CONTINUOUS STIMULATIONS VERSUS TRANSITIONAL 

 SHOCK IN THE PHOTOTACTIC RESPONSE 



S. J. HOLMES 

 From the Zoological Laboratory of the University of California 



In a paper by Miss K. W. McGraw and the writer (1) a de- 

 scription is given of several experiments which were designed to 

 ascertain whether the phototactic response is due to the con- 

 tinuous stimulating effect of light or to the stimuli hat result 

 from changes in the intensity of light. The ordinary photo- 

 tactic movements of animals afford an opportunity for both 

 these forms of stimulation to come into play. An animal going 

 toward or away from the light naturally deviates more or less 

 from a straight path and hence subjects its photosensitive sur- 

 faces to more or less frequent changes of light intensity. In 

 order to separate the effect of continuous stimulation from that 

 of the shock of transition the attempt was made to eliminate 

 the latter so far as possible and thus to observe the influence 

 of constant stimulation alone. 



In one set of experiments insects with one eye blackened over 

 with asphalt varnish were placed within a small circular enclosure 

 in the center of a cylindrical container whose sides were lined 

 with uniformly colored paper. Light was admitted through 

 the center of the upper side which was also uniformly colored, so 

 that in whatever way the insect turned the amount of light 

 entering the eye would be approximately the same. In general, 

 insects that were positively phototactic performed circus move- 

 ments toward the normal eye when placed in the enclosure, 

 while negatively phototactic insects turned about in the reverse 

 direction. The movements of the insect doubtless produced 

 some changes in intensity of photic stimulation, but they were 

 slight; nevertheless the behavior of the insects within the en- 

 closure was practically the same as when they were outside and 



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PSYCHOBIOLOGY, VOL. I, NO. 2 



