Tropisms and Instincts as Adaptations 385 



ordinary diffuse light, as well as in lamplight. It is stated 

 that the animals orientate themselves towards the light more 

 quickly when it is strong than when it is weak. They turn 

 their bodies toward the light, and then move forward in the 

 direction from which the rays come. It can be shown by a 

 simple experiment that the aphids are turned by the direction 

 of the light, and not by its intensity. If they are placed in a 

 tube, and the tube laid obliquely before a window in such a 

 way that the direct sunlight falls only on the inner end of the 

 tube, the aphids will, if started at the inner end of the tube, 

 first crawl toward the outer surface of the tube, and then wan- 

 der along this wall, passing out of the region of sunlight into 

 the end of the tube nearest the window, where they come to 

 rest at the end. They have moved constantly towards the 

 direction from which the rays come, passing, as it were, from 

 ray to ray, but each time toward a ray nearer the source of 

 the light. 



If the tube be turned toward the window, and the window 

 end be covered with blue glass, the aphids crawl into this 

 end of the tube, as they would have done had the tube been 

 uncovered. If, on the other hand, the end of the tube be 

 covered with red glass, they do not crawl into the part of the 

 tube that is covered, unless they are very sensitive to light. 

 Even in the latter case they may remain scattered in the red 

 part, and do not all accumulate at the end, as they do when 

 blue glass is used. In other words, while they respond to blue 

 as they do to ordinary light, they behave toward red as they do 

 towards a very faint light. 



In diffuse daylight the aphids, as has been said, crawl 

 toward the light, but if they come suddenly into the sun- 

 light they begin to fly. Thus they remain on the food plant 

 until the sun strikes it, and then they fly away. 



The aphid also shows another response ; it is negatively 

 geotropic, i.e. it tends to crawl upward against gravity. If 

 placed on an inclined, or on a vertical, surface, it will crawl 



