Holmes, Phototaxis in Crabs. 495 



of their movements were something unexpected. In a crab with 

 crossed eyes the behavior of objects in its visual field consequent 

 on its movements is different from what it is accustomed to. Under 

 normal conditions fiddler crabs probably possess binocular vision 

 so far as may be judged by the behavior of the eye-stalks and the 

 arrangements of the facets of the eyes, but when the eyes are 

 crossed the animals have two visual fields which give quite dif- 

 ferent impressions, thus adding further to their confusion. The 

 visual world of such crabs doubtless seems a hopelessly mixed up 

 affair, and it is not surprising that the animals often sulk as if dis- 

 couraged with their efforts. Crabs kept for several days in order 

 to ascertain if they would improve in the appropriateness of their 

 responses gave only negative results. 



The phototactic reactions of fiddler crabs are very easily checked 

 or overcome by fear. When a light is brought near the animals 

 they often scurry away in great haste, and at first one might be 

 led to interpret this behavior as a manifestation of negative photo- 

 taxis, but it is a very different phenomenon. Even when showing 

 a strong positive reaction the fiddlers often flee in apparent alarm 

 upon a shght movement of the light. If the movement is a sudden 

 one they are more apt to beat a retreat, while they follow slower 

 movements without fear. When exposed to strong illumination 

 for some time they become more insensible of their surroundings 

 and are dominated almost entirely by the stimulus from the hght 

 without regard to movements made near them which at first would 

 send them scurrying off in great haste. They get, as it were, 

 "warmed up" to the work, becoming not only less responsive to 

 other stimuli, but more vigorous in their phototactic activity, I 

 have described similar phenomena in the water-scorpion, Ranatra, 

 and other observations have shown that it is a not uncommon trait 

 in tropic responses. 



I do not wish to add further to the confusion that exists in the 

 use of the term tropism, but I believe that the retreat of the fiddlers 

 from a moving light cannot properly be described as a tropic reac- 

 tion. It may be related to tropic reactions as vision is according 

 to Radl, but it is not so much a response to the light per se as to 

 a sudden movement or appearance of the light. If we described 

 as negative phototaxis all movements however caused which were 

 directed away from the light we should have to include the flight 

 of the fiddlers under this designation, and say that a sudden move- 



