REACTIONS OF LAND ISOPODS TO LIGHT 215 



h. The results which have been analyzed are typical for the 

 species Oniscus asellus. The reaction of nearly all individuals is 

 negative, with only occasional reversals to positiveness. The 

 constancy of these results will be shown in the experiments to 

 be described later in this paper. 



c. Nothing has been said as yet about the reactions of Por- 

 cellio. The two species of Porcellio are also negative to light, 

 but the reaction is less consistent than that of Oniscus. The 

 difference between the two genera will be shown repeatedly in 

 the following pages, and, finally, will be summarized in the sec- 

 tion on the comparison of Oniscus and Porcellio. 



C. MODIFIABILITY OF THE LIGHT REACTIONS 



A consideration of the ecological importance of isopod be- 

 havior involves the question whether the behavior is constant or 

 is easily modified by environmental changes. Shelf ord has 

 pointed out the fallacy of the assumption that the behavior of a 

 given species is a constant characteristic, whatever the environ- 

 mental conditions. In recent studies of fresh-water annuals 

 (Shelf ord, '14) and marine fonns (Shelf ord, '16), he has shown 

 that behavior characters belong to a community even more than 

 to a species, and that behavior differences between individuals 

 of a single species from different habitats may be greater than 

 those between widely diverse species which occupy the same 

 habitat. 



As an illustration of this principle, Allee ('12) has shown dif- 

 ferences in the rheotaxis of the fresh-water isopod Asellus com- 

 munis Say, according as the individuals tested came from ponds 

 or from streams. 



Modification of behavior, however caused, is shown in many 

 ways, of which the three following are important: 1) by a par- 

 tial or entire failure to respond to the stimulus; 2) by an inten- 

 sification of response; 3) by a reversal of response. Allee ('12, 

 '14) obtained the first two of these modifications by subjecting 

 Asellus to various experimental conditions which changed the 

 metabolism of the animal. He found that conditions similar to 



