Habits, Reactions, and Associations in Ocypoda arenaria. i<) 



The results of these experiments and observations leave little doubt 

 that in Experiment F the movement of flies around the coral was a factor in 

 determining the behavior of the crab. 



The eyes of Ocypoda are stimulated much more strongly by moving 

 objects and probably also by still objects on days when the sun is shining 

 brightly than when it is cloudy; also much more during the middle of the 

 day than early in the morning or late at night. It was found that the ap- 

 proach of man on cloudy days did not cause the crabs to retreat to their 

 burrows as quickly as on days when the sun was shining brightly ; also that 

 they did not run as promptly after fish thrown to them early in the morn- 

 ing or late in the afternoon as they did after those thrown during the 

 brighter part of the day. 



During the night-time it was found that Ocypoda would easily approach 

 a man lying on the sand and even crawl up upon him. The writer has had 

 them enter his pockets and on several occasions has received their rather 

 severe pinches. They show but little fear at night, when the contrast in 

 intensity of light on objects is small, but in the day-time such behavior does 

 not occur or at least very seldom. 



Bethe (1895, 1897) ^^^ observed that when a light is directed against 

 the eye of Carcinus mccnas, or when a dark object is placed in front of the 

 eye, the crab reacts by a movement of the first pair of antennae. Such a 

 movement does not take place in the case of Ocypoda, but it was found that 

 when a dish containing several individuals was brought from the direct 

 sunlight outside of the laboratory into the subdued light inside, they reacted 

 by a sudden jump the moment the shadow of the door was reached and 

 that when the dish was taken out they reacted again in the same manner as 

 soon as the edge of the shadow was reached. After the change had been 

 made rapidly and many times the crabs failed to react, but after allowing 

 them to rest for a few minutes and then repeating the experiment the indi- 

 viduals began again to react. 



While the anterior, posterior, outer lateral, and inner lateral surfaces are 

 sensitive to the differences in intensity of light, the dorsal surface does 

 not seem to be, or at least only slightly so (fig. 7). The writer was able to 

 stand during the middle of the day at the open window of the laboratory 

 mentioned in a previous section, without disturbing the crabs below in the 

 least; not even the most exaggerated motions, such as the swinging of the 

 arms or the waving of large objects, would cause them to run to their bur- 

 rows or even move away. This was not a case of inhibition, for the same 

 movements performed on the sand by the experimenter at the same distance, 

 or a much greater distance, always caused a run for shelter. 



Many attempts were made to study the behavior of Ocypoda after the 

 eyes were cut off, but they would not live long enough after the operation to 

 recover sufficiently from the shock. Much care was used in amputating the 

 eyes, but in no case was it possible to keep the individuals alive for more 



