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. [Illustration: FIG. 136.—Cross-section of wing of Pronuba.—After Spuler. ] [Illustration: FIG. 137.—Cross-section of wing of Pieris: _s_, insertions of scales.-After Spuler. ] The structure of a complete vein is described by Spuler. In a cross-section of a noctuid moth (_Triphæna pronuba_, Fig. 136) the chitinous walls are seen to consist of two layers, an outer (U) and inner (_c_), the latter of which takes a stain and lies next to the hypodermis (_hy_). In the cavity of the vein is the trachea (_tr_), which shows more or less distinctly the so-called spiral thread; within the cavity are also Semper’s “rib” (_r_) and blood-corpuscles (_bc_), which proves that the blood circulates in the veins of the completely formed wing, though this does not apply to all Lepidoptera with hard mature wings. We have been able to observe the same structure in sections of the wing of Zygæna. A cross-section of a vein of _Pieris brassicæ_ shows that the large trachea is first formed, and that it extends along the track between the protoplasmic threads connecting the two hypodermal layers.