PHOTOTAXIS IN FIDDLER CRABS AND ITS RELA- 

 TION TO THEORIES OF ORIENTATION. 



BY 



S. J. HOLMES 



{From the Zoological Laboratory of the University oj Wisconsin.) 



In the phototactic movements of animals which orient them- 

 selves definitely to the light it is almost always the longitudinal 

 axis of the body which is kept parallel to the rays. It is therefore 

 not without interest in relation to the much discussed "theory of 

 tropisms" to find an animal which orients itself sidewise instead 

 of in the usual manner. Such an animal is the common fiddler 

 crab of the Atlantic coast, Uca pugnax (Smith), which shows a 

 very decided positive phototaxis, especially in strong Hght. Like 

 many other crabs the members of this species run sidewise, and 

 in fact they are so constructed that they would find great difficulty 

 in any other form of locomotion. When recently brought into the 

 laboratory they seem remarkably attracted by a bright light. They 

 gather on the side of their enclosure nearest the light and often 

 struggle for a long time to get nearer the source of attraction. By 

 changing the position of the light the crabs may be made to follow 

 it about in any desired direction, changing their course promptly 

 whenever the hght is moved. There is lateral instead of longi- 

 tudinal orientation to the direction of the rays. 



When the crabs reach the side of the dish they usually do not 

 maintain their lateral orientation. They frequently face the light, 

 holding their eyes erect and moving from side to side in the en- 

 deavor to get as near the light as they can. Very often they settle 

 down facing the hght, remaining there for a long time as if spell- 

 bound. There seems to be no tendency to get into a position of 

 lateral orientation once the animal attains to a position of prox- 

 imity to the light. This orientation is necessary if a certain object 

 is to be followed, but, so far as I can determine by watching its 

 behavior, the crab does not tend to assume it except during loco- 



