98 Studies in Animal Behavior 



behavior in general, but it is very probable that the 

 region selected represents at least a rough approxi- 

 mation to the optimum. 



Intimately related to the effect of intensity of 

 light on phototaxis is the influence of previous ex- 

 posure. Groom and Loeb in their observations on 

 Balanus larvae found that these organisms were posi- 

 tive in the morning, even to quite strong light, and 

 that later in the day they became negative, even 

 though the light to which they were exposed became 

 reduced in intensity. That this is not the result 

 of a diurnal rhythm is shown by the fact that posi- 

 tive larvEe placed for a few hours at any time of 

 day in a dark enclosure will be positive when first 

 exposed to light. Similar behavior is shown by 

 Daphnia magna, except that in moderate light the 

 animals become indifferent and show a negative re- 

 action when the intensity of the light is increased. 

 After exposure to a stronger light the Daphnias 

 again become indifferent and require a still stronger 

 light to produce a negative response. 



The close connection between the influence of in- 

 tensity of light and time of exposure is shown by 

 the fact that forms that are positive in moderate 

 light frequently do not immediately become nega- 

 tive when strong light is thrown upon them. Stras- 

 burger found that the swarm spores of Ulothrix 

 which are positive in weak light remained positive 

 for a very short time when exposed to strong light. 

 Balanus larvae rendered positive by being kept in 



