In order fully to understand the bearing of the following remarks it is necessary to know something of the anatomy and nomenclature of butterflies. Fig. 3 is an ideal butterfly. The wing margins are described as the _Costal_, which is the upper strong edge of the wing, the _Hind_ margin, forming the outside, and the _Inner_ margin, forming the base. The nervures consist of four principal veins; the _Costal_, a simple nervure under the costa, the _Sub-costal_, which runs parallel to the costal and about halfway to the tip emits branches, generally four in number; the _Median_ occupying the centre of the wing and sending off branches, usually three in number, and the _Sub-median_ below which is always simple. There are thus two simple nervures, one near the costal the other near the inner margin, and between them are two others which emit branches. Between these two latter is a wide plain space known as the _discoidal cell_. Small veins called the _discoidal_ pass from the hind margin towards the cell, and little transverse nervures, known as sub-discoidal, often close the cell. By these nervures the wing is mapped out into a series of spaces of which one, the discoidal cell, is the most important. The nervures have two functions, they support and strengthen the wing, and being hollow serve to convey nutritive fluid and afterwards air to the wing. The wings are moved by powerful muscles attached to the base of the wings close to the body and to the inside of the thorax, all the muscles being necessarily internal. "There are two sets which depress the wings; firstly a double dorsal muscle, running longitudinally upwards in the meso-thorax;[28] and, secondly, the dorso-ventral muscles of the meso- and meta-thorax,[29] which are attached to the articulations of the wings above, and to the inside of the thorax beneath. Between these lie the muscles which raise the wings and which run from the inner side of the back of the thorax to the legs."[30] When we consider the immense extent of wing as compared with the rest of the body, the small area of attachment, and the great leverage that has to be worked in moving the wings, it is clear that the area of articulation of the wing to the body is one in which the most violent movement takes place. It is here that the waste and repair of tissue must go on with greatest vigour, and we should, on our theory, expect it to be the seat of strong emphasis. Accordingly we commonly find it adorned with hairs, and in a vast number of cases the general hue is darker than that of the rest of the wing, and so far as we have been able to observe, never lighter than the body of the wing. Even in the so-called whites (Pieris) this part of the wing is dusky, and instances are numerous on Plate IV.