226 CHARLES HARLAN ABBOTT 



SO dry that few individuals of Oniscus survive, Oniscus requir- 

 ing more moisture in its surroundings than does PorcelHo. For 

 a maximum, the substratum can easily be kept saturated with 

 water. 



A comparison of the reactions, after keeping the isopods for 

 over a month in these two extreme habitats, is shown in figures 

 11 and 12. 



According to figure 11, Oniscus gave essentially the same 

 negative reaction regardless of preceding moisture conditions. 

 Both curves are typically negative and resemble each other 

 closely. 



On the other hand, Porcellio scaber (fig. 12) from the dry 

 habitat was somewhat less negative than the same species from 

 moist surroundings, but both curves for Porcellio show a con- 

 siderable degree of indifference to. the stimulus. 



There is, therefore, little evidence that the reaction varies 

 greatly according to the amount of moisture in an isopod's en- 

 vironment. The chief observable difference in the effect of the 

 two habitats is an increase of activity under dry conditions, 

 which has no apparent relation to the character of the photo- 

 taxis. It is interesting that, although, of the two genera, Por- 

 cellio is the more adapted to survive atmospheric dryness, lack 

 of moisture should apparently affect its behavior more than that 

 of Oniscus. 



5. Reaction in water 



In studying the amphipod, Orchestia agilis, which spends 

 most of its life out of water. Holmes found ('16, p. 105) that, 

 when thrown into water, these normally positive Crustaceans 

 became at once strongly negative. As sowbugs, when placed in 

 water, will remain active for some time before showing ill effects, 

 it was possible to try a similar test with them. Five individuals 

 each of Oniscus and Porcellio rathkei were placed in a rectangular 

 jar of water and exposed to horizontal light of an intensity close 

 to that used in most of the experiments previously described. 

 The isopods moved about to a considerable extent, but there 

 was no orientation with respect to the position of the light. 



