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Fig. 136.—Cross-section of wing of Pronuba.—After Spuler.

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.