June, '16] PARKER: DISPERSION OF MUSCa DOMESTICA LINNAEUS 347 



or evening. The height of the point of Hberation may well affect the 

 initial flight, but scarcely more. 



This discussion concerning the factors of dispersion has necessarily 

 been brief in order that it might not be unduly complicated. Other 

 stimuli, such as phototropism and heliotropism might have been con- 

 sidered; undoubtedly they play their part in a minor way. Nor have 

 all the ideas suggested by dealing with dispersion as a result of external 

 stimuli been discussed in detail. Three further points should be men- 

 tioned. First, that stimuli acting at any given time may be consid- 

 ered as active or passive according as they do or do not cause a re- 

 action on the part of the fly. Passive stimuli might well be termed la- 

 tent. A stimulus active at one time may be passive at another and 

 of several existing at the same time one may be active, the others 

 passive. And again, active stimuli may be divided on the basis of 

 their effect on movement or dispersion, — that is, some stimuli cause 

 77iovements or tropic responses (stimuli from feeding and breeding areas, 

 etc.), which result in dispersion, while others may cause inactivity 

 (low temperatures, rain, etc.) which delays dispersion. The former 

 are termed inciting, the latter inhibiting. When active some stimuli 

 may be always inciting, while others may be always inhibiting, or the 

 same stimulus may sometimes be inciting, sometimes inhibiting, de- 

 pending on its quality and intensity as compared with other stimuli. 



The second point concerns the standpoint from which the question 

 of stimuli is viewed. The writer has considered them to be external 

 and resultant movements as not in any way instinctive. Others, 

 however, might choose to say that the stimuli which govern a fly's 

 movements are of internal origin. As far as can be foreseen any 

 arguments in support of this opinion can be equally well explained 

 on the basis of external stimuli. For example, one might say that 

 when the ova become mature the fly instinctively seeks a suitable 

 breeding ground; but, on the other hand, it may well be suggested 

 that the maturing Ova cause some physiological change which makes 

 the fly more receptive to the stimuli "thrown out" from such areas. 

 Indeed, physiological changes must constantly be taking place and 

 probably often determine the manner in which a fly reacts to the 

 complex of stimuli which are constantly at work. 



The third point concerns the fact that the results of this investiga- 

 tion have indicated that the distance to which flies may spread from 

 anj^ given breeding area under city conditions in Montana is con- 

 siderably greater than that found in previous experiments in other 

 localities. Considering that dispersion has to do with the spread of 

 flies from a breeding ground, it at once becomes apparent that no 



