94, INTRODUCTION TO 



always produce the imago at the same time; a I 

 difference of many months, even of years, has heen 

 observed in certain instances, an anomaly which we 

 have hitherto found no means of explaining, although 

 it is not difficult to perceive that it may often tend to 

 the benefit and even the preservation of the species- 

 The manner in which butterflies and moths make 

 their escape from their pupa-case, when about tc 

 become denizens of the air, has been already ex- 

 plained,* and it only remains for us to say a few 

 words respecting the mode in which this operation is 

 accomplished among other tribes. The incomplete 

 pupse have a comparatively easy task to perform, 

 as their limbs are each in a separate envelope, and 

 when one is free it can assist in the extrication of 

 the others. But the coarctate pupse are enclosed in 

 a common case, the texture of which is more than 

 usually rigid and unyielding; unless, therefore, 

 special provision had been made for their liberation 

 their condition would have been nearly hopeless 

 This provision consists for the most part of a circula 

 suture near the anterior end, where the head lies 

 which, so weakens the adhesion of the end to the 

 body of the puparium, that it can be pushed open 

 from within like a kind of lid, and afford sufficien 

 room for the inmate to escape ; this lid, in some 

 instances, consists of two semicircular pieces, whic] 

 open like a pair of folding doors. Reaumur has made 

 us acquainted with the singular fact that some kind 



* See volume of Nat. Lib. formerly referred to. 



