WATER 



187 



thereby the water released by the breaking 

 down of sugars or other carbohydrates and 

 also that produced by the oxidation of 

 hydrogen or carbon in the body of the ani- 

 mal. Fat is rich in hydrogen as well as 

 carbon and is poor in oxygen, and so is a 

 potent source of this last-mentioned kind of 

 water of metabolism. 



Certain insects, including Tribolium con- 

 fttsum and Dermestes vulpinus, eat more 

 food at lower humidities to produce a given 

 unit of body weight; the length of the lar- 

 val period increases, and the weight of the 

 pupae decreases. With such insects at such 

 humidities, the greater part of the body 

 water is derived from the oxidation of food 

 (Fraenkel and Blewett, 1944). 



Some animals are able to Hve indefinitely 

 without water beyond that furnished by air- 

 dry food. Forms like the drywood termites 

 (Cryptotermes) and powder-post beetles 

 are examples. Others combine the use of 

 metabolic water with other kinds of water 

 supply; the ability of the desert-adapted 

 camel to go eleven or more days with- 

 out drinking comes from its being able to 

 use water of metabolism obtained, in part, 

 from the oxidation of the fat in its hump, 

 as well as to store water in special compart- 

 ments of its stomach. 



As a result of combinations of these dif- 

 ferent water-producing and water-conserv- 

 ing abilities, desert mammals, such as 

 antelopes and many rodents, can exist for 

 months without taking liquid water other 

 than the often copious desert dew. The 

 combination of a dry, impervious integu- 

 ment, internal lungs or tracheal tubes, dry 

 feces, and the excretion of crystalline uric 

 acid, often combined with burrowing and 

 nocturnal habits, make reptiles, birds, and 

 many insects well fitted to withstand life in 

 dry habitats. 



Color may be aflFected by humidity, or by 

 humidity and heat. The correlations are 

 summarized as Gloger's rule (Hesse, Allee, 

 and Schmidt, 1937; Dobzhansky, 1941). 

 Exceptions aside, races of birds or mam- 

 mals living in cool, dry regions are lighter 

 in color (have less melanin pigment) than 

 races of the same species living in warm, 

 humid areas. The same rule holds among 

 insects, except that pigmentation increases 

 in humid cool climates and becomes less 

 in hot, drv ones. Appropriate changes fre- 

 quently follow rearing under controlled 

 experimental conditions and seem more 



afiFected by the humidity than by tempera- 

 ture. 



Insects and Moisture 



In many ways insects present a special 

 case in their relation to environmental 

 moisture, especially in relation to atmos- 

 pheric humidity. Insects are all small when 

 judged by vertebrate standards, and many 

 of them are tiny even when considered in 

 relation to their fellows. Once again we 

 have to deal with the principle that the 

 bulk of an animal increases as the cube, and 

 the surface increases as the square of the 

 length. The ratio of surface to body bulk 

 is large in the small to tiny insects, and this 

 has \atal importance in the water conserva- 

 tion of the more minute insects that have 

 only a thin chitinous covering. For these, 

 the loss of water quickly becomes acute. 

 Kennedy (1927) recognized this relation- 

 ship for insects and concluded that the 

 most outstanding adaptation to equalize 

 the chance of survival of such an insect in 

 a drying environment lies in its sensitive- 

 ness to changes in the humidity of 

 its surroundings, particularly when the 

 minimum toleration point is approached. 

 Such sensitiveness cannot save insects in 

 marginal habitats; a series of dry years de- 

 creases the area inhabited by the pale west- 

 ern cutworm (Porosagrotis) by hundreds 

 of square miles (Cook, 1924). 



The rate of development of some insects 

 varies with the vapor pressure of the atmos- 

 phere—that is, with absolute, rather than 

 with relative, humidity. The "cotton stainer" 

 insect, Dysdercus howardi, shows such a 

 relationship fairlv well for the egg stage 

 (Fig. 41). 



There is factual support (Headlee, 1917, 

 1921) for the commonsense suggestion that 

 an optimum humidity exists for each spe- 

 cies and varies from stage to stage of the 

 life history. The optimum humiditv depends 

 apparently on the concentration of the body 

 fluids and on the energy relations at evap- 

 orating surfaces. The latter have never 

 been measured for any animal, and, accord- 

 ing to Adolph (1932), the vapor tension of 

 the skin of the living frog cannot be meas- 

 ured. This is the more important, techni- 

 cally, since the frog is a good experimental 

 animal for such purposes; size alone makes 

 it much more favorable than are most in- 

 sects. Until methods are available to approx- 

 imate, at least, the vapor tension of living 



