340 JOURNAL OF ECONOMIC ENTOMOLOGY [Vol. 9 



That is, on the basis of averages, the counterbalancing effects of cer- 

 tain stimuli will incidentally balance each other if long enough periods 

 are considered. 



For the purposes of discussion those so-called factors of dispersion 

 which are themselves stimuli or the sources of stimuli may be sepa- 

 rated into three divisions — physical, physiographical and physiolog- 

 ical.* 



Physical 



Wind. — As previously noted, Hewitt has suggested that flies will 

 travel with the wind (negative anemotropism), while Hindle claims 

 that they fly against or across the wind (positive anemotropism). 

 From an unbiased standpoint both statements seem reasonable, 

 ^iven the proper conditions. Hewitt's observations would indicate 

 that, under certain conditions, flies are stimulated to fly with the wind, 

 but was not this stimulus dominant because others were compara- 

 tively less intense? In the case of Hindle's observations, the reac- 

 tion of the flies was positively anemotropic in result, but the stimulus 

 may have been due to odors borne by the wind so that the actual 

 reaction was really chemotropic, the wind acting as an agent to con- 

 vey the stimulus. 



In conducting the summer's work an attempt was made to deter- 

 mine to just what extent, under city conditions, wind served as a 

 stimulus to migratory movement. Careful examination of data has 

 failed to give proof of a positive character. For illustration let us 

 examine the data of Tables III and IV. The former is a record of 

 stained flies recovered at Station 216 during a period of twenty days 

 from July 30 to August 19. A total of 132 such flies were captured, 

 seventy-four from the Sales Yards, twenty-six from the Laboratory, 

 eighteen from the Washington School and fourteen from the City 

 Dump. During the same period Table IV shows that the wind va- 

 ried from north to south through east, the majority of the records 

 reading north, northeast or east. 



Station 216 is about centrally located with the Laboratory and 

 Washington School release points to the east and the City Dump and 

 Sales Yards release points to the west -(See Plate 23). The wind, then, 

 as far as records show, blew from the direction of the Laboratory and 

 Washington School toward this station and the other release points 

 beyond for a considerable part of the period in question, but never 

 from the opposite direction. Considering Sales Yards versus Labora- 

 tory flies, we find that 248,140 flies were released from the Sales Yards 



* This division is not exact. It might be better if vital were substituted for physio- 

 logical, as the stimuli herein referred to as physiological arise from sources vital to 

 fly life, that is, from feeding areas and breeding areas. 



