The veins in both the fore and hind wings of butterflies may be divided into simple and compound veins. In the fore wing the simple veins are the costal, the radial, and the submedian; in the hind wing, the costal, the subcostal, the upper and lower radial, the submedian, and the internal are simple. The costal vein in the hind wing is, however, generally provided near the base with a short ascending branch which is known as the precostal vein. In addition to these simple veins there are in the fore wing two branching veins, one immediately following the costal, known as the subcostal, and the other preceding the submedian, known as the median vein. The branches of these compound veins are known as nervules. The median vein always has three nervules. The nervules of the subcostal veins branch upwardly and outwardly toward the costal margin and the apex of the fore wing. There are always from four to five subcostal nervules. In the hind wing the subcostal is simple. The median vein in the hind wing has three nervules as in the fore wing. Between the subcostal and the median veins, toward the base in both wings, is inclosed the cell, which may be wholly or partially open at its outer extremity, or closed. The veinlets which close the cell at its outward extremity are known as the discocellular veins, of which there are normally three. From the point of union of these discocellular veins go forth the radial veins known respectively as the upper and lower radials, though the upper radial in many genera is emitted from the lower margin of the subcostal. An understanding of these terms is, however, more readily derived from a study of the figure in which the names of these parts are indicated (Fig. 40). [Illustration FIG. 40.--Wing of _Anosia plexippus_, showing the names of the veins and nervules: _C_, _C_, costal veins; _SC_, subcostal vein; _SC_1_, etc., subcostal nervules; _UR_, upper radial; _LR_, lower radial; _M_, median veins; _M_1_, _M_2_, _M_3_, median nervules; _SM_, submedian veins; _I_, internal veins; _PC_, precostal nervule; _UDC_, _MDC_, _LDC_, upper, middle, and lower discocellulars.] Butterflies generally hold their wings erect when they are at rest, with their two upper surfaces in proximity, the under surfaces alone displaying their colors to the eye. Only in a few genera of the larger butterflies, and these tropical species, with which this book does not deal, is there an exception to this rule, save in the case of the _Hesperiidæ_, or "skippers," in which very frequently, while the anterior wings are folded together, the posterior wings lie in a horizontal position. _Internal Organs._--Thus far we have considered only the external organs of the butterfly. The internal organs have been made the subject of close study and research by many writers, and a volume might be prepared upon this subject. It will, however, suffice for us to call the attention of the student to the principal facts.