﻿NORTH AMERICAN SPHING1DJE. Ill 



mine form or resemblance in lepidoptera, and as there exists in them a typical struc- 

 ture, which enables us to bring together in a group, the individuals whose character- 

 istics accord perfectly with the conception of a family, we need not look further than 

 this, and the agreement in general structure, to determine easily, and naturally, the 

 existence of this relationship between perfect individuals. 



PTEKOLOGY. 



This leads naturally to the consideration of this subject, and the special characters 

 of the wings in the present group. The treatment of it must, however, be brief and 

 concise, else these preliminary remarks will far exceed the limits originally marked 

 out. 



The mode of escape of the imago from the pupa case, and the protections with 

 which it is frequently provided, has been a fruitful source of conjecture and supposi- 

 tion amongst naturalists. It is not necessary to notice these in detail, further 

 than to observe, that not one of them furnish a rational or probable explanation of 

 the process as it occurs in the imago, whose transformation is subterranean. The 

 escape of the perfect insect from the cocoon takes place in two ways ; in one, the 

 pupa case is first thrust forth by the bending and extension of the the abdominal seg- 

 ments, and the case subsequently ruptured along the dorsum by the action of the 

 thoracic muscles of the contained insect ; in the other, the pupa case remains within 

 the cocoon, and the insect makes its way through the substance, at a point predeter- 

 mined by the larva in its construction. In this latter case, the act of insect parturi- 

 tion takes place in the following manner. The imago enveloped in a cocoon, escapes 

 from the pupa case in the usual mode. In obedience to an essential, instinctive 

 prompting, the larva, during its final metamorphosis, places its head in the direction 

 and opposite to the avenue of intended escape, after its work has been completed, 

 whether this be the construction of a subterranean cell or a silken cocoon ; and as 

 soon as the escape from the pupa case is effected, the liberated imago engages itself in 

 this avenue, using the portion of the thorax to which the wings are attached, or the 

 wing-shoulders — not the tegulse — as fixed points, on which the thoracic muscles operate 

 to effect the perfect delivery of the body. The motions of which the thorax is sus- 

 ceptible are of two kinds. The one is lateral expansion, by which dilatation of the 

 avenue is effected, and the resisting structures ruptured ; and the other, is an extru- 

 sive impulse which takes place after this has been effected, and during which, the 

 wing-shoulders are thrown forward to give fixed points of support or operation, from 

 which the prothorax is impelled against the resisting portions. In those species which 

 construct dense silken cocoons, there is an abundant secretion poured forth from the 

 salivary vessels during the delivery, and this has possibly some solvent power over 

 the gum which unites the threads, but none whatever on the threads themselves. 



