REACTIONS TO LIGHT IN VANESSA ANTIOPA 389 



the 'continuous action theory' it should be ten times greater in 

 the former intensity than in the latter. Moreover, this theory 

 is not supported by the results obtained when the illumination 

 is gradually increased after the animals have become oriented in 

 a beam of light. The condition mentioned is fulfilled in the 

 experiments already described in which the insects with one 

 functional eye were tested in a beam from a stationary glower. 

 In figures 4 and 9 it can be seen that after the butterflies had 

 become orient.ed and were moving toward the source of light 

 at a definite angle with the rays they were gradually approaching 

 the glower and consequently the illumination was at the same 

 time being gradually increased. On the basis of the 'continuous 

 action theory/ one would expect the butterflies while in the beam 

 to curve gradually toward the functional eye, increasing the angle 

 of curvature as they approached the glower. On the contrary, 

 they moved while in the beam in fairly straight courses, and the 

 angle of deflection remained practically unchanged as the organ- 

 isms drew nearer to the source of light. 



This work shows conclusively that the angle of deflection does 

 not vary with the intensity of the light, thus indicating most 

 strongly that orientation in Vanessa is not necessarily dependent 

 upon the relative amounts of light energy received by the retinas. 

 It also seems to show that the assumption that the tension of 

 the muscles controlled by the two retinas varies with the amount 

 of light energy received, does not hold. This conclusion is further 

 supported by the fact that Vanessa with only one eye functional 

 can reorient toward either side and so follow a source of light 

 as its position is changed, as is shown in the following section. 



4. Reorientation after changing the direction of the beam of light 



To ascertain whether or not butterflies with but one func- 

 tional eye can reorient they were placed into a horizontal beam of 

 light and after they had assumed a definite axial position and were 

 moving toward the source of light at a definite angle to the direc- 

 tion of the rays, as previously described, the source was moved 

 to a second position at the same distance from the animal, but 



