288 Journal of Comparative Neurology and Psychology. 



Case J2 — The previous experiment was further modified by 

 reversing the Y-tube so that the arms were directed away from the 

 window (Fig. 21, D). Larvae which were giving a negative reaction 

 were employed. They were placed in the end a, and the light 

 was admitted. After the usual body-orientation had taken place, 

 the progression away from the window began. When the larvae 

 reached the point x, and had come under the influence of the w^hite 

 ground bounding one side of the tube they would swing their heads 

 toward the right and continue their progress until all were gathered 

 in arm c. This was somewhat unexpected. It eventually trans- 

 pired, however, that the white ground bordering the outer surface 

 of the tube did not act as a reflector or intensifier of the light rays, 

 but as an opaque shield, cutting off" the rays which would other- 

 wise have entered the arm c. Thus, as in Case 30, the negatively 

 reacting larvae had merely grouped themselves in the arm where the 

 light was least bright. When the Y-tube was so placed that half 

 of arm c rested upon a sheet of white paper the result was diff^er- 

 ent. The larvae congregated in arm />, which was, under these con- 

 ditions, the region of least light intensity. 



Case 55 — The four cases mentioned above were supplemented 

 by other experiments involving the use of colored glass plates. 

 As described in Experiment 15, these plates were so placed over the 

 arms of the Y-tube that a difference in the intensity of light striking 

 one arm was caused by interposing a red, orange or yellow glass plate 

 between that arm and the source of illumination. In these cases 

 the positively reacting larvae gathered in the arm where the light- 

 intensity was the greater, while the negatively reacting larvae 

 grouped themselves in the arm where the light was least bright. 

 As a rule, the larvae of earlier stages seemed to be more susceptible 

 than the others to slight diff'erences in the intensity of light at the 

 entrance to the arms. 



Thus is explained the tendency for positively reacting larvae 

 to gather in regions of greater light-intensity, and on the other 

 hand, the tendency of negatively reacting larvae to congregate in 

 regions of lesser light-intensity. This condition of affairs has, 

 no doubt, given many investigators reason to believe that such 

 reactions are but manifestations of a positive photopathy; and that 

 photopathy and phototaxis are fundamentally the same. We now 

 know, however, that the reaction just described in Case 5 is due 

 to the combined efi^ects of two tendencies; the one to turn the head 



