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The wings morphologically, as their development shows, are simple sac-like outgrowths of the integument, i.e. of the free hinder edge of the tergal plates, their place of origin being apparently above the upper edge of the epimera or pleural sclerites. Calvert^[24] however, regards the upper lamina of the wing as tergal, and the lower, pleural.

The wings in most insects are attached to the thorax by a membrane containing several little plates of chitin called by Audouin articulatory epidemes.

The wings, then, are simple, very thin chitinous lamellate expansions of the integument, which are supported and strengthened by an internal framework of hollow chitinous tubes.

The veins.

These veins are in reality quite complex, consisting of a minute central trachea enclosed within a larger tube which at the instant the insect emerges from the nymph, or pupa, as the case may be, is filled with blood (Fig. 136). Since these tubes at first contain blood, which has been observed to circulate through them, and since the heart can be most easily injected through them, they may more properly be called veins than nervures. The shape and venation of the wings afford excellent ordinal as well as family and generic characters, while they also enable the systematist to exactly locate the spots and other markings of the wings. The spaces enclosed by the veins and their cross-branches are called cells, and their shape often affords valuable generic and specific characters.