242 THE EFFECT OF TEMPERATURE 



The reason of this is probably that a temperature of 

 0.5 is so low that it paralyses all tissue changes in the 

 pupae, and at the end of ten weeks the stage of develop- 

 ment is no further advanced than at the beginning. 

 Thus the time of emergence of these pupse, after trans- 

 ference to a temperature of 32, was just as long as for 

 those kept only at this temperature. 



Arguing from his experiments on two moths, Selenia 

 illustraria and Ennomos autumnaria, Merrifield * 

 came to the conclusion that, in their case at least, the 

 markings were chiefly affected by the temperature ex- 

 perienced during the earlier part of the pupal period, 

 whilst the colouring was " chiefly affected during the 

 penultimate pupal stage, i. e., before the colouring of 

 the imago begins to show." A low temperature dur- 

 ing this latter period causes darkness, and a high tem- 

 perature the opposite effect. Thus, by difference of 

 treatment, it was found possible to obtain from the same 

 brood individuals showing (1) summer markings with 

 summer colouring; (2) summer markings with an ap- 

 proach toward spring colouring; (3) spring markings 

 with summer colouring, and (4) spring markings with 

 almost spring colouring. 



We see, then, that in some cases seasonal dimorphism 

 is a direct response to temperature, or is a somatic 

 modification, whilst in other and perhaps the majority 

 of cases it is only indirect, the temperature acting as a 

 stimulus to arouse a blastogenic variation. When the 

 response is direct, low temperature generally induces a 

 darkening of colour, as, for instance, in V. urticcz, Las- 

 ciocampa quercus (and calluncu), Arctia caja and E. 



* Trans. Ent. Soc. 1891, p. 55. 



