-catcher) 

 and Brady ornis murinus (mouse-coloured fly-catcher). From the continuous 

 nature of the attacks, and the immense number of house-flies {Mtisca domestica) 

 that tame drongos of my own have shown themselves capable of devouring in a 

 very short time, there could be very little doubt that the birds were making con- 

 siderable inroads into the male fly population, and it was likely that the practical 

 disappearance sometimes of this cluster was in part due to them. The birds perched 

 on the low trees bordering the vlei and dropped to the tsetses in the grass below. 



In my general experiments I found that many birds disliked Muscid flies, including 

 Stomoxys, replete or empty (actual Glossina not tested), but that drongos, fly- 

 catchers, stonechats, swallows, bee-eaters and some commoner small searching 

 birds (Crateropiis, Phyllastrephus, Apalis) liked them much ; drongos and swallows 

 (certainly also fly-catchers) continued to feed on them when so replete. as to refuse 

 all non-Dipterous insects. The digestion in at least two of these last three groups 

 is so rapid that (in my experiments) even when the birds were replete three or four 

 minutes' rest made room for two or three or more flies. These birds must be rela- 

 tively formidable enemies to a fly the males of which display themselves so freely, 

 and it is a pity that the population of such enemies must be limited by the dry 

 season food supply. I have sometimes seen swallows hawking continually after 

 insects on grass-stems and grass-blades, picking them off while flying, so that 

 male morsitans is a likely enough object for attack by these birds. Birds are in 

 the habit of paying special attention to insects, otherwise acceptable, that are present 

 in numbers together, so that in localities in which male morsitans is being kept 

 scattered by the presence of much game it must lose a far smaller proportion of 

 its population through the attacks of birds. 



* The presence of the birds at this spot was noted practically daily ; they were only 

 continuously watched on about two days. 



