386 Evolution and Adaptation 



upward. Such an experiment is best made in the dark, 

 since in the light the aphid also responds to the light. If 

 put on a window it crawls upward never downward. 



Aphids are also sensitive to heat. If they are placed in a 

 darkened tube and put near a stove, they crawl away from 

 the warmer end ; but if they are acted upon by the light at 

 the same time, they will be more strongly attracted by the 

 light than repulsed by the heat. We thus see that there are 

 at least three external agents that determine the movements 

 of this animal, and its ordinary behavior is determined by 

 a combination of these, or by that one that acts so strongly 

 as to overpower the others. 



The swarming of the male and female ants is also largely 

 directed by the influence of light. Loeb observed that when 

 the direct sunlight fell full upon a nest in a wall the sexual 

 forms emerged, and then flew away. Other nests in the 

 ground were affected earlier in the day, because the sun 

 reached them first. These ants, when tested, were found to 

 respond to light in the same way as do the aphids. The 

 wingless forms, or worker ants, do not show this response, 

 and the winged forms soon lose their strong response to light 

 after they have left the nest. Thus we see that the helio- 

 tropism is here connected with a certain stage in the develop- 

 ment of the individual ; and this is useful to the species, as it 

 leads the winged queens and males to leave the nest, and 

 form new colonies. Even the loss of response that takes 

 place later may be looked upon as beneficial to the species, 

 since the queens do not leave the nest after they have once 

 established it. 



It is familiar to every one that many of the night-flying 

 insects are attracted to a lamplight, and since those that fly 

 most rapidly may be actually carried into the flame before 

 they can turn aside, it may seem that such a response is 

 worse than useless to them. The result must be considered, 

 however, in connection with other conditions of their life. 



