Water 85 



for hibernation by loss of water from their blood (Rasmussen, 

 1917) . Insects survive longest in dry air at high temperatures 

 when they can cool their bodies by evaporation; this means that only 

 insects of some size can live under such conditions, and that small 

 insects are confined to moist, cool situations (Mellanby, 1932) . 

 Frogs which have the subdermal spaces full of water can remain 

 some time in the atmosphere, but such a reserve of water is soon 

 exhausted and must then be renewed. A desert lizard is well 

 insulated against the loss of water and adapted to high temperatures. 



Water 



Aquatic habitats differ from those on land in that they show 

 no change corresponding to humidity. The ability of many types 

 of animals to live on land is dependent to a considerable degree 

 on the amount of water present in the atmosphere or soil. Water 

 vapor is lighter than air in the ratio of 5 to 8, and damp air is 

 therefore lighter than dry air. Air has 1/800 the density of water. 

 It takes 4 to 5 times as much heat to raise air 1 ° C. as to heat dry 

 soil the same amount (Fowler, 1928) . Some animals absorb water; 

 others lose it at all temperatures. Most insects have an optimum 

 humidity that is little below saturation (Ludwig, 1945) ; many 

 are therefore nocturnal or become dormant when moisture decreases. 

 Desert animals have special adaptations for acquiring and con- 

 serving water. 



"The necessity of water in biologic processes is universal" 

 (Rountree, 1922) . Water not only furnishes material for build- 

 ing living substance but aids in solution, circulation, oxidation, 

 hydration, hydrolysis, lubrication, excretion, and other activities. 

 Every animal must have a continual supply of water to carry on 

 its life processes. If water cannot be obtained in free form, it 

 must be acquired from food or even by the breaking down of 

 the tissues of the organism that uses it. 



Animals along the seashore are adjusted to various degrees in 



