Holmes, The Reactiojis of Ranatra to Light. 337 



at right angles to the rays, the Hght falHng on its right side as 

 before (position C). In each of the ten trials made it turned 

 towards the right as in the previous experiment. Then it was 

 placed with its head pointing obliquely away from the light 

 (position D), when it still turned to the right in each of ten suc- 

 cessive trials. Finally, it was put back into its original position, 

 A, with its head pointing directly away from the light. Instead 

 of turning to the left, as it did at first, it turned to the right as 

 in the experiment immediately preceding, and it repeated the 

 performance in the same way ten times in succession. It was 

 then placed in a slightly oblique position so that its left side 

 was exposed to the light. Twice it turned to the right as 

 before, but in the third and several subsequent trials it turned 

 to the left. It would then continue to do so when placed back 

 in its original position, or even with its right side slightly turned 

 towards the light. 



The experiments afford unmistakable evidence that when 

 Ranatra has turned in any direction once it tends, ipso facto, to 

 turn in the same direction a second time. The habit thus 

 formed may even overcome the tendency to turn towards a par- 

 ticular side which is caused by blinding one eye. 



IV. General Considerations on the Phototactic Response. 



The behavior of Ranatra in relation to light is, I believe, not 

 without interest in relation to the general theory of the photo- 

 tactic response. Many features of the phototaxis of this insect 

 seem to afford strong support to the reflex theory. The per- 

 fectly definite and regular movements of the head in response to 

 changes in the position of the light and the fact that these move- 

 ments take place under all conditions in exactly the same way 

 indicate that they are as machine-like as the most devoted parti- 

 san of the reflex theory of tropisms could wish. There is no 

 evidence of choice, properly so called, in the performance of 

 these actions. It is true that during the death feint they are no 

 longer performed, but this affords no ground for regarding them 

 as in any sense voluntary, since, as is well known, undoubted 



