286 PRINCIPLES OF ANIMAL BIOLOGY 



ponds) or be blown about by the wind. Eggs of Crustacea and rotifers, 

 similarly covered with heavy shells, may likewise be dried without all 

 being killed. One family of rotifers may be dried in the adult stage, as 

 may also certain roundworms. Earthworms burrow deeper in the soil 

 as moisture disappears near the surface, and eventually they roll up in 

 balls to conserve their moisture. 



Excess of moisture is often as injurious as dearth of it. Soil organisms 

 may be drowned in wet seasons because the air is driven from the soil 

 by water, and they are unable to obtain their required oxygen from 

 water. The sugai'-beet root louse suffers most damage from excess 

 moisture at the time of hatching from the egg and at the periodic shedding 

 of the skin as it grows. So much damage is done at these times that the 

 louse multiplies in dry soil more than fifteen times as fast as in soil 

 moistened from below, and nearly thirty times as fast as in soil moistened 

 by water falling from above. 



Insects that suck the sap of plants are more or less independent of 

 moisture in the air around them, as long as their host plants can maintain 

 themselves. Indeed, in such animals the water may be regarded as a 

 waste material to be eliminated. The white fly, common on many 

 greenhouse plants, ejects water from its rectum in frequent bubbles 

 that burst and spray over the surrounding leaf surfaces at considerable 

 distances. Aphids are similarly supplied with excess water. 



Among the higher animals, the water requirements differ enormously. 

 Mammals that lose much water through sweat (man, horse) or con- 

 siderable excretion of urine or milk (cattle) must make good the loss by 

 drinking. Most mammals are included in one or more of these categories, 

 but some manage to get along with very little water except that taken 

 with their food. Camels are the classical illustration of the ability to 

 do without water, since they can subsist a week with only dry food, and 

 if they are fed green plants they can avoid other water for a month or 

 more. Mountain goats, prong-horn antelopes, mule deer, jack rabbits, 

 gazelles, jumping mice, and some of the ground squirrels are said to use 

 only the water that is eaten with other food. Such animals are peculiarly 

 fitted for regions where there are few or no bodies of water. 



One important ecological function of water in the protoplasm of 

 animals is its modification of the effect of temperature. Relatively 

 dry protoplasm endures high temperatures — even above that of boiling — 

 without coagulation, and low temperature without freezing. It is not 

 necessary that the water be actually removed from protoplasm to pro- 

 duce this effect, but merely that the amount of free water be reduced. 

 Thus, in the pupa of the polyphemus moth, which is covered with a thick 

 horny coat, there is little actual evaporation of the water, but as winter 



