Chap. 5 ANIMALS AND THEIR ENVIRONMENTS 85 



logical low gear for which little or no food is needed. In winter the water is 

 warmer than the air; frogs stay in muddy pond bottoms but do not drown 

 because they take in enough, oxygen through the skin for their lowered metab- 



FiG. 5.18. Bubbles of nitrogen gas (black) 

 collect at the joints when a person, e.g., a deep- 

 sea diver, rises suddenly into greatly lowered 

 pressure. 



olism. Insects go through a special cold-hardening, partly by loss of water and 

 the production of bound water which does not freeze except in extraordinarily 

 low temperatures. Earthworms burrow below the frost line and gather in clus- 

 ters conserving heat and moisture. Lady beetles spend the winter in companies 



Fig. 5.17. A, midsummer temperature of a lake. Water contracts with cooling 

 and becomes heavier but only to 4° C. (39.2° P.). When warmer or colder than 

 this it becomes lighter. 



Water takes its place in layers according to its weight which is dependent on 

 temperature. 



B, sections of a lake showing the seasonal changes in temperature. Summer. 

 The light is stronger but the diatoms decrease probably because of inadequate 

 nourishment and perhaps of silica since the thermocline seems to bar the way to 

 chemical substances that might otherwise well up from the bottom. Autumn. 

 With the mixing of the water and disappearance of the thermocline there is an 

 upward diffusion of nutrient salts. Another increase of diatoms occurs, not so great 

 as in spring since the sunlight is weaker. Winter. The lake is covered with ice 

 which is water at its lightest and coldest. Spring. Light increases and with it an 

 increase of diatoms called the spring pulse, of great importance in the food supply 

 of all young animals. 



