No. 629] HABITAT RESPONSES OF WATER-STRIDER 503 



The responses of crabs living in shallow water differed 

 from the responses of those living in deeper water. AVhen 

 the former were placed on the beach, they displayed a 

 very definite hydrotropism, but the latter, under similar 

 experimental conditions, evinced no such definiteness of 

 response. Drzewina considered such responses to be 

 adaptive in character. She seems to infer that the char- 

 acter of the behavior, already acquired, must be taken into 

 consideration in the interpretation of their present re- 

 sponses. These observations are described by her as 

 follows (1908, p. 1011) : 



4. Bole of Vision. — On tlie several occasions that I have 

 observed tlie drying up of isolated stream pools, having 

 on their surfaces trapped Gerris remigis, I liave watched 

 carefully in order to detect whether the sense of siiiiit wms 

 the principal factor in aiding these a(inati(' liiiu- to Hiid 

 other bodies of water. The role played, directly. ])y 

 vision, is probably not of immediate importance dui ing 

 their responses in this connection, except in those in- 

 stances when the ground is flat and level and the gerrids 

 are comparatively close to the water. There are various 

 obstacles that modify the possi])ilities of such an ex].1aiia- 

 tioii. [f there are two or more bodi.'S of water in tlie im- 

 mediate vicinity, it ha^ l.een ol.-erved that the -cnids 

 are just as likely to move toward the farthest one, a^ they 



