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 ina 
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. 
