348 JOURNAL OF ECONOMIC ENTOMOLOGY [Vol. 9 



limit can safely be placed unless the time concerned is at least as long 

 as average fly life and the number of flies in some measure approaches 

 that number which we would expect to find emerging from a medium 

 sized breeding area during the period of time concerned. In fact, to 

 obtain the results desired, the number of flies used would have to be 

 greatly in excess of this. But when we consider that in all previous 

 experiments probably less than 50,000 flies were used and that the 

 largest number used in any one experiment was 25,000, it is at once 

 suggested that the difference in the numbers of flies used explains the 

 difference in results (nearly 390,000 in this work). The explanation 

 is as follows. It is a fair assumption that under normal conditions 

 flies from any breeding area will spread in all directions, but will be 

 most abundant at nearby feeding areas. From this "zone" of 

 greatest abundance the ultimate result of fly migration will be out- 

 ward though the number in the "zone" itself would be kept reasonably 

 constant by continual additions from the breeding ground. It is 

 apparent that, in a dispersion experiment using small numbers of flies, 

 as they spread outward they become constantly fewer and fewer for 

 any given unit of territory and consequently more and more scattered 

 until a point is finally reached where the chances of recapture become 

 reduced to infinity even though flies be actually present. But if the 

 number of flies released is indefinitely increased, the possible number 

 of flies per unit of territory at long distances is increased and con- 

 sequently the possible limit of capture becomes constantly more and 

 more distant until finally a "true" limit is reached, the actual location 

 of which is dependent on the length of life of the average fly, while its 

 actual determination must be based on experiment continued for this 

 length of time and the use of a sufficiently large number of flies to 

 permit their recapture at the dispersion limit. While this idea is 

 merely offered as a possible explanation of why the results secured at 

 Miles City differ so materially from earlier observations, it suggests 

 as well that, by using a sufficiently large number of flies for a suffi- 

 ciently long time and by making observation over a sufficiently large 

 area, results could perhaps be obtained that would have more or less 

 general application under balanced conditions. 



Concerning Dispersion Over Open Country 



No evidence of particular value was collected concerning dispersion 

 across open country. On the 14th of August, 5,000 flies marked with 

 gentian violet were liberated at a point midway between Fort Keogh 

 and Miles City (PI. 24, fig. 3). The prairie was barren for the interven- 

 ing mile in each direction. A trap from the Sales Yards examined on 

 August 21 contained thirteen of these flies and one from Keogh. 



