138 



By clearing in the zone infested by immigrating flies the effect is local 

 at best. A small amount of clearing at the centres of infestation 

 may be much more effective than a large amount of clearing away 

 from them." 



In discussing the degree of density that may be considered " ob- 

 noxious/' Fiske expresses the belief that a density, " as measured by 

 the males to be caught ' per boy per hour/ " of 6-0 " is ordinarily safe 

 and sanitary, but that one of 15-0 or 20-0 would ordinarily be dan- 

 gerous. Admittedly it would be desirable for purely sanitary reasons 

 to reduce density to everywhere, but the expense would be so 

 enormous as to render it completely impracticable. Practical measures 

 must combine efficiency with economy, and the most economical 

 measures are such as do not carry reduction in density of the pest 

 beyond what is necessary for sanitary reasons." 



In the course of some suggestions for the limitation and destruction 

 of G. morsitans in Nyasaland, contained in a paper published in 1914, 

 Dr. J. O. Shircore (139) writes as follows, after stating that, from his 

 own personal observation, the " proclaimed Sleeping Sickness area/' in 

 Dowa District, " contains at least four ' primary centres ' which 

 harbour tsetse-flies." 



" The main reasons why these are defined as ' primary centres ' are : 

 (1) Fly are found here abundantly throughout the year. (2) They are 

 present in these situations when there are none or extremely few 

 elsewhere, when the whole country is bare of grass, the trees without 

 foliage, and the ground baked hard. (3) These are the only areas 

 where in the dry season water is actually above the earth's surface or 

 at no great depth below. For example, at the Lingadsi River the 

 centre is almost definitely limited in length by the water and moisture 

 in the river-bed. At Patsanjoka the water is present throughout the 

 dry season. At Kuti and Chitawa it dries late, but here it is seen that 

 although the marshes are superficially dry, yet during late October and 

 early November, before any rain, fresh grass already springs up and 

 trees begin to put on green foliage in the low-lying country near the 

 marshes ; this is in marked contrast to other parts of the district, e.g., 

 the portions between Nsadzu, Waya and Matumba to Mtalamanja, 

 that being somewhat raised undulating broken country with a fairly 

 rocky surface. (4) It is in these situations that the fly can best feed 

 during the long season of drought ; there are herds of eland and 

 buffalo at the Lingadsi ; waterbuck, hartebeeste and eland at Nyansato ; 

 and various species of large and small game at Patsanjoka and Kuti 

 dambos, and numbers of these can be seen at various times resting in 

 the shade during the heat of the day. (5) In these places there is 

 light forest with fairly short grass, here and there open glades, and 

 water at no great distance, a combination which is ideal for both game 

 and fly. The latter feed as the game come to and from water, and the 

 light forest enables them to see and follow the game without difficulty. 

 Heavy forest and thick high grass impede the flight of tsetses, and, 

 moreover, they are unable to see any distance. They are sometimes 

 found in such situations, but not in any numbers. 



" The fact that the fly are found at the above-mentioned centres all 

 the year round, and that they still exist there at the height of the dry 

 season, when there are few or none elsewhere, makes it quite reasonable 

 to suppose that they breed at these places, and that it is from these 

 centres that they extend into the surrounding country along connecting 

 forest as soon as the conditions become suitable. This is actually 

 observed to take place. During May, June, and early July, tsetse 



