MOVEMENTS OF DAPHNIA 257 



However, the sunlight is much reduced at that season, causing 

 a tendency toward positive phototaxis. Also, the low tem- 

 perature of the water causes a tendency toward negative geo- 

 taxis and positive phototaxis. All of these factors tend to 

 keep the animals near the surface at all times and so cut down 

 the range of migration at that time of year. 



In different lakes the conditions of temperature will be very 

 varied and this will have a great effect in causing the observed 

 variations in the range and the peculiarities of the diurnal 

 migration found in different lakes. The differences in the tem- 

 peratures are particularly great between shallow and deep 

 lakes. 



As stated above, the low water temperatures of winter will 

 have a tendency to keep the daphnids near the surface. In 

 summer the increase in temperature of the surface layers will 

 cause the animals to occupy lower levels. Diurnal movements 

 may take place in summer as well as in winter, but in summer 

 the limit of movements will be at lower levels than in winter. 



Changes in temperature, then, are not the primary cause of 

 diurnal movement, but difference in temperature of the water 

 at different levels in a lake may operate to shorten the distance 

 of the diurnal movement. Changes in water temperature, par- 

 ticularly of the surface layers, at different seasons of the year 

 is the principal factor determining the seasonal movements. 

 This result is chiefly due to the effect of different degrees of 

 temperature on geotaxis, although the effect of temperature 

 on phototaxis has a minor effect. 



MOVEMENTS CAUSED BY MECHANICAL STIMULATION 



The mechanical stimulation most likely to affect the plankton 

 Crustacea will be the action of the waves. It has been stated 

 in a previous section that mechanical stimulation, while it con- 

 tinues, tends to inhibit the locomotor activity of the animals 

 so that they fall passively through the water. After descending 

 to a depth where the wave action could no longer be felt the 

 daphnids would tend to rise again until stopped by reaching a 

 region of perceptible wave action. The result is that the daph- 

 nids should be found at a lower level than normal on stormy days, 

 and by this reaction escape the damaging action of strong waves. 



