JOSEPH KRAFKA, JR. 451 



DISCUSSION. 



Temperature as a Factor in the Mechanism of Development. 



Direct Effect of Temperature upon Growth, Size, Number of Parts, 



Structure, and Color. 



Ordinarily temperature is not a factor capable of modifying struc- 

 ture to any marked extent. Certainly structural variations are in 

 no way comparable with the variations in rate at which they are 

 brought about. The capacity to develop specific color, size, and form 

 is an heritable characteristic; e.g., the present study involves the 

 white-eyed mutant of Drosophila, of which all individuals are white- 

 eyed regardless of the temperature at which they develop. 



Many organisms, however, exhibit variations in their structural 

 characteristics which may be considered as a direct response to 

 temperature. 



Among the earlier investigators of temperature effects on structure 

 were Merrifield, Weismann, Standfuss, Fischer, and Dorfmeister.^ 

 The chief object of their experiments was the production by environ- 

 mental manipulation of the various racial and polymorphic forms in 

 the Lepidoptera. 



Vernon (1895) found that the size relations between various parts of the 

 echinoderm larvae could be modified in response to different temperatures at which 

 they developed. Standfuss (1895) found a reduction in the size of the imagos, 

 as a result of rearing lepidopterous larvae at high temperatures. This he ascribed 

 to the indirect effect of insufficient nourishment. 



Tower found that by subjecting larvae of Leptinotarsa decemlineaia to various 

 temperatures he could affect the amount of pigmentation in the adult. His re- 

 sults are unique in that an increase from the mean temperature range of the 

 species (22.5°C.) had the same result as a decrease. He obtained an increase in 

 melanism down to 16° and up to 28° followed by a decrease to albinism beyond 

 these temperatures. 



Shelford found a tendency toward melanism with an increase in temperature 

 due to the reduction in size of the unpigmented areas on the elytra of the tiger 

 beetles. 



All these reactions are complex and the materials do not lend them- 

 selves to close quantitative study. It is obvious that no simple tem- 

 perature relations can be worked out for them. 



^ For a complete review, see Bachmetjew. 



