SENSATION AND REACTION 93 



stimulation from their surroundings, and nobody has 

 seriously suggested that the growth of a plant shoot towards 

 the light is accompanied by sensation in the organism. 

 According to Loeb's interpretation of phototropic reaction, 

 the incidence of light on one side of the sensitive head region 

 of the insect leads to a corresponding one-sided action of the 

 body-muscles so that the creature is necessarily turned in 

 the direction of the rays of light. Then the '' luminous 

 intensity being the same on both sides, there is no reason 

 for the animal to turn from this direction either to the right 

 or to the left." It must now directly approach or recede 

 from the source of light. It is noteworthy that the fly- 

 maggots taken as examples of negative phototropism have 

 no eyes, and there is no reason for attributing any definite 

 visual sensation to them. But insects Hke moths that fly 

 into a lamp have eyes, and even though their reaction to the 

 light be automatic and inevitable, it does not necessarily 

 follow that they have no appreciation of the light towards 

 which they are irresistibly drawn. The apparently suicidal 

 action of a moth flying at last into a flame may be due to 

 the " pull " of the light in rapid alternation on either side 

 as at close quarters the insect turns to and fro. 



Other responses to stimulation mentioned in this chapter 

 may be recognised as referable to the group of tropic 

 actions. The male moth " allured " by the caged female, 

 or the female fly attracted by carrion to her egg-laying, is 

 positively chemotropic, while the black aphids which climb 

 as high as possible on the bean-shoots where they feed are 

 negatively geotropic ; they turn from the earth with its 

 gravitational attraction. The consideration of these and 

 similar actions leads naturally to the wide subject of 

 behaviour. 



