254 LEE RAYMOND DICE 



to which the daphnids are neutral must extend over a consider- 

 able depth. 



It has also been shown that the light intensity affects the 

 geotaxis. The action of the positive geotaxis induced by strong 

 or moderately strong light will probably be to cause the daphnids 

 to go down until further motion downward is stopped by the 

 beginning of positive phototaxis. 



The most important change in light intensity acting on a 

 lake is the change from sunlight to darkness occurring every 

 day. If we suppose the daphnids to be at the surface at night, 

 the first rays of light in the morning will cause a positive geo- 

 taxis which will overcome to some extent the positive photo- 

 taxis and the animals will begin to move downward. As the 

 light increases in intensity they will continue to move down- 

 ward, owing to the continued increasing positive geotaxis, and 

 will keep near the lower limit of neutral phototaxis. The down- 

 ward movement then is due to the action of light on the geo- 

 taxis, but the limit of downward motion is determined partly 

 by the phototaxis. In the evening as the daylight fades the 

 daphnids move upward, owing partly to the negative geotaxis 

 caused by lowered light intensity, and partly due to the positive 

 phototaxis caused by weak light. At night the daphnids remain 

 near the surface due to the negative geotaxis existing in the 

 absence of light. For the sake of simplicity in the above state- 

 ment no account has been taken of the gradual changes in 

 light intensity throughout the day. 



It is evident that in the case of Daphnia pulex the change 

 of geotaxis caused by changes in light intensity is the most 

 important factor causing the diurnal migrations, while change 

 of phototaxis is a factor merely limiting the extent of the move- 

 ments. If there were no change in geotaxis there would be 

 only a feeble indication of a diurnal migration in this species. 



The reversal of phototaxis caused by the action of the ultra- 

 violet rays might under some circumstances be a factor causing 

 diurnal movement, but it seems that in this case the positive 

 geotaxis caused by the action of the visible rays will keep these 

 daphnids down below the action of the ultra-violet rays, so 

 that the action of these rays will not be a factor in the vertical 

 movements. 



The light intensity effective on any given lake will vary at 



