HELIOTKOPISM OF ANIMALS 47 



nently on the side of the test-tube which is turned toward 

 the source of light. 



If direct sunlight comes through the window, and the 

 tube containing the animals is so placed that one-half lies in 

 the direct sunlight, while the other half is in diffuse day- 

 light, and if the latter half is nearer the plane of the win- 

 dow than the former, the animals will move to the window 

 side of the vessel, like the caterpillars of Porthesia chry- 

 sorrhoea; they leave the direct sunlight and move into dif- 

 fuse daylight in order to follow as nearly as possible the 

 direction of the rays. The result is the same when the dif- 

 fuse daylight first passes through dark-blue glass. The 

 animals are compelled to go to the window side of the tube 

 under all conditions, no matter whether the test-tube is 

 covered entirely or only in part by the blue glass, or 

 whether the blue glass is placed over the window or the 

 room side of the tube. The less refrangible rays which 

 pass through deep-red glass are not very effective. In con- 

 sequence, if the test-tube is entirely covered with red glass, 

 the animals, if not very sensitive, distribute themselves 

 evenly over the whole test-tube, just as in the dark; or, if 

 more sensitive, they collect after a long time on the window 

 side of the test-tube. But even then they do not quite 

 behave as under blue glass. While under blue glass they 

 collect in a very small area on the window side of the tube, 

 under red glass they occupy a much larger area. If only a 

 part of the test-tube is covered with red glass, the animals 

 collect at the window side of the uncovered portion of the 

 test-tube, as the less refrangible rays have only a minimal 

 effect. When I placed a test-tube containing highly sen- 

 sitive plant lice on a horizontal table perpendicular to the 

 plane of the window, and covered the window side of the 

 test-tube with a bright-red glass, the animals collected at the 

 boundary between the uncovered and the covered part of the 



