allee] simple experiments IN ANIMAL BEHAVIOR 179 



Results, — ^Under normal conditions animals that are positive 

 to white light will collect in the blue light. Those positive to 

 fainter white light will be found in the green and blue light. 

 Animals negative to most intensities of white light will collect in 

 the red light and if given time enough will travel from one end to 

 the other as the glasses are moved. Isopods and amphipods will 

 collect in the red, while the common and fairy shrimps will be 

 found in the blue light. 



Discussion. — The action of colored light is very complex and 

 much debated. The most plausable view is that positively photo- 

 tactic animals are positive to the more chemically active blue 

 rays while the same rays repel negatively phototactic animals. 



4. Purpose. — To demonstrate the reactions of small entomostra 

 cans to light. 



Materials. — ^A thin walled, straight sided, glass timibler and 

 black paper. 



Procedure: — Cover the bottom and sides of one timibler with 

 black paper so as to exclude all light. On one side cut a slit ]/^ 

 inch wide. For this purpose use a sharp knife point. The edges 

 of the slit should be clear cut. Fill with water containing a num- 

 ber of small entomostracans ; copepods, ostracods and cladocerans ; 

 the more, the better. Adjust a black paper cap so that it can be 

 easily removed. Place the glass with the slit exposed to bright 

 light (direct sunlight is best) for one or two minutes. Without 

 moving the tiimbler quickly remove the cap and observe the 

 position of the animals. Arrange another tumbler in exactly the 

 same way except that no light is allowed to enter. How does the 

 position of the control entomostracans compare with those in the 

 experiment ? 



Results. — In the experiment the entomostracans will be found 

 at the side of the glass nearest the light. They will not be col- 

 lected in the most intense light but just at its margins. Oc- 

 casionally one will move out into the direct light. When by 

 chance this happens the rate of movement is increased but the 

 direction is entirely random. The animal may go up or down 

 but, sooner or later, its movements bring it into less intense light, 

 and then the rate of reaction decreases. In the control the 

 animals will be scattered. 



Discussion. — This experiment may be varied by arranging a 

 cylinder of black paper as a light screen and leaving the upper end 



