344 Journal of Comparative Neurology and Psychology. 



during their progress they change their direction of locomotion 

 so as to continue going towards it. If the light is not carried 

 too far to one side the insect may be made to follow it around 

 in either direction. In the specimens employed care was taken 

 that every other part of the eyes was thickly covered except a 

 small portion of the surface of one side, so there is no doubt 

 that the movements of the insect were directed only by light 

 entering at this point. The side of the eye in this case doubt- 

 less functions as it does in a normal individual, but it is difficult 

 to explain satisfactorily the orientation of the specimen either 

 through direct reflexes, or by the method of trial and error. 

 Were the insect so constituted as to respond to an increase of 

 light entering the left eye by a turn to the left and to a decrease 

 of light by a turn to the right, we can understand how, when 

 once pointed towards the light, a straight course might be pre- 

 served. If the insect turned towards the right there would be 

 an increase of light entering the left eye which we might sup- 

 pose stimulates the insect to turn in the opposite direcrion. 

 Deviations to the left would cause a diminution of light entering 

 the left eye, which we might suppose acts as a stimulus to turn 

 to the right side. The right eye may be supposed to act, vm- 

 tatis mutandis, in a similar manner. The numerous cases of re- 

 action to shadows {Scattenempfindlichkeit) which are found 

 among several groups of animals show that diminution in the 

 intensity of light may act as a stimulus as well as an increase in 

 intensity. If it be the variations in the intensity of light which 

 afford the stimuli for turning in the one or the other direction, 

 we can attribute to all parts of the eye essentially the same 

 function. If the posterior side of the left eye is all that is ex- 

 posed and a Ranatra that is facing the light turns to the right 

 side the exposed part of the eye receives an increased stimulus 

 which we may suppose brings about a turn to the left. A turn 

 to the left of the median position, up to a certain point, would 

 probably (owing to the body intercepting a part of the rays) 

 diminish the light entering the posterior side of the eye, thus 

 causing a turn to the right. In this way the creature might be 

 supposed to maintain a straight course towards the light. A 



