3i6 Journal of Comparative Neurology and Psychology. 



of orientation it reaches over to one side with the anterior leg 

 nearest the light and pulls the body over until it is parallel 

 with the rays. 



2. The Negative Reaction. While under ordinary circum- 

 stances Ranatra is positively phototactic it may in certain con- 

 ditions become strongly negative. If Ranatras which have 

 been kept for several hours in the dark are brought back again 

 into the light they usually show a marked negative reaction, 

 but this negative phototaxis is never so violent as the positive 

 sometimes becomes. Instead of appearing to be the result of 

 reflexes which are comparatively direct and involuntary, the 

 behavior of Ranatra gives the impression of being caused by the 

 desire to escape from a situation which is unpleasant. It is 

 perhaps remotely analogous to the action of a man when, after 

 having been for some time in the dark, he instinctively turns 

 away from the sudden glare of a strong light. The behavior 

 of specimens after having been kept in the dark may be illus- 

 trated by the following experiments : 



Twenty-five Ranatras which had been in a dark room for twelve hours were 

 placed, one at a time, in a glass trough through which light was passed from an 

 incandescent lamp situated a foot from one end. Each specimen was placed in 

 the water in the center of the trough at rivht angles to the direction of the rays 

 and let go in that position. If the specimen swam to the negative end, and 

 within one minute returned to the other end when the light was changed it was 

 classed as negative; and a similar criterion was employed for the positive reac- 

 tion. If a specimen swam to one end of the trough and did not return to the 

 other end within one minute after the light was changed it was classed as doubt- 

 fully positive or negative according to which end it first reached. Seventeen of 

 the specimens proved to be negative; one was positive; four were doubtfully 

 negative and two doubtfully positive. Out of the twenty-five specimens employed 

 only one was unmistakably positive in its reaction, and this one swam to the 

 positive end of the trough upon change of the light several times in succession. 

 One spectmen did not swim to either end for five minutes. The Ranatras were 

 then left exposed to the light of an incandescent lamp placed three inches from 

 the end of the trough for one hour and forty minutes. At the end of this time 

 all of the specimens were positive, The light was then held eight feet from the 

 trough and the positive reaction still continued. 



One of the specimens which showed a very marked negative phototaxis was 

 singled out. During the time the others were being experimented with it was 

 exposed to the light, and when again studied (after about a half hour's exposure) it 

 showed a very evident negative phototaxis, but not so strong as before. After a 

 time the negative reaction became so faint that it was scarcely distinguishable. 



