[Illustration FIG. 36.--Magnified representation of arrangement of the scales on the wing of a butterfly.] [Illustration FIG. 37.--Androconia from wings of male butterflies: _a_, _Neonympha eurytus_; _b_, _Argynnis aphrodite_; _c_, _Pieris oleracea_.] _The Wings._--The wings of butterflies consist of a framework of horny tubes which are in reality double, the inner tube being filled with air, the outer tube with blood, which circulates most freely during the time that the insect is undergoing the process of development after emergence from the chrysalis, as has been already described. After emergence the circulation of the blood in the outer portion of the tubes is largely, if not altogether, suspended. These horny tubes support a broad membrane, which is clothed in most species upon both sides with flattened scales which are attached to the membrane in such a way that they overlap one another like the shingles on a roof. These scales are very beautiful objects when examined under a microscope, and there is considerable diversity in their form as well as in their colors. The males of many species have peculiarly shaped scales arranged in tufts and folds, which are called androconia, and are useful in microscopically determining species (Fig. 37). The portion of the wings which is nearest to the thorax at the point where they are attached to the body is called the _base_; the middle third of the wing is known as the _median_ or _discal area_, the outer third as the _limbal area_. The anterior margin of the wings is called the _costal margin_; the outer edge is known as the _external margin_, the inner edge as the _inner margin_. The shape of the wings varies very much. The tip of the front wing is called the apex, and this may be rounded, acute, falcate (somewhat sickle-shaped), or square. The angle formed by the outer margin of the front wing with the inner margin is commonly known as the _outer angle_. The corresponding angle on the hind wing is known as the _anal angle_, and the point which corresponds to the tip or apex of the front wing is known as the _external angle_ (Fig. 38). A knowledge of these terms is necessary in order to understand the technical descriptions which are given by authors.