REACTIONS OF ARTHROPODS TO LIGHTS 



491 



About 15 to 20 indi\aduals were thus transferred to each of 

 about 25 small 4-ounce \^dde-mouth jars for each of the series 

 of tests. The flies were not reared in the dark room but were 

 always dark-adapted before they were used in any of the light 

 experiments and, as in the case of the adult blow-flies, they were 

 never used a second time immediately after having been exposed 

 to the light of the apparatus. 



2. Methods. The apparatus used for the Drosophila was essen- 

 tially the same in principle as that used for the blow-flies and 

 differed only in some minor details. The tube leading from the 

 funnel in this apparatus was smaller and its opening into the 

 illuminated C3dinder was partially obstructed by two pieces of 

 cork which prevented the flies from going upwards too rapidly. 

 There was also a device to enable the experimenter to close 

 completely the aperture when so desired. 



3. Results. To determine whether all the hghts used were 

 effective in stimulating the animals the flies were permitted to 

 enter the cylinder illuminated by a single monochromatic light 

 from one end only. In each of these tests about 200 individuals 

 were used, which were found to react as represented in table 5. 

 This experiment demonstrates the strong directive effect that 

 each of the colored lights used in these tests have on Drosophila, 

 and gives further evidence of the strong positive phototropism 

 of these insects. A greater percentage of the individuals were 

 negative when tested with the yellow or red lights than when 

 the blue and green were used. This result suggests the fact 

 subsequently established that the lights having the more refran- 

 gible rays exert the greatest directive stimulus on the pomace flies. 



