June, '16] PARKER: DISPERSION OF MUSCA DOMESTICA LINNAEUS 339 



But since the season totals show that only one of every 366.5 flies 

 stained was recovered the possibility of error becomes insignificant. 

 Furthermore, such flies would have been stained twice and if captured 

 their color reactions would probably not have been normal and such 

 records would have been disregarded. 



Another sou'rce of error was the fact that the number of flies released 

 was estimated in all cases. At the Laboratory the number of each 

 species was estimated and these figures are perhaps the nearest cor- 

 rect. The Sales Yards figures are small, rather than large, but the 

 numbers in the City Dump and Washington School series are likely 

 to have been overestimated. The flies from these traps were released 

 at successive intervals of two, two and three days. They were usu- 

 ally well filled and many flies must have died because of their long cap- 

 tivity in such close quarters. This fact was considered when estimates 

 were made, but the writer feels that the latter, nevertheless, were 

 too high.* 



The possibility of carrying stained flies on clothing was guarded 

 against by careful examination before leaving the various release 

 points. 



Concerning Factors of Dispersion 



The following factors have been suggested by previous writers as 

 concerned in fly dispersion: wind, temperature, state of weather, nature 

 of locality, height at which flies are liberated and the time of day at 

 which liberated. When we consider that dispersion concerns the sum 

 total of flies migrating from a breeding ground during the breeding 

 season, these terms seem to have less significance than may be the 

 case when dealing with short periods of time. The writer prefers to 

 consider seasonal dispersion as the combined result of tropisms, or 

 responses to stimuli, which are active during the whole season, but 

 vary in intensity and duration. Observations of short duration may be 

 of value to demonstrate the dominance of some one stimulus for a 

 short time, but from the standpoint of seasonal results this stimulus 

 may be reduced to a point of insignificance. For instance, a certain 

 stimulus (wind, for example) may cause flies to migrate in one direc- 

 tion on one day, in another the- next and so on, but when the results 

 are examined in toto for the experiment in its whole duration it is 

 found that flies are more or less equally distributed in all directions 

 which would presumably have been the case if this stimulus had 

 never existed. This factor or stimulus, then, can be eliminated as 

 having any more than an incidental effect on the ultimate distribution. 



* During the season of 1914 the writer and his assistant counted large numbers of 

 flies in Hodge traps and became accustomed to estimating the numbers of flies con- 

 tained. 



