MOVEMENTS OF DAPHNIA 235 



Frisch and Kupelweiser (1913) state that after a short expos- 

 ure Daphnia magna and less evidently Daphnia pnlex become 

 indifferent to the light and show no decided phototaxis so long 

 as the intensity remains the same. When the light intensity is 

 decreased the animals become temporarily positive while on 

 increase of intensity they become temporarily negative. After 

 a few minutes exposure to a new intensity the daphnids again 

 become indifferent to the light and uniformly distributed through- 

 out the dish. It has been shown above that in weak light or 

 in light of moderate intensity Daphnia pulex remains constantly 

 positive. Within the range of these intensities I have been 

 unable to reverse even temporarily the phototaxis by sudden 

 change of intensity. However, within the limits where the light 

 intensity is strong enough so that the daphnids are normally 

 indifferent, sudden changes of intensity produce the effects 

 noted by the above mentioned authors. The tendency to these 

 effects is probably present on any sudden change of intensity, 

 but in weak light this tendency is overpowered by the normal 

 phototaxis. It does not seem likely that these temporary 

 changes of phototaxis can be of much importance in causing 

 the vertical migrations of Daphnia. 



PHOTOTAXIS IN RELATION TO TEMPERATURE 



Loeb (1906) found that daphnids which were indifferent to 

 light at 19° C. became positive when the temperature was re- 

 duced to 11° C. When the temperature was raised to 25° C. 

 they became again indifferent or weakly negative. My experi- 

 ments show the same results. Daphnids indifferent to sunlight 

 at 20° C. show positive phototaxis to the same light at low 

 temperatures. This first becomes pronounced at about 12° C. 

 Animals positive to diffuse light showed a slight reduction of 

 the positive phototaxis on heating to 32° C. 



In diffuse light to which Daphnia pulex was positive at 22° C. 

 Yerkes (1900) found no change of phototaxis on raising the 

 temperature, although this was carried to the point where all 

 the daphnids died. In weak or moderately weak electric light 

 I have been unable to detect a variation in phototaxis on any 

 amount of heating. We may then conclude that decrease of 

 temperature causes a tendency to increase of positive photo- 

 taxis while increase of temperature causes a tendency to decrease 



