454 PHOTOREACTIONS OF "WTIIP-TAIL SCORPIONS 



path followed by the animals is greater than the radius of the observa- 

 tion circle of the apparatus. When the induced asymmetry is ex- 

 treme, typical circus movements of small radius result. The point of 

 emergence of the trails of larger radius on the standard observation 

 circle, gives an index of the curvature of the arc of locomotion in 

 quantitative terms which are comparable with the other reaction meas- 

 urements. The constant curving of the path of locomotion in ani- 

 mals which are asymmetrically sensitive stands in sharp contrast to 

 the locomotion of normal animals under bilaterally unequal illumina- 

 tion. In the latter case the path of locomotion is straight once the 

 direction of crawling becomes such that the inequahty in illumination 

 in the field is equalized in its effectiveness on the photosensitive areas 

 of the animal by greater exposure of the sensitive areas on the side of 

 less intense illumination, and lesser exposure of the sensitive areas on 

 the side of more intense illumination (5). Asymmetrically sensitive 

 animals under vertical illumination continue in a curving path 

 of locomotion because it is impossible through changes in axial posi- 

 tion to equahze bilaterally the effective illumination. Under equal 

 and opposed horizontal illumination slight bilateral inequalities in 

 sensitiveness may be compensated for by the assumption of an axial 

 position inclined toward the less intense light. When the asymmetry 

 of sensitiveness is made extreme the effective illumination cannot thus 

 be brought into bilateral equilibrium and the curved path of locomo- 

 tion is continued. 



A feature deserving further comment is the way in which the am- 

 plitude of the deflections increases as the degree of asymmetry in 

 sensitiveness is increased. The cumulative effect produced by black- 

 ening m.ore than one receptor on the same side of the head is obvious 

 from the "figures. Less apparent but equally significant is the fact 

 that the unbalance in reaction is greater when all the receptors except 

 a given one are eliminated, than when the same receptor is the only 

 one covered. For example, the deflection induced when the median 

 eye on one side is capped averages 15.6°; when both lateral eyes, the 

 cutaneous sensitive areas on both sides, and the median eye on one 

 side are blackened (leaving functional only one median eye), the de- 

 flection averages 27.1° (see Table III). The unbalanced factor in each 

 experiment is a median eye, but when the median eye alone is ehm- 



