PHOTOTAXIS IN RANATRA. 199 



PHOTOTAXIS IN RANATRA. 



S. J. HOLMES. 



The response of Ranatra to light is very inarli:e(l. When placed in a 

 dish of water near a, window or an artificial light, Ranatras persist in 

 attempting to get as near the light as possible for a long time. If one 

 of these insects is picked np and laid on a table it will remain motion- 

 less for several minntes. ]\rost of the experiments on these forms were 

 carried on in a darkened room, the only source of ilhimination being an 

 ordinary incandescent light fastened to the end of a flexible cord so as to 

 be freelv movable in anv desired location. If the light is moved about over 

 a specimen which is feigning death, the duration of the feint is much less 

 than under ordinary conditions. If the light is passed back and forth 

 over the body at right angles to its long axis, the head soon follows the 

 light by rotating so that its upper surface is kept as nearly as possible 

 perpendicular to the direction of the rays.' If the light is passed back 

 and forth over the body in a longitudinal direction, the head follows by 

 bending down when the light is in front and by bending upward when 

 it is behind. By passing the light around the animal in a circle the head 

 follows by twisting around in a circle also. All these movements occur 

 with a regularity and precision which is almost machine like. 



After responding tolhe stimulus of the light only by movements of the 

 head, Ranatra rises rather awkwardly and begins to follow the mo- 

 tions of the lamp by swaying the body. When the light is passed across 

 the long axis of the body the back is turned so as to be more nearly per- 

 pendicular to the rays. These swaying movements are accompanied by 

 bending the legs, the joints being flexed on the side toward the light and 

 extended on the opposite side. When the light is held just behind the 

 body the anterior end of the animal is raised high in the air and the 

 head is upturned. If the light is placed in front the anterior part of the 

 body is lowered and the head turned down. These movements are also 

 very regular and precise. One may cause the animal to assume almost 

 any attitude by holding the light in the proper position. 



Soon the animal begins to follow the light, at first by Avalking slowly 

 and unsteadily, but later by moving more rapidly. After a while its efforts 

 to go towards the light become positively violent ; it shows no fear nor 

 and tendency to action save the one overmastering phototactic impulse. 

 S. J. Holmes^ 



Zoological Laboratory, University of Michigan, 

 Ann Arbor. 



