REACTIONS OF DAPHNIA PULEX TO LIGHT AND HEAT. 365 



body is parallel with the rays of light. Because of the slight depth of the water in 

 the box it was impossible for them to move far in a straight line when thus oriented 

 without reaching the surface of the water or the bottom of the box. As a result of 

 this condition the animals orient themselves with their heads toward the higher 

 intensity region and swim upward at an angle of about 45° with the surface until 

 they reach the surface; they then cease swimming and by reason of gravity sink 

 toward the bottom of the box; in a few seconds they again swim upward. This is 

 repeated, and thus the animal approaches, by a very indirect, zigzag path, the region 

 of greatest illumination. 



To the highest intensity (about 20 candle-power) which was obtainable with 

 the apparatus from a 100-candle-power lamp, the Daphnia were uniformly positive 

 in their phototaxis. And apparently when they had reached the + end of the band 

 of light they were still trying to move on to a higher intensity. There is therefore 

 no evidence from these experiments of an "optimal intensity." 



As it seemed possible that a higher intensity might reveal the "optimal," further 

 tests were made by simply placing a 100-candle-power lamp at one end of the glass 

 experiment-box. Under these conditions the animals moved directly to the + end of 

 the box. When no adiathermal screen was interposed between the lamp and the box 

 the heat was so great as to kill them within a few seconds after they reached the point 

 nearest to the lamp. But notwithstanding the fact that they were thus directed by 

 the light into a region of exceedingly violent thermic stimulation the Daphnia never 

 turned back. When a screen was used the animals after reaching the + end remained 

 there, moving about in a very irregular jerky manner as if under the influence of a 

 strong stimulus. They have been observed to move about in this way for as long 

 as thirty minutes, within a few centimetres of a lamp whose light was almost too 

 bright for the human eye to endure, without giving any signs of a tendency to become 

 negative to the light. To all appearances the organisms are wholly unable to resist 

 the directive influence of light, even though it lead them into intensities of stimula- 

 tion far greater than those to which they are accustomed, or into the presence of 

 positively harmful stimuli. In a previous paper (Yerkes, :00, p. 419) I have described 

 the phenomenon of Daphnia being directed by light into harmful chemical solutions. 



From these observations we learn that Daphnia is positively phototactic to light 

 up to an intensity of 100 candle-power, and that there is no evidence of an " optimal 

 intensity" between and 100 candle-power. 



2. The Influence of Heat Accompanying Light. — The following experiments were made 

 to determine (1) whether the radiant heat which accompanies the light from a 16- 

 candle-power incandescent lamp has any influence upon the motor reactions of 



