346 JOURNAL OF ECONOMIC ENTOMOLOGY [Vol. 9 



they may be considered as substations, so to speak, which aid and abet 

 distribution and further increase the final radius of dispersion. Sev- 

 eral reasons that may influence flies to leave such localities to which 

 they have been attracted may be suggested. It is known that flies 

 seek "shelter" at night. Where abundant the writer has seen them 

 collect in the late afternoon by the thousands under the eaves and on 

 the walls, especially on the south and west sides, of houses and build- 

 ings. It is apparent, then, that to reach such places a fly must leave 

 the immediate vicinity of any feeding ground or breeding ground at 

 which it was busy and there is no particular reason apparent why a 

 fly should return to the same place when it resumes its activity in the 

 morning. Indeed, whatever stimulus a feeding or breeding place may 

 exert at midday under a strong sun would seem likely to be less in- 

 tense in the cool of the morning. Similarly after a rain, stimuli may 

 be less intense. Showers and changes of temperature may also cause 

 flies to seek "shelter" and again it is a reasonable question to ask if 

 there is any stronger reason why a fly should return to the place of its 

 activities just before such an interruption than that it should go in some 

 other direction? The direction of wind may have changed, and if 

 stimuli may be wind-borne, which there is no good reason to doubt, 

 may not the fly receive a stimulus from an entirely different direction? 

 Other possible reasons for migration may be disturbance (flies in a 

 business portion of a city are constantly being disturbed) and the 

 removal of garbage, refuse, manure, etc., which. serve as an attraction. 

 Whether or not the fly can be said in general to leave a feeding or 

 breeding place in response to other stimuli from similar places may be 

 questioned. Indeed, an innate wandering instinct may sometimes 

 be the cause but even then may not the direction of flight be con- 

 trolled by external stimuli and since it is at feeding and breeding areas 

 that flies are commonly found in abundance, is it not unreasonable to 

 suppose that flies are sooner or later attracted by stimuli emanating 

 from them, though such stimuli may perhaps not be correlated with 

 the reason which initiated the movement? There is no apparent 

 reason to assume that flies will necessarily stop at feeding or breeding 

 areas nearest at hand. 



Hindle has suggested the height of the point of liberation and the 

 time of day are factors influencing the distance of dispersion. Bearing 

 in mind that dispersion is primarily a problem of spread from breeding 

 areas and that the life of a fly is a matter of several weeks, it is hard to 

 assign a true significance to these points. Certainly it seems as if it 

 would make very little difference in the ultimate distance a fly will 

 have traveled whether it emerged from its puparium in the morning 



