The wing area may be divided into the strong costal margin, the hind margin, the nervules, and the spaces; and, however complex the pattern may be, it is always based upon these structure lines. In the majority of insects the costal margin is marked with strong colour. This may be noticed in _Papilio Machaon_, _P. merope_, _Vanessa antiopa_, and the whites in Plate IV. The extreme tip of the fore-wings is nearly always marked with colour, though this may run into the border pattern. This colour is dark or vividly bright, and we know no butterfly, not even dark ones, that has a light tip to the wings. Sometimes, it is true, the light bead-border spots run to the tip, but these are not cases in point. The development of tips has been traced in Chapter VI., and need not be repeated. The hind margin of both wings is very commonly emphasized by a border, of which _V. Antiopa_, Pl. III. Fig. 3, is a very perfect example. The border pattern may consist of one or more rows of spots, lines, bands, or scallops;[31] and there is frequently a fine fringe, which in many cases is white, with black marks, and to which the term bead-pattern may be applied. A definite relation subsists in most cases between the shape of the hind margin and the character of the border-pattern. The plain or simple bordered wings have plain border patterns, and the scalloped wings have scalloped borders; or rather scalloped borders are almost exclusively confined to scalloped wings. In our English butterflies, for instance, out of the 62 species:-- 33 have plain margins to the wings. In all the border is plain, or wanting. 20 have the fore-wings plain, and the hind-wings scalloped, and in all the hind-wings are scalloped and the fore-wings plain, or with slightly scalloped border-patterns. 9 have scalloped margins and scalloped border-patterns. Another relation between structure and pattern is found in those insects which have tailed hind-wings, for the tail is very frequently emphasized by a spot, often of a different colour from the rest of the wing as in the Swallow-Tails, Plates IV. and V. Yet another point may be noticed. In each wing there is a space, the discoidal cell, _j_ Fig. 3, at the apex of which several nervures join, forming knots. These are points at which obstacles exist to the flow of the contents, and they are almost always marked by a distinct pattern. We thus have a discoidal spot in very many butterflies, in nearly all moths; and in the other orders of winged insects the decoration is even more pronounced, as any one may see who looks at our dragon-flies, wasps, bees, or even beetles.