256 LEE RAYMOND DICE 



perature changes, then, cannot have an important influence in 

 causing the diurnal movements. 



In summer the surface waters of an ordinary lake become 

 warmed while, if the lake be deep enough, the lower levels 

 remain cool (Table II). 



TABLE II 



Temperatures of Lake Cochituate, Mass. 

 (From Fitzgerald, 1893, Trans. Amer. Soc. Civ. Eng. 34( p. 96.) 

 (In degrees Centigrade calculated from the original Fahrenheit.) 



Averages.... 16.9 16.6 16.0 14.2 11.5. 10.3 9.3 8.3 7.9 7.2 



It is evident that in performing any considerable migration near 

 the surface in summer the daphnids will be subjected to a con- 

 siderable variation of temperature. If we consider them moving 

 downward in the early morning in response to increase of light 

 intensity they will be moving into water at lower and lower 

 temperatures. These low temperatures will have a tendency 

 to produce negative geotaxis and positive phototaxis. These 

 tendencies will operate to reduce the range of the downward 

 movement. On moving upward in the evening the animals 

 will be reaching increasing temperatures of water. These 

 higher temperatures will have a tendency to produce positive 

 geotaxis. If the surface temperature is in the neighborhood of 

 25° C. there will be a slowing up of the locomotor activity after 

 a short exposure to that amount of heat. Irregular wandering 

 away from the high temperature may also be operative. These 

 factors will then work together to lessen the distance of upward 

 travel in summer. Then, in a lake where the surface water is 

 much warmer than the under layers we would expect that the 

 range of diurnal migration would be much reduced. 



In winter, when all the water of the lake is at nearly the 

 same temperature we would expect rather large migrations 



