REACTIONS OF ARTHROPODS TO LIGHTS 477 



which was left in a warm dark room of comparatively uniform 

 temperature. The larv^ae selected for experimental purposes^ 

 were about five or six days old, an age at which they seemed 

 most active and most responsive to light. The animals were 

 always dark-adapted and were carefully guarded from light for 

 several hours before the experiments. 



2. Methods. The accessory apparatus for the experiments on 

 the blow-fly larvae was comparatively simple, being merely a 

 small table 18 cm. in height, supporting a thin piece of slate 

 30 cm. long and 25 cm. wide. This piece of apparatus was 

 placed inside the dark chamber midway between the two gener- 

 ators. The apparatus was arranged so that an animal placed 

 at the center of the slate was illuminated by lights from two 

 opposite sources, each of equal intensity. The surface of the 

 slate was frequently moistened with warm water to facilitate 

 the movements of the larvae and to prevent them from following 

 the old courses of other individuals. In addition to these tests, 

 the larvae were made to trace their own courses on paper with 

 dilute solutions of methylene blue as described by Herms ('11, 

 p. 189). The rate at which the larva moved was indicated b}^ 

 marking on the paper the position of the animals at the end of 

 each ten seconds. 



Different individuals, even when of the same age and reared 

 in the same culture under identical conditions, varied more or 

 less in their responsiveness to light. The cause of this difference 

 was not ascertained but probably depended on some physiologi- 

 cal condition at present unknown. 



3. Results. The animals were first tested with each of the 

 four individual colors to determine whether they were responsive 

 to the various wave lengths of light of an intensity used in these 

 experiments. In each test the larva was placed next the side 

 of the slate nearest the source of light. Its subsequent course 

 after orienting was traced on the slate or else on the paper with 

 the methylene blue solution. Each individual was given two 

 tests, the first with tlie source of hght on one side, the second 

 with it on the other. The larva was then put aside and a new 



