32 FRESH-WATER BIOLOGY 



organs. Yellow is brightest to the human retina. In addition to 

 color animals react to direction and to intensity of light. Prob- 

 ably the majority of fresh-water animals react more strongly to 

 direction than to intensity. Hydropsycke and Argia do not react 

 to intensity at all but react to direction very sharply. Experi- 

 mental conditions in which direction away from source accompanies 

 a sharp decrease in intensity gives sharpest reactions with most 

 aquatic animals. 



Animals react to intensity with reference to an optimum. The 

 optimum usually corresponds to the usual Kght in their natural 

 environments. The organism may often be modified by changes 

 in the chemical character of the water, or even by rough handling 

 {Daphnia, Ranatra), so that it selects a different optimum, or re- 

 verses its reaction to direction of rays. 



Many animals react to shadows or small areas of illumination. 

 Thus frogs will hop to a shadow in the middle of a sunny field and 

 Amblystoma will follow a person along a sunny road. This type 

 of behavior is doubtless an important thing under water but has 

 been but little investigated. 



One of the topics which has absorbed much of the attention of 

 limnologists is the daily depth migrations of certain Crustacea. 

 They usually accumulate near the surface at night and in deeper 

 water during the day. The causes of these migrations are very 

 complex and light is an important factor. Dice has recently dis- 

 cussed the matter in full. Light is probably important in confin- 

 ing certain animals in deep water, in turbid streams, under stones 

 and logs and in caves, ground water, etc. 



The early invention of the thermometer has led to quite com- 

 plete investigation of temperature and an over-estimation of its 

 importance in the direct control of the distribution of life in water. 

 The tendency of modern investigation is to weaken the belief in 

 its direct importance. 



Stream temperatures are probably about the same at the various 

 points in any cross-section, except the shallow sluggish margin on 

 warm summer days. The extent to which daily, seasonal, and 

 weather fluctuations in atmospheric temperature affect a lake is 

 determined by the depth and size. Small lakes with incomplete 



