192 



Scientific Proceedings (54). 



121 (817) 



Experiments on the light and tactile reactions of a cave variety 

 and an open water variety of an amphipod species. 



By A. M. Banta. 



[From the Station for Experimental Evolution, The Carnegie 

 Institution of Washington.] 



The amphipod, Eucrangonyx gracilis (Smith), occurs in the 

 waters of many caves in southern Indiana and is very generally 

 and abundantly distributed in small surface streams in the same 

 region. The individuals occurring in caves almost without excep- 

 tion have very little or no body pigment though possessed of the 

 normal pigment in the eyes. Those living in surface streams 

 have the usual amount of pigment for a crustacean. In other 

 structural characters the forms are apparently alike. 



In an attempt to determine whether there was a physiological 

 difference between the two forms they were tested for light and 

 tactile reactions. These amphipods are negative to all intensities 

 of light to which they respond. Extended series of light experi- 

 ments using horizontal illumination were conducted during which 

 every effort was made to maintain as precise conditions as possible. 

 The cave form was found to be somewhat less responsive to light 

 than the form living outside caves, but this difference was not 

 large and was distinctly observable only with relatively small 

 intensities of light. 



In order to test the tactile reactions a delicate camel's hair 

 fastened to the end of a glass rod was used to stimulate various 

 parts of the body of several individuals of the cave form and the 

 same number (of equal size) of the surface form. Complete tabu- 

 lations of the nature and vigor of each response, when a response 

 occurred, were kept and failures to respond were likewise recorded. 

 This data showed a greater number of failures to respond and on 

 the average a slightly less vigorous response on the part of the 

 surface form as compared with the cave form. 



Hence the cave form appears to be less responsive to light and 

 more responsive to tactile stimulation than its outside relative. 



