94 
INTRODUCTION TO 
always produce the imago at the same time; a 
difference of many months, even of years, has been 
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 to 
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 
pupce 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 pupae are enclosed in 
a common case, the texture of which is more than 
usually rigid and unyielding ; unless, therefore, a 
special provision had been made for their liberation, 
their condition would have been nearly hopeless. 
This provision consists for the most part of a circular 
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 sufficient 
room for the inmate to escape ; this lid, in some 
instances, consists of two semicircular pieces, which 
open like a pair of folding doors. Reaumur has made 
us acquainted with the singular fact that some kinds 
See volume of Nat. Lib. formerly referred to. 
