HELIOTROPISM OF ANIMALS 31 



the tube. If at the beginning of the experiment the animals 

 were on the room side of the test-tube, they move until they 

 reach this boundary. 



We therefore get the same results by using red glass 

 that we got by using opaque pasteboard in a previous 

 experiment. Taken together with the preceding ones, this 

 experiment proves that pre-eminently the more refrangible 

 rays of mixed daylight are heliotropically effective. Although , 

 as we have just seen, the rays passing through red glass or a 

 red solution are not absolutely ineffective, yet the weak light 

 which is reflected from the walls of the room, and which 

 contains some blue rays, is more effective than the diffused 

 light reflected from the sky after it is filtered through red 

 glass. It is for this reason that the animals on the window 

 side under the red cover migrate to the boundary of the red 

 screen where they are held by the rays of diffuse daylight. 



Experiment 4. If, as before, we place the test-tube with 

 the longitudinal axis perpendicular to the window, and cover 

 it with red glass on the window side and with blue glass on 

 the room side, the animals collect under the blue glass at its 

 boundary with the red glass. 



Experiment 5. If we place the test-tube with its longi- 

 tudinal axis parallel to the window, the animals scatter over 

 the whole length of that part of the tube which is covered by 

 blue glass. 



From all these experiments it follows that it is chiefly the 

 more refrangible rays which determine the orientation of 

 the caterpillars of Porthesia chrysorrhoea toward light. 



The only difference between the heliotropism of these 

 animals and the heliotropism of plants is this, that the less 

 refrangible rays are not so completely ineffective in the 

 case of the caterpillars of Porthesia chrysorrhcea as 

 apparently are in many plants. This point must, howevei 

 be studied more accurately with the aid of a spectrum. 



