Holmes, TJie Reactions of Ranatra to Light. 319 



time its movements become more vigorous and it turns to walk 

 away from the light ; it can then be driven about in any direc- 

 tion at will. 



The difference in the swaying movements of positive and 

 negative specimens is mainly brought about by the different 

 movements of the legs at the femoro-tibial joint. In negative 

 specimens the legs on the side toward the light are extended at 

 this joint while those on the opposite side are flexed ; in posi- 

 tive specimens the reverse relation occurs. The rolling of the 

 body is the same in both cases so that the muscles extending 

 between the legs and the body are similarly affected in both 

 kinds of reaction. The same relation probably obtains with 

 the muscles between the joints of the upper parts of the leg. 

 The vertical swaying movements of the body which involve the 

 employment of these muscles are the same in both kinds of 

 response. When light is behind a negative specimen the ante- 

 rior part of the body is held high in the air as the insect walks 

 away. When the light is held in front of the insect the ante- 

 rior end of the body is lowered as it is turned from the light. 



Only a part of the organism is subject to a change in the 

 sense of its phototactic response. The head reflexes and sway- 

 ing movements of the body are always such as to bring their 

 upper surfaces more nearly at right angles to the direction of 

 the rays, whether the organism as a whole is going towards or 

 away from the stimulus. No matter how strong or how weak 

 the light, or whatever may be the condition of the anirrial, these 

 responses, if made at all, always occur in the same way. 



^. The Effect of Contact on Phototaxis. As shown by the 

 following records of experiments, Ranatra may be made nega- 

 tively phototactic by means of contact stimuli. In one experi- 

 ment seven specimens that were swimming against the side of 

 the dish towards the window were picked up by their breathing 

 tubes and dropped back into the water. At first they remained 

 quiet but soon showed a negative reaction, swimming vigor- 

 ously against the side of the dish away from the light. In 

 about twenty minutes all but two had become positive again. 

 They were all picked up by the breathing tube a second time 



