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 explained,[1] 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 pupæ 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 pupæ 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.
