Holmes, The Reactions of Ranaira to Light. 345 



similar explanation might be applied to the orientation of a 

 Ranatra with only the anterior half, or in fact any other part of 

 the eye exposed. The difficulty with this explanation is that 

 an insect that started with an oblique orientation to the direc- 

 tion of the rays would tend to continue in that position, since 

 a departure from it towards either side would be followed by a 

 compensatory movement. If, however, light produces a con- 

 stant effect upon the muscular tonus of the body, irrespective 

 of affording stimulation by variations in intensity arising from 

 turning towards different sides, we can better explain the fact 

 of orientation. Take the case of a Ranatra with only the lateral 

 surface of one eye exposed. Light entering the eye tends to 

 increase the action of the flexors on the same side, and that of 

 the extensors on the opposite side of the body. Through this 

 action alone the insect, so far as light directs its movements, 

 would continue to go around in a circle indefinitely. But as a 

 matter of fact, as its circus movements bring the eye away from 

 the light, they become checked and are followed by a turn in 

 the opposite direction. What is the stimulus to this turn? Ob- 

 viously there is a diminution of light received by the eye as it 

 turns away, and we may regard this diminution as a stimulus 

 to a movement in the opposite direction. This stimulus may 

 be conceived, then, to overcome the tendency to the perform- 

 ance of circus movements, and thereby bring about an approxi - 

 mate re-orientation of the creature to the direction of the rays. 

 That there is a conflict of impulses as the insect turns away from 

 the light towards the side with the functional eye is evinced by 

 the hesitancy, the swaying this way and that which often occurs 

 when the creature has reached a position such that the amount 

 of light received by the eye is materially diminished. The effect 

 of light upon the tonus of the muscles of which the behavior of 

 Ranatra gives so much evidence, fails to account for the fact of 

 compensation in the movements of the insect. The responses 

 to variations in the intensity of light, on the other hand, do not 

 adequately account for the preservation of efforts to attain a 

 parallel orientation to the direction of the rays. Each of these 

 factors may, however, supplement the other in such a way as to 



