Reactions of the Pomace Fly to Odorous Substances 535 

 III THEORETIC DISCUSSION 



The experimental results recorded in this paper show very con- 

 clusively that the reactions of Drosophila to odorous mattnais 

 are by no means uniform, but vary in method under different 

 circumstances. When the stimulus is very w^eak little more than 

 random movements are excited, but w^hen the stimulus is some- 

 what stronger trial and error movements gradually prevail, where- 

 by the fly becomes approximately oriented toward the odorous 

 material, much as has been emphasized for many lower animals by 

 Jennings ('04). Finally the orientation to the odorous material 

 becomes very accurate and the fly may be said to take an almost 

 direct course to it. It is clear that the latter part of the course is 

 accomplished by methods in the main free from anything that can 

 be described as trial and error. Since under a like degree of 

 stimulation flies, after the loss of an antenna, carry out circus 

 movements with great regularity, it seems impossible to explain 

 the movements under these conditions in any other way than on 

 the basis of the tropism theory. This theory has been stated in 

 several ways. As applied to chemical stimulation Verworm ('99, 

 p. 429) declares: "The word chemotaxis is applied to that prop- 

 erty of organisms that are endowed with the capacity of active 

 movement by which when under the influence of chemical stimuli 

 acting unilaterally they move toward or away from the source of the 

 stimulus. Where there is an approach to the source of the stimu- 

 lus, there is positive chemotaxis, where there is a removal from the 

 source negative chemotaxis. Unilateral stimulation with chemical 

 stimuli is only realized when the concentration of the substance 

 in question gradually increases from the living object in one direc- 

 tion. 



The method by which Drosophila finds its food is directly com- 

 parable to that observed by Harper ('05, p. 33) in the reactions 

 of Perichaeta to weak and strong light. This earthworm orients 

 away from the source of a weak light stimulus by frequent random 

 movements, /. e., by the trial and error method. But when the light 

 stimulus is greatly increased the orientation is direct, random 

 movements toward the light are suppressed altogether and the 



