The ribs of a butterfly's wings are by some authors described as veins, whilst others style the main ones nervures, and the branches nervules. Fig. 9, B, represents the venation, or neuration of the Black-veined White, and the numeral system of indicating the veins has been adopted, as it is the most simple. In another method of referring to the venation, and one that has been much in use, vein 12 of the fore wing would be styled the costal nervure, or vein; veins 11, 10, 9 (absent in figure), 8, and 7 would be the subcostal nervules 1, 2, 3, 4, and 5; 6 would become the upper radial, and 5 the lower radial; 2, 3, and 4 would be the median nervules 1, 2, and 3; vein 1 would be the submedian nervure, or vein. On the hind wing, vein 1_a_ would be the internal vein; 1 the submedian; 2, 3, and 4 the median nervules; 5 the lower and 6 the upper radials; 7 the subcostal, and 8 the costal nervures. Just near the base of the hind wing will be noted a short recurved vein (p.c.); this is the precostal vein, and so named because it comes before the costal. It is always absent in some species. Comparing the venation of A and B, it will be seen that in A the fore wing has 12 veins and the hind wing 8 veins, whilst in B there are only 11 veins on the fore wing, but the hind wing has one vein more than that of A. In the Black-veined White, vein 9 is absent on the fore wing, and on the hind wing there is one internal vein. [Illustration: FIG. 10. =Arrangement of Scales.= (_After Holland._)] Dust-like as they appear to the naked eye, the scales from a butterfly's wing seen under the microscope are found to be exceedingly interesting structures and very varied in shape. Dr. Sharp describes them as "delicate chitinous bags." Chitin, it may be mentioned, is the horny substance of which the chrysalis shell is formed, and this was adverted to when discussing the chrysalis stage as a varnish-like ooze. As seen on the wings, the scales are flattened and the upper and under sides are then almost, or quite, brought together. They are attached in lines on the membrane or covering of the wing by short stalks which fit into sockets in the membrane. The arrangement of the scales, which has often been stated to resemble that of the slates on a roof, is shown in Fig. 10. Colour is chiefly due to pigment contained in the scale or adhering to the interior of its upper side. Pigments, according to Mayer, are derived, by various chemical processes, from the blood while the butterfly is still in the chrysalis. Some scales have minute parallel lines (_striæ_) on their upper sides, and rays of light falling on these are turned aside or broken up, and so produce changes in the colouring of a wing, according to the angle from which it is looked at.