Holmes, TJie Reactions of Ranatra to Light. 341 



whatever changes in the nervous system may be due to the 

 formation of associations or the effect of habit afford a further 

 element in determining the nature of the phototactic response. 



Now, while the light reactions of Ranatra take place to a 

 large degree in a stereotyped and mechanical way, there is much 

 to indicate that the insect seeks stimulation by light much as a 

 bit of food or other object of interest is sought by a higher 

 animal. A dog seeing a piece of meat on the other side of a 

 fence makes a variety of efforts to get the prize. He runs up 

 and down looking for a hole where he can get through or per- 

 haps a low place where he can jump over. We cannot class his 

 actions as direct reflexes in response to outer stimuli. His con- 

 duct is determined by the effort to secure a gratifying experi- 

 ence and various methods are employed to gain that end. A 

 Ranatra seeking the light is much less resourceful than a dog 

 trying to get on the other side of a fence, but, to a certain 

 extent, the behavior of both animals is determined by interest 

 in the object sought. As we have seen, Ranatra is able to go 

 towards the light despite obstacles of various kinds. Even 

 when one eye is totally blackened over and all but a small part 

 of the posterior surface of the other is covered, many Ranatras 

 are able to go towards the light in nearly a straight line. How 

 is this done ? A Ranatra in this condition is in much the same 

 situation a man would be if one eye were blindfolded and he 

 were attempting to reach a light by walking backwards. A 

 man would accomplish this, if he had not previously seen where 

 the light was situated, in a very indirect manner. By moving 

 his head from side to side he could determine the darkest part 

 of his visual field, and by facing in that direction he would 

 finally be brought by walking backwards to the light. The 

 darkest part of the visual field in this case is a rather indefinite 

 thing as anyone may readily assure himself by trying the ex- 

 experiment. A man's course towards the light would proba- 

 bly be very indirect, and it is not a little remarkable that so 

 benighted a creature as Ranatra can reach the goal by so direct 

 a course. 



The two sides of the compound eyes must function differ- 



