MOVEMENTS OF DAPHNIA 255 



different seasons of the year. The lower limit of neutral light 

 intensity for phototaxis should be at a much lower depth at 

 midday in summer than at the same time of day in winter. For 

 this reason the lower limit of daily movement in Daphnia should 

 be deeper in summer than in winter. However, the range of 

 motion may be less in summer than in winter owing to the 

 effect of the higher temperatures of the surface waters in summer 

 on the geotaxis. It seems that the change of the midday light 

 intensities with the seasons will have a much smaller effect in 

 determining the mean daily depth than will the changes in 

 temperature. 



Cloudy weather should have the effect of raising the mean 

 vertical position of the daphnids. In partly clouded weather 

 the passage of a cloud across the sun should reduce the light 

 intensity enough to cause a tendency to negative geotaxis and 

 positive phototaxis which would cause an upward movement 

 of the animals. Clear sunshine following would produce a 

 tendency to positive geotaxis and neutral phototaxis resulting 

 in movement downward. 



Very strong moonlight might cause a slight tendency to 

 positive geotaxis and produce a small movement downwards. 



Different lakes will be subjected to great differences in light 

 intensity due to their position in latitude and altitude. The 

 light will penetrate to very different depths in different lakes 

 owing to the great variation in the clearness of the water. We 

 should then not expect to find the same migration behavior in 

 every lake. 



VERTICAL MOVEMENTS CAUSED BY CHANGES IN 

 TEMPERATURE 



Few observations are at hand to show the variation during 

 the day of the water temperatures at different levels in a lake, 

 but this variation is much greater in shallow than in deep lakes. 

 A table given by Hankinson (1907, p. 202) shows a variation 

 of three degrees in the surface temperature, during May 29, 

 in a shallow part of Walnut Lake, Michigan. The daily varia- 

 tions in temperature of some shallow lakes must be greater than 

 this, but in most lakes the variation in the temperature of the 

 surface water will be very small and there will be almost no 

 diurnal variation in the temperature of the lower levels. Tern- 



