486 ALFRED O. GROSS 



changed are shown in figure 23 and are similar to those shown 

 in figure 21, with the exception that three larvae which were 

 started in the red were not turned in the reverse direction by the 

 blue. They were turned at an angle when exposed to the blue 

 but did not turn completely before creeping off the paper. When 

 such larvae are allowed to continue on another sheet, they usu- 

 ally orient in the course of time in a direction away from the 

 blue light. Such slight irregularities, which may occur in any 

 series, are probably due to corresponding differences in the 

 responsiveness of the organisms. For this reason experiments 

 i^i which the larva is started first in one light and then in the 

 other are highly important in serving as an effective check 

 throughout these records. In each diagram (figs. 22-33), though 

 the records are plotted all on one side, they represent records 

 one-half of which are from larvae started on the right side and 

 the other half on the left side in the respective colors of each 

 pair. A greater number of observations were made for the green 

 and blue pairs than for any other, since the relative differences 

 of the stimuli produced by these lights, is apparently less than 

 that in the lights of the other pairs. 



These combined records of the blow-fly larvae reactions sub- 

 stantiate the results previously shown in the reproduction of 

 the paths plotted by the larvae, namely, green is the most effec- 

 tive and the red the least so, of the four monochromatic lights, 

 in orienting the larvae. The blue and the yellow are intermediate 

 between these extremes but the blue light is much stronger in 

 its effect than the yellow. The effectiveness of the four colors, 

 therefore, is in the order, beginning with the strongest; green, 

 blue, yellow, red. 



B. Calliphora erythrocephala Meigen {adult) 



1. Material. The adult blow-flies used in these experiments 

 were reared in the laboratory from the cultures of larvae used 

 in the foregoing experiments. The flies were easily kept alive 

 and in good condition by feeding tliem on sugared water. They 

 were placed in a large screened cage provided with a device for 



