Habits, Reactions, and Associations in Ocyf^oda arcnaria. 39 



the vibrations of sound produced during the movement of tliis stridulating 

 ridge against the basal joint of the chela are probably felt by other ocypodas. 



The tactile sense is well developed in Ocypoda. With the body orient- 

 ated in a fixed position, the animal can move in practically any direction. 

 It runs with a considerable degree of accuracy and undoubtedly has a sense 

 of position and distance. In locomotion these crabs are guided by differ- 

 ences in the lighting of surfaces, by tactile stimuli, by differences in mus- 

 cular effort and by the stimulation of the equilibrating organs resulting from 

 a tilting of the body. 



During most of the time Ocypoda lives on land, only going to the water 

 occasionally for the purpose of moistening the gills. When exposed to the 

 direct sunlight without water for more than 4 hours individuals usually 

 die. They can live in their burrows without sea-water for at least 48 hours. 



The " Aufbaum Reflex " described by Bethe is an attitude of defense. 

 Ocypoda often hides from man by simply settling down and throwing sand 

 over its body. It is sometimes found in a resting or " sleeping " condition 

 when it does not react to many of the ordinary stimuli. The death- feigning 

 reaction is exhibited under certain conditions. 



This brachyuran forms associations and habits. 



