246 LEE RAYMOND DICE 



the locomotor activity increases there will be a tendency for 

 the animal to move to higher levels while a decrease of activity 

 will allow it to sink. 



Yerkes (1900) has shown that an increase of light intensity 

 increases the speed of locomotion in Daphnia pulex. It can 

 easily be observed that on a great increase of light intensity 

 there is a decided increase of activity. 



The increase of locomotor activity caused by increase of light 

 intensity will have a tendency to cause the daphnids to move 

 upwards, while a decrease of intensity slows down their activity 

 and allows them to sink to lower levels. 



LOCOMOTOR ACTIVITY IN RELATION TO TEMPERATURE 



In studying the effect of temperature on locomotor activity 

 a number of experiments were performed to determine the speed 

 of motion toward a weak light at various temperatures. This 

 method is not very satisfactory because the change in tempera- 

 ture causes changes in the phototaxis as well as in the locomotor 

 activity and it is difficult to separate the effects of the two 

 factors. Further, different individuals give very different and 

 often contradictory results. The number of animals used in 

 the averages is too small to give very great accuracy in con- 

 clusions. However, it is thought that some of the relations 

 between temperature and rate of activity are indicated. 



The method used was to mark off transverse lines 10 centi- 

 meters apart on the bottom of a glass dish. In the darkroom 

 a 50 watt Edison lamp was placed 25 centimeters horizontally 

 from each end of the dish. A daphnid was then introduced 

 into the dish and allowed to swim toward one light. As soon 

 as the animal passed the line marked off on that end of the dish 

 this light was turned off and at the same time the light at the 

 other end of the dish turned on. An observation consisted of 

 8 trips, 4 each way, of the animal across the dish. The time 

 was recorded in minutes and seconds. In comparing the effects 

 of change of temperature two dishes were used with the water 

 kept at the different temperatures. Then the animals were 

 gently shifted from one to the other by a large pipette. 



In Experiment 16 each individual shows a decided increase 

 of speed of travel toward a weak light on first raising the tem- 

 perature 5 degrees. The averages for five individuals are 4' 02" 



