It has been said that the wings of the bee have a perfectly symmetrical action, and that they have a single fixed direction, moving up and down, always at right-angles with the line of the thorax. Under the microscope each of the four wings is seen as a transparent, impervious membrane, intersected with fine ribs. The front wing, however, has a much stronger and stiffer rib running the entire length of its upper edge, and it is on this main rib that almost the entire force of the flight-muscles is concentrated. If you look farther, you will see that the under wing has a row of fine hooks along its top edge, while the lower edge of the upper wing is flanged or folded back. In flight the hooks on one wing engage with the flange on the other, and thus the wings on each side are automatically locked together, forming one continuous air-resisting surface. This combined wing is very flexible throughout, except at its upper edge, where it is stiffened by the main rib. In action, therefore,—the force being applied practically to the edge alone, which resists the air while the rest of the wing bends to it—the result is that the whole wing becomes an oscillating, inclined plane, whose inclination, forward on the down-stroke, is still forward on the up-stroke, because the plane-inclination reverses itself automatically. From this it will be understood how the flexible wings of the bee are used in straightforward flight; but, seeing that the wings themselves are incapable of independent or irregular action, it is not yet clear how the bee contrives to steer herself, rising or descending, or turning sideways, just as the mood seizes her. It is here that the reason for the peculiar construction of her body becomes plain. The fine link which unites her abdomen to her thorax is really an universal joint, actuated by a series of powerful cross-muscles, and the bee steers herself through the air by using the weight of the lower half of her body as a counterpoise. By swinging her heavy abdomen forward or backward, or from side to side, she changes her centre of gravity, and the line of force of her aëroplanes, at one and the same time. Actually her body keeps its vertical position, being her heaviest part, and it is the lighter wing-supporting thorax which is deflected. But the result is the same, and every variety and direction of flight is accomplished by the bee on what seems a far more simple plan than that evidenced in the flight of birds.