456 EFFECT OF TEMPERATURE UPON FACET NUMBER 



Period during Which the Character of a Certain Structure Is Determined 

 as Shown by the Temperature Effects. 



Vernon^ sums up the work of this nature on Lepidoptera. Dorfmeister con- 

 cluded that the temperature had its greatest effect during the change from larva 

 to pupa. According to Weismann, temperature acted at the beginning of the 

 pupal period in Vanessa prosa. Merrifield concluded that the markings are chiefly- 

 affected during the early part of the pupal period, while coloring is affected 

 during the penultimate pupal stage. 



Hoge showed that exposure of the eggs to cold produced the greatest percent- 

 age of reduplication of legs in the imago of the fruit fly. Evidently the materials 

 which determine the structure of the legs are differentiated in early embryonic 

 development. 



The bar eye factor comes into play after about three-fourths of the larval period 

 is finished. 



These last two cases are of interest in showing that some of the reactions 

 which are involved in the differentiation of specific adult structures may occur at 

 very early stages of immature life. Environmental stimuli must therefore be 

 applied at definite periods in order to modify the organization of the adult. 



The Direct Effect of Temperature upon the Mechanism of Inheritance. 



Plough has shown that temperature has a definite effect on the percentage of 

 crossing over between the hereditary materials of the second chromosome in 

 Drosophila. He gets a maximum percentage at 13° and at 31° with a minimum 

 from 22-27°. This curve Plough compares to the curve of Howell for the amount 

 of contraction of the frog muscle at varying temperatures under constant stimu- 

 lus. It is decidedly not a van't Hoff curve. Plough refers the phenomenon 

 further to Lillie's results on the activation of the starfish eggs at various tem- 

 peratures and concludes that temperature "probably causes some alteration in 

 the physical basis of the egg." 



The temperature effect on actual facet number seems to have nothing in com- 

 mon with the above results. If, however, we consider the per cent of increase or 

 decrease per °C. some very interesting relations appear. Here, too, we find 

 that the maximum change comes at the extreme high and low temperatures, with 

 a minimum between, much as in the above reactions. With Plough, I leave the 

 significance of the similarity of the cases to future research. 



Individual Variation as Afected by Temperature. 



In his book, Vernon^ makes the statement that "variability becomes steadily 

 greater as the environment becomes more unfavorable." 



2 Vernon (1903), p. 241. 



3 Vernon (1903), p. 218. 



