274 BRADLEY M. PATTEN 



muscle groups; and (3) that light may operate as a sthnulus 

 effective in bringing about kinetic and tropic responses, both 

 through changes of intensity and constant intensity. 



I wish to express my indebtedness to my father, Dr. Wm. 

 Patten, for procuring the specimens of Mastigoproctus on which 

 this work was done, and to Prof. W. M. Wheeler who identified 

 the species for me. The greater part of the work was done at 

 Woods Hole. It is a pleasure to express my appreciation of the 

 courtesies and facilities extended to me by the Marine Biological 

 Laboratory. 



SUMMARY 



The photic reactions of whip-tail scorpions were studied with 

 a view to establishing quantitatively, certain characteristic 

 responses. No attempt was made to treat exhaustively all 

 phases of their behavior under the influence of light. The 

 object was rather to obtain such reaction measurements as 

 A^ould best serve as a basis of comparison for subsequent work 

 directed toward determining the relative effectiveness of the 

 various parts of their complex photoreceptive mechanism. 



Reactions to photic stimuli of known intensities were re- 

 corded in terms of the induced angular deflections from an 

 initial direction of locomotion. The results obtained may be 

 summarized as follows: 



1. The threshold for the kinetic effect of light was at about 

 0.16 candle meters. 



2. The response was clearly negative to all directive illumi- 

 nation which induced locomotion. Up to an intensity of 1 candle 

 meter the amplitude of the reactions increased rapidly. In the 

 intensities above 1 candle meter the increase in deflection was 

 much more gradual. 



3. When started heading away from the source, in a horizontal 

 beam of light of 120 candle meters, animals continued to move 

 along the path of the rays. In 40 trials the average was within 

 0.6 of a degree of the central ray of the beam. 



