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Chapter XIII. 



Transformation of Pupae into Perfect Insects. 



The period which pupae require to come to maturity 

 seems to depend mainly on temperature and size, 

 though there are several other causes at work appa- 

 rently inscrutable to human research. Those who 

 adopt, after Swammerdam, the untenable theory of 

 evaporation being all that is requisite to bring an 

 insect to maturity, tell us that these two circumstances 

 will account for all the phenomena ; but we shall 

 immediately have to record several curious facts quite 

 at variance with such a conclusion. " It is plain," 

 say Kirby and Spence, " that this necessary transpi- 

 ration, other circumstances being alike, must take 

 place sooner in a small than in a large pupa. Since 

 the more speedy or more tardy evaporation of fluids 

 depends upon their exposure to a greater or less 

 degree of heat, we might, a priori, conclude that 

 pupae exposed to a high temperature would sooner 

 attain maturity, even though larger in bulk, than 

 others exposed to a low one : and this is the fact. 

 The pupa of a large moth, which has assumed that 

 state in the early part of summer, will often disclose 

 the perfect insect in twelve or fourteen days ; while 

 that of an ichneumon, not one-hundredth part of its 

 size, that did not enter this state till late in autumn, 

 will not appear as a fly for seven or eight months. 

 But this is not the whole. The very same insect, 

 according as it has become a pupa at an earlier or a 

 later period of the year, will at one time live but 



