ANIMALS AND THEIR ENVIRONMENT 283 



Among the aphids or plant lice some individuals have wings, other do 

 not, and it has been sho^vn that temperature helps to determine whether 

 wings develop. No general rule can be given for the control of wing 

 production, since different strains respond differently, even within the 

 same species. In one strain maximum wing production is obtained if 

 the parents are reared continuously at low temperatures. Since the 

 aphids generally alternate between two host plants during a season, and 

 since ^\ings are the easiest means of effecting their migrations, it is impor- 

 tant to them that uings develop in at least some individuals at the right 

 time. Another insect that responds developmentally to temperature 

 is the vinegar fly, Drosophila. One of its mutant varieties has vestigial 

 wings (Fig. 204, page 236) which are quite useless for flight. At very 

 high temperatures, however, the wings of this variety are nearly normal. 

 This response happens to be of very little use to the flies for two reasons : 

 first, the temperature which induces full wing development is so high 

 that it is otherwise detrimental and flies seldom meet exactly that 

 temperature, and, second, inability to fly is not this mutant's worst 

 handicap, since it is physiologically weak and never matures so rapidly or 

 in so large numbers as do the normal flies of the species. Color in butter- 

 flies is likemse known to be affected by temperature, and it seems certain 

 that the differences between the northern and southern varieties of a 

 species are sometimes thus determined. 



Genetic and evolutionary effects of temperature are known in a few 

 organisms. Mutations in Drosophila have been produced by heat in 

 experiments by Goldschmidt and others. The amount of separation and 

 recombination by characters in this fly due to breakage and reconstitu- 

 tion of chromosomes by exchange of pieces (page 236) is increased by 

 high temperature. And Seller has found that whether a given sex- 

 determining chromosome in a certain moth goes into the polar body or 

 remains in the egg at the meiotic division depends partly on the tem- 

 perature. The effect is such that more females are produced at high 

 temperatures. 



Light. — The most obviously important influence of light upon the 

 ecology of animals is its effect upon green plants upon which the animals 

 feed. These plants are dependent on photosynthesis for their own 

 nutrition and can maintain themselves only where sufficient light is 

 present. Animals that live in caves must therefore subsist on plants 

 that do not carry on photosynthesis or on other animals whose food 

 chains do not end in green plants. In moderately deep lakes, as is pointed 

 out later, green plants are limited to the surface water, if floating, and 

 to a strip along the shore, if rooted (Fig. 258). Animals dependent 

 on such plants for food must spend part of their time in the regions 

 indicated. 



