240 CHARLES HARLAN ABBOTT 



first, the delay sets in on the average after ten responses, so 

 that in nearly every ordinary series of twenty trials some re- 

 sponses occur only after an interval. 



When an animal does not respond to the initial light stimulus, 

 there are three possible explanations why it responds later. 

 These are: l) a delayed response to the initial stimulus; 2) a 

 response to the continuous stimulus, and 3) a response due to 

 causes wholly other than light stimuli. Whichever of these is 

 true, the difference between immediate and delayed response is 

 a difference in photokinesis rather than in phototaxis, because 

 orientation is quite as definite when the response is delayed as 

 when it is immediate. 



The opportunity for observing the influence of continuous 

 stimulation after delayed reaction is found in those instances 

 when orientation occurs before locomotion, whether or not it is 

 accompanied by locomotion. Under these circumstances, the 

 first locomotor movement of the animal after a delay is a turn- 

 ing movement, and the cause of the turning cannot be found in 

 any change of intensity which has occurred since the initial 

 stimulus on the eyes of the animal. That is, the reaction under 

 these conditions must be due either to the continuous constant 

 stimulus or to an after-effect of the shock produced by the 

 initial stimulus. 



This raises the further question: Would an isopod give an 

 orienting response after a delay if the stimulus were not con- 

 tinued? Mast ('12) tested fireflies from this point of view, and 

 in his summary says: 



The males do not orient when exposed to continuous illumination. 

 They respond only to flashes of light and the reaction does not begin 

 until after the light has disappeared. Removal of the female imme- 

 diately after she glows has no effect on the reaction. Thus orientation 

 may take place in total darkness, and it is surprising how accurately 

 these animals turn through the proper angle in the total absence of the 

 stimulating agent. Here we have a case in which it is clearly demon- 

 strated that light does not act continuously in the process of orientation 

 as demanded by Loeb's theories, a case in which it is also clearly dem- 

 onstrated that the continuous action of the stimulating agent is not 

 necessary to keep the organism oriented. 



