Holmes, The Reactions of Ranatra to LigJit. 343 



tience when a Ranatra that shows a strong positive reaction has 

 deviated considerably from a straight path towards the Hght. 

 When the normal eye looks away from the light the specimen 

 may sway from side to side, back off, then go ahead again, 

 growing continually more excited until finally it takes to flight. 

 If the wings are fastened down to prevent their being used, the 

 insect frequently spends several minutes before getting out of 

 its dilemma. Sooner or later a fortunate movement is made 

 which brings the creature into a situation such that the tendency 

 to turn to a particular side meets with little opposition. Out of 

 many trials made in this state of perplexity a successful one is 

 finally made and followed up. Individuals vary greatly in their 

 conduct in such a situation. Many correct their course before 

 getting far out of orientation ; others, when they get out of line, 

 turn back again with comparative readiness. 



We might explain the function of the posterior side of the 

 eye in orientation on the supposition that a movement which 

 brightens the visual field brings about a reflex that causes a turn 

 in the opposite direction. In this way deviations from the po- 

 sition that kept the darkest part of the field of vision in focus 

 would be checked and the insect would, therefore, travel towards 

 the light. It is more difficult, however, to explain the orienta- 

 tion of an insect in which a small area of the lateral surface of 

 one eye is the only part exposed. If the insect moved so that 

 the eye would look toward the darkest part of the visual field it 

 would place its body with the blind side towards the light. If, 

 on the other hand, it moved so that the eye would receive the 

 maximum amount of stimulus, the opposite side would be 

 brought toward the light. It is obvious that the insect moves 

 so that the eye receives neither the maximum nor the minimum 

 amount of stimulus, but is kept exposed to light of an interme- 

 diate degree of intensity. The degree of intensity varies, more- 

 over, in every step of its course, so that there is no justification 

 for explaining the orientation of the insect through the effort to 

 keep a certain intensity of light constantly before the eye. It is 

 remarkable that Ranatras so treated frequently go towards the 

 light in nearly a straight line. Moreover, if the light is moved 



