Tropisms Relation to Modes of Behavior 57 



ous amphipod Crustacea of that region and particu- 

 larly the reactions of the terrestrial species com- 

 monly called sand fleas. It is a somewhat curious 

 circumstance that the aquatic amphipods are nega- 

 tive to light and tend to keep in the darkest part of 

 their environment while the terrestrial ones are usu- 

 ally positive. Positive phototaxis is the most pro- 

 nounced in the most terrestrial of the species, the 

 large Talorchestia longicornis, which lives in holes 

 in the sand high up on the beach. When dug out 

 of the sand these crustaceans usually lie curled up 

 in a death feint, but when they become active they 

 manifest a very strong tendency to hop toward the 

 light. When brought into a room they may keep 

 hopping toward a window with intervals of rest 

 during the entire day. If they are placed in a dish 

 one-half of which is shaded while the other half 

 is exposed to the direct sunlight they will keep 

 hopping toward the light until they are overcome 

 by the heat of the sun's rays. 



The smaller Orchestia agilis, which lives nearer 

 the water's edge and frequently manifests a nega- 

 tive reaction to light, shows the same fatal degree 

 of positive phototaxis when exposed for some time 

 to strong sunlight. Does light orient these forms 

 automatically and involuntarily as is apparently the 

 case with the larvae of Arenicola? There are sev- 

 eral facts which favor such an interpretation. The 

 persistent and apparently unreasonable nature of 

 the response, its sudden reversal by certain external 



