mesothoracic pair is called the anterior, upper, or primary wings; the metathoracic pair, the posterior, under, or secondary wings. When a single pair only exist, they are attached to the mesothorax, except in one recorded instance, the Aschiphasma annulipes, which has posterior wings without the least trace of an anterior pair.
In general form, structure, and relative size, they undergo a great variety of modifications in the different orders; and it is from these modifications that the respective orders derive their names. Sometimes all the four are of similar membranous structure, and either all equal in size, (Neuroptera,) or the anterior pair largest, (Hymenoptera, &c.) At other times, the anterior are rigid and horny throughout their whole extent, (Coleoptera,) or only on one-half of their surface, (Hemiptera Heteroptera.) In some instances, (Orthoptera, &c.) the anterior wings are what is called pergameneous—that is, of a substance between coriaceous and membranous, or somewhat resembling parchment.
The harder the substance of which wings are composed, the less active is the share they take in flight. When they consist entirely of a hard horny substance, (as among the Coleoptera,) they can be subservient to this use in a very trifling degree; their primary and important function being to protect the inferior wings and the abdomen. They are, accordingly, distinguished by a name significant of their use, namely, elytra, sheaths or coverings. In proportion as the membranous portion of the wing gains upon the denser part the wing becomes a more efficient
