234 CHAELES HARLAN ABBOTT 



structure seems to be suited to terrestrial as well as aquatic life. 

 However, land isopods are confined to a limited, protected en- 

 vironment, and are not suited, like insects, to become domi- 

 nant land animals. The fact that they remain in the environ- 

 ment in which they are fitted to survive is probably due chiefly 

 to another set of characters, those which make up the behavior. 

 These characters will now be considered. 



3. Reactions of land isopods 



Although the entire behavior complex is involved in adapta- 

 tion, it is possible to make a partial analysis into reactions to 

 various classes of stimuli. Among the most important factors 

 of the environment of land animals are humidity, evaporation, 

 contact, and light. 



a. Reactions to humidity and evaporation. So far as observa- 

 tion shows, the effect on land isopods of exposure to a dry at- 

 mosphere, including the first effect in desiccation experiments, 

 is an increase of activity. This is a useful adaptation, provided 

 the activity carries them to other regions where moisture condi- 

 tions approach more nearly the optimum. 



The problem whether these animals have a definite reaction 

 to differences in humidity and evaporation has not been inves- 

 tigated. The most satisfactory method yet devised for making 

 such tests is doubtless that of Shelford for testing reactions to 

 the evaporating power of air. Among the species of animals in 

 which he has found such a response (Shelford, '13 a) are several 

 commonly .associated with sowbugs, including ground beetles, 

 salamanders, snails, and the yellow-margined millipede (Fon- 

 taria corrugate Wood). No similar expermients have thus far 

 been recorded for land isopods, but it is quite possible that 

 they, too, give a negative reaction to increased evaporating 

 power of air, and as a result remain in places where evaporation 

 is at a minimum. The somewhat unusual occasions when they 

 visit exposed places may possibly be only at times when there 

 is a large amount of atmospheric humidity. 



