262 STATES OF INSECTS. 



the Ichneumones minuti L. ; amongst the Lepidoptera, the 

 subcutaneous tribes ; and the majority of the Diptera, — 

 remain as pupa? only a few days or weeks : while the 

 larger species in all these orders commonly exist in the 

 same state several months — many even upwards of two 

 years. There are, however, numerous exceptions to 

 this rule ; for some large pupae are disclosed in a much 

 shorter time than some others not a twentieth part of 

 their bulk. 



The reasons both of the rule and of the exceptions to 

 it are sufficiently obvious. And first, as to the rule : — 

 If you open a pupa soon after its assumption of that state, 

 you will find its interior filled with a milky fluid, in the 

 midst of which the rudiments of its future limbs and or- 

 gans, themselves almost as fluid, swim. Now the end to 

 be accomplished during the pupa's existence is, the gra- 

 dual evaporation of the watery parts of this fluid, and the 

 development of the organs of the inclosed animal by the 

 absorption and assimilation of the residuum. Reaumur, 

 by inclosing a pupa in a stopped glass tube, collected a 

 quantity of clear and apparently of pure water, equal to 

 eight or ten large drops, which had evaporated from it, 

 and was condensed against the sides of the tube, and it 

 was found to have lost an eighteenth part of its weio-ht a . 

 It is plain, therefore, that this necessary transpiration, 

 other circumstances being alike, must take place sooner 

 in a small than in a large pupa. Next, as to the excep- 

 tions : — 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 



a Reaum. i. 383. 



