Carpenter, Reactions of Drosophila. 485 



conditions of the experiment remaining the same, a similar tropic 

 reaction followed. The flies were consistently negative to a tem- 

 perature of from 5° to 6° C. Occasionally the creeping excursions 

 toward the light were prolonged beyond the lower water-line, but 

 only in very infrequent cases did the flies reach the upper water-line. 



As a control for both this and the previousexperiment the water 

 vessel of the apparatus was filled with water at nearly room tem- 

 perature. The flies then, following the light, crept to the end of 

 the immersed portion of the box. 



Reaction to unilateral light stimulation. The regular curved 

 paths described by Drosophila in its response to the repelling 

 effect of heat and cold suggested the possibility that this reaction 

 might be explained by the ''local action theory of tropisms." It 

 was conceivable that the unsymmetrical stimulation might act 

 locally on the organs of locomotion, presumably through the 

 nervous system. Those locomotor organs on the side subjected 

 to the greater stimulus might move more rapidly than those on 

 the opposite side until the insect should be turned so as to head 

 directly away from the region of stimulation. The two sides of 

 the body would then be equally affected by the heat or cold, and 

 the organs of locomotion would, therefore, move with equal rapid- 

 ity, and carry the fly away in a straight line. 



The adequacy of this simple explanation of the temperature 

 reactions might have been tested if the stimulus could have been 

 confined to one side of the body only. The tropism theory would 

 call for circus movements by the fly as long as the unilateral stimu- 

 lation was maintained. No satisfactory method for applying this 

 test to the temperature reactions occurred to me. The light re- 

 action, however, presented fewer difficulties, and furnished quite 

 as critical a reflex for the purpose. The light stimulus is effective 

 through the paired eyes of the insect. If one eye is covered circus 

 movements are to be expected under the theory of tropisms, with 

 the uncovered eye toward the centers of the circles, since Droso- 

 phila is positively phototropic. Such reactions in insects with one 

 eye blinded have been recorded by Holmes ('01) for the blue- 

 bottle fly, two species of bees, a robber-fly, a horse-fly, and a 

 species of syrphid. Parker ('03) has observed similar behavior 

 in the mourning-cloak butterfly (Vanessa antiope). 



In preparation for this test experiment, one eye of each of 

 several insects was covered by an opaque cap of lampblack and 



