Chai>ter XIII. 



Transformation of Pupae into perfect Insects. 



The period which piipec require to come to matur- 

 ity 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 immedi- 

 ately have to record several curious facts quite at vari- 

 ance with such a conclusion. ^ It is plain,' say Kirby 

 and Spence, ' that this necessary transpiration, 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 pupa^ exposed to a high tem- 

 perature 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, v/ill at one time live but 



