REACTIONS OF WHIP-TAIL SCORPION TO LIGHT 273 



of locomotion exhibited under stich conditions on the basis of 

 the single sudden change of intensity produced on both sides 

 of the head, in starting the experiment. 



When we attempt to account for the persistence of locomo- 

 tion on the assumption that light of constant intensity does not 

 operate as a stimulus, we are confronted by other difficulties. 

 It has already been demonstrated that light of constant inten- 

 sity acting vertically stimulates to kinetic activity. It would 

 be weird physiology to assume that when light of the same 

 intensity acts horizontally, inducing a directive reaction, con- 

 stant intensity no longer stimulates. Moreover it ha^ been 

 shown (1) that locomotion ceases in the absence of stimulation, 



(2) that in the reactions to equal opposed lights, once orientation 

 is attained, it is so accurately maintained that no considerable 

 changes of intensity on the photoreceptors are encountered, and 



(3) that locomotion under such conditions is indefinitely per- 

 sistent. In view of these facts it is clear that the assumption 

 that changes of intensity are the only effective photic stimuli 

 is not in accord with the facts established concerning the reac- 

 tions of Mastigoproctus. The conception that light acts as a 

 stimulus both thi'ough changes of intensity and constant inten- 

 sity is in accord with what is known concerning the general 

 physiology of stimulation by light, and with the specific reactions 

 here described. 



The conclusions we would seem to be justified in drawing 

 with regard to the orientation of Mastigoproctus may be briefly 

 summarized as follows. The bilaterally symmetrical distribution 

 of the stimulus on the photoreceptors which in all cases exists 

 when the animals are in orientation cannot be regarded as inci- 

 dental to an alignment of the body dependent on other factors. 

 The behavior of the animals under all the conditions of illumi- 

 nation worked with, is in accord with the hypotheses (1) that 

 the animals so move that stimulation is brought into bilateral 

 equilibrium and there maintained; (2) that the muscular re- 

 sponses which bring about the bilateral balance of stuniulation 

 result from a proportional transmission (over paths at present 

 unknown) of the impulses initiated in the receptors to definite 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 23, NO. 2 



