502 OTHER BLOOD-SUCKING FLIES 



confined to patches of brush along water courses, is the most 

 valuable measure in connection with their local destruction. 

 As said before, these flies seldom go over 50 yards from such 

 brushy borders of streams except when following prey, in which 

 case they may go several hundred yards. If brush is cleared 

 away and low branches of trees cut out for a distance of 30 

 yards from the edge of water in the vicinity of fords, villages, 

 washing places, etc., the flies quickly disappear, and do not re- 

 appear as long as the cleared area is kept clear. The effective- 

 ness of this method of extermination has been demonstrated 

 especially well by the Portuguese Sleeping Sickness Commission 

 on the Island of Principe where tsetse flies were almost, though 

 not entirely, exterminated in a four years' campaign. In ad- 

 dition to clearing margins of bodies of water, the beds of the 

 water courses were straightened and leveled to make the clear- 

 ing easier, and forests were completely cleared away on a large 

 scale where they seemed to harbor tsetses. In addition some of 

 the men employed in these operations wore on their backs black 

 cloths smeared with sticky bird-lime, thus being converted 

 into active traps for capturing flies. Nearly half a million flies 

 were thus caught, and the number caught daily gave a good in- 

 dex to the effectiveness of the preventive measures being used, 

 and must of itself have been a supplementary means of destruc- 

 tion which was of value. The eradication of Glossina mor si- 

 tans is a much more difficult problem, since its habitats, though 

 sharply confined to " belts," are not so closely Umited to the 

 edge of water, and are therefore more difficult to clear. Since, 

 however, the areas occupied are usually not over a few square 

 miles at the most, complete deforestation of such areas when 

 near villages or highways would often be feasible. 



The destruction of pupse of tsetse flies by natural enemies 

 undoubtedly aids in limiting their numbers, but the instinct 

 which leads tsetses to deposit their offspring where birds cannot 

 scratch gives the pupae a high degree of immunity to this class 

 of natural enemies and to artificial means of destruction. The 

 newly deposited larvae are covered by a slimy secretion which 

 apparently protects them against the attacks of the ants which 

 almost always abound in the tsetse breeding places. The pu- 

 pse are attacked by parasitic insects (Fig. 238), but apparently 

 not to a sufficient extent to seriously reduce their numbers. 



