ENVIRONMENTAL RELATIONSHIPS 



of water, with emphasis on prevention of evaporation, and on reclamation 

 and reutiHzation of water involved in the process of excretion. Mammals, 

 in which evaporation of water from the skin is an important aspect of temper- 

 ature regulation, must make good this water loss by commensurate intake of 

 water. 



In terms of the availability of water, as with other environmental factors, 

 each species of terrestrial animal exhibits a specific range of tolerance, beyond 

 which it cannot survive. This is clearly illustrated by experiments with 

 developmental stages of various insects which deposit their eggs on surfaces 

 exposed to air. During the embryonic period within the eggshell, atmos- 

 pheric humidity largely determines gain or loss of water by the insect through 

 the shell and its membranes. Similar series of eggs, maintained at a constant 

 temperature but under diflferent conditions of atmospheric humidity, show 

 very definitely the range of tolerable humidities, and in some cases a narrower 

 zone of optimum humidity, for embryonic development. In some species 

 development and hatching occur only in an atmosphere of 100 per cent 

 relative humidity, saturated with water vapor. In other species, as illustrated 

 by Figure 19.3, such a high humidity may be slightly less favorable than 

 lower ones. The eggs of some species are so well protected against evapora- 

 tion that development proceeds normally even in a perfectly dry atmosphere. 

 In such conditions young larvae often die of desiccation as soon as they have 

 ruptured the eggshell at hatching. Similarly, larvae may be unable to survive 

 in saturated atmospheres after completing their embryonic development. In 

 the experiment illustrated by Figure 19.3, more larvae developed than were 

 able to survive after hatching, at all degrees of humidity, but particularly at 



Fig. 19.3. Effects of different rela- 

 tive humidities, at a constant tem- 

 perature, on the development and 

 hatching of eggs of a moth, Telea 

 polyphemus. (From D. Ludwig and 

 J. M. Anderson, 1942, Ecolooy, 

 vol. 23, reprinted by permission.) 



100- 



80 



60 



f^ 40 - 



20 



I I Per cent hatching 



^1 Additional per cent developed, 

 failing to hatch 



10 16 56 76 100 

 Per cent relative humidity 



603 



