AGGREGATIONS OF HIGHER ANIMALS lOQ 



colonial governments have been active in attempts 

 to control the density of these fly populations. In 

 general they are restricted to damp, low-lying forest. 

 In districts where this is confined to the borders of 

 water-courses, and hence where the fly belt has nat- 

 urally a definite limit and is restricted in size, an 

 ingenious fly trap has been used successfully. The 

 trap takes advantage of the natural reactions of the 

 tsetse fly. These are strongly positive to a slightly 

 moving dark object a few feet above ground. With 

 appropriate screening they can be caught as they fly 

 toward such an object; they will fly up and fall back 

 until they literally wear themselves out. It was at 

 first thought that such a trap would be helpful chiefly 

 in reducing the excess fly population; then, to the 

 delight of the control officials, they found that when 

 in these restricted fly belts the tsetse flies had been 

 trapped down to a certain minimum population 

 there was no need to catch the very last flies; below 

 the minimum level those remaining disappeared 

 spontaneously from the area. Nor did they return 

 unless brought back in considerable numbers accom- 

 panying movements of game, or as a result of the 

 slow extension of range from other infested areas. 

 The work of the control officials in such regions thus 

 was very much easier than had been anticipated. 

 Two pertinent cases concerning the minimum 



