The black lines are elastic, horny tubes, over which the membranes are spread and stretched, like the silk of an umbrella by its ribs. The upper membrane is firmly attached to the tubes (which are called nervures); the lower has but a slight adhesion, and is easily stripped from them. The nervures originate in the body, and diverge like a fan to various points of the tip, and to the upper and lower edges; some of them, however, terminate in the substance of the wing without reaching the edge, and some send off cross branches by which two are connected together. They generally maintain the same thickness throughout, but there are enlargements where the branches join the main trunks. These nervures are hollow, and are, during life, filled with a subtile fluid, which is supplied from the vessels of the body. They contain also ramifications of the exquisite spiral air-vessels. In this wing of the bee all of these structures may be seen to greater advantage. Unlike the fly, which has but a single pair of wings, the bee has two pairs, of which the fore pair is the larger and more horny, the hinder pair seeming to be, as it were, cut out of the hinder and inner side of the fore ones. The two edges--the hinder edge of the fore pair and the front edge of the hind pair--then correspond, but it is necessary that, during flight, when the wings are expanded, the two wings on each side should _maintain_ this relative position, neither overlapping the other, but together presenting one broad surface, wherewith to beat the air. There must be, therefore, some contrivance for locking together the two edges in question, which yet shall be capable of being unlocked at the pleasure of the animal; for the wings during repose slide over one another. This contrivance is furnished by a series of hairs or spines running along the front edge of the hindwing; they are bent up into strong semicircular hooks, arching outward, looking, under a high power, like the hooks on a butcher’s stall. On the other hand, the margin of the forewing is strengthened, and is turned over with a shallow doubling, so as to make a groove into which the hooks catch; and thus, while the forewings are expanded, the hooks of the other pair are firmly locked in their doubled edge, while, as soon as flight ceases, and the wings are relaxed, there is no hindrance to the sliding of the front over the hind pair.