REACTIONS OF DAPHNIA PULEX TO LIGHT AND HEAT. 369 



conclude that there is no decisive evidence of any influence upon the movements of 

 Daphnia of the heat accompanying light. 



To the second question, Does the directive influence of light tend to diminish 

 during the experiments? we are able to answer, No; for throughout the period 

 of observation the rate of movement, in each of the five animals, gradually 

 increased. This was apparently due to an increase in sensitiveness to changes in the 

 intensity of the light. As the experiments with an individual were continued there 

 was a noticeable quickening in the orientation movements, and an increase in the 

 rapidity of the swimming. From the evidence of the experiments it may be said 

 that under the conditions with which we are dealing the sensitiveness of Daphnia 

 to changes in light intensity and to the directive influence of light does not diminish 

 but, on the contrary, increases. 



In studies of Daphnia made by Davenport and Cannon ('97, p. 31), and by me 

 (Yerkes, :00, pp. 407-414), the rate of movement was found to increase with the 

 intensity of the light. Davenport and Cannon concluded that the difference was due 

 almost entirely to difference in the precision of orientation. Although it is true that 

 precision of orientation is the chief cause of the increase in rate, I find that the rapidity 

 of the swimming movements also varies with the changes in the intensity of the light. 



In view of the well-established fact that Daphnia moves more rapidly in strong 

 than in weak light, how are the results of the experiments just described to be 

 explained? In them, it will be remembered, the time for the "Second Half" of the 

 trip toward the light was longer than that for the "First," when just the opposite 

 was to be expected. It might be said that as the animals approached the light the 

 intensity became too great for them, hence a hesitation which increased the tune. 

 This explanation is, moreover, supported by the fact that, whereas the path taken 

 in the "First Half" of a trip was fairly straight, it became zigzag in the "Second 

 Half." 



But there is another fact to be considered. It should be remembered that when 

 the animal in an experiment reached the end of the trough toward which it had been 

 swimming, it was in a region of very strong light ; then suddenly the light was turned 

 off at that end, and turned on at the opposite end. This sudden and great change 

 in the stimulus always caused the animal to start off toward the light at high speed. 

 It seemed not improbable, therefore, that the shortness of the time for the "First 

 Half" as compared with the "Second" might be due to this initial stimulus. For 

 the purpose of settling this matter, observations were made in which the animals 

 were started toward the light without the initial stimulus of the sudden change from 

 strong to weak light. These experiments proved conclusively that the time for the 



