REACTIONS OF ARTHROPODS TO LIGHTS 487 



removing them to small glass jars as required. In order to 

 dark-adapt the flies, the jars containing two to five individuals 

 each were placed in the dark at least an hour before using them 

 in the experiments. 



2. Methods. The essential part of the apparatus used in these 

 tests was an elongated glass cylinder or tube 6 cm. in diameter 

 and 40 cm. long supported on a base 14 cm. high. The base 

 was a black box, IG cm. wide and 20 cm. long, open on one side 

 to allow the experimenter to place or remove the glass jars con- 

 taining the flies. This apparatus was placed midway between 

 the generators in such a way that the center of the cylinder, 

 was directly in the center of the field of light and its axis parallel 

 to the direction of the rays. A funnel opened into the cylinder 

 from the dark chamber of the box below. Since the flies are 

 strongly negatively geotropic, they readily crawl upwards when 

 freed in the funnel. As they leave the narrow opening leading 

 into the illuminated region they are oriented to one side or the 

 other depending on which light exerts the greater stimulus. 

 Mechanical counters were used for recording the number of flies 

 as they left either end of the apparatus. The flies were not 

 used a second time but a fresh lot of dark-adapted individuals 

 was taken for each new set of records. 



S. Results. The flies were tested first with single lights to 

 determine whether they were responsive to each of the four 

 colors of an intensity used in these experiments. In all of these 

 tests the direction of light rays was interchanged for each set 

 of individuals, but for a matter of convenience all the records 

 of any one color are combined. When the single lights are thus 

 compared with darkness the flies were found to be distributed 

 as shown in table 1. The records of the experiments shown 

 in table 1 demonstrate the strong positive phototropism of the 

 blow-fly to each of the four monochromatic lights. A greater 

 number of flies were negative or indifferent to the red or yellow 

 than there were to the blue or green, a result which indicates 

 that the more refrangible rays are more effective than those at 

 the opposite end of the spectrum. This relative efficiency of 



