THE TSETSE PROBLEM IN NORTH MOSSURISE. 361 



he is idle. When watching tsetses in my net experiments I relatively seldom 

 noticed the approach of any that attacked, though their attentions were sufficiently 

 annoying to make me wish to do so. A male in search of females approaches boldly 

 enough and is not silent, but the attack of flies intent only on feeding is often 

 remarkably surreptitious. 



I was never bitten through my clothing by G. morsitans, but very frecjuently 

 indeed by hungry G. brevipalpis, both through thin shirt and vest and strong khaki 

 trousers, though I occasionally watched flies probe for a time as though unable 

 to penetrate. The scrotum (when one squatted) was almost always the point 

 of attack, and this accorded well with this fly's habit in relation to cattle. It 

 always attached itself to their bellies, more or less in the median line, or between 

 the fore or hind pair of limbs. The kicking of its belly by a beast in brevipalpis 

 country almost invariably indicates that it has been attacked by that fly. The 

 position chosen has probably as much to do with considerations of shade as with 

 the thinness there of the skin, and some very hungry brevipalpis released by me 

 behind running cattle fed at the first points reached, including the flanks, whence 

 one was seen to be licked off — suggesting a further reason for the selection of the 

 belly by a fly that is less active in avoiding such reprisals than is G. morsitans. The 

 smaller tsetses attacked more on the flanks and forequarters and lower down. 



I carried out an experiment in trapping tsetse by fastening skins smeared with 

 bird-lime on the goats and under the animals' bellies. The fly-catching properties 

 of the lime, made from Loranthus berries, were excellent, but it speedily became 

 so masked with leaves and grass blades as to be useless. 



I have referred already to the low poise and sidelong, running, almost Olfersia- 

 like movements of the fly in evading reprisals. This is well seen when the natives 

 try to catch it, as they frequently do, by " treading on its toes " with the edge 

 of an assegai or knife. Occasionally a leg is damaged in the process and this, with 

 the attacks of birds, makes unreliable the method of marking by snipping off a 

 portion of a leg that was adopted at first. I later used paint from water-colour 

 tubes, placing with l, small brush a dab of white or colour on the back of the thorax 

 and varying its position and size. This proved excellent. It enabled me in some 

 cases to recognise individuals again and again, and the conspicuousness of the 

 mark brought me reports from the natives as to the spots at which such flies 

 reappeared. It doubtless also carried with it the disadvantage of laying the flies 

 open to detection by birds — a consideration that is of less importance in relation 

 to the males, which, in any case, trust more to activity and less to concealment 

 than the other sex. 



XIII. — ^Proportion of the Sexes. 



The following statements represent the result of day-to-day observation and 

 analysis in the field — the only reliable method. 



(1). Wherever we had to deal with male crowds or queues awaiting females or, 

 in the case of brevipalpis, the bush immediately bordering on the path on which 

 the male flies would line up at sunset, we took, as might be expected, practically 

 nothing but males. 



