In walking, as seen in the accompanying figure (Fig. 8), three legs are thrown forward at a time, two on one side and one on the other. Flies and many other insects can walk upside down, or on glass, as easily as on a level surface. A fly's foot, as in most other insects, consists of five joints (tarsal joints), to the last one of which is appended a pair of stout claws, beneath which is a flat, soft, fleshy cushion or pad, split into two (sometimes three) flaps, beset on the under surface with fine hairs. A part of these hairs are swollen at the end, which is covered with "an elastic membranous expansion, capable of close contact with a highly polished surface, from which a minute quantity of a clear, transparent fluid is emitted when the fly is actively moving." (T. West.) These hairs are hence called holding, or tenent, hairs. With the aid of these, but mainly, as Mr. West insists, by the pressure of the atmosphere, a fly is enabled to adhere to perfectly smooth surfaces. His studies show the following curious facts. "That atmospheric pressure, if the area of the flaps be alone considered, is equal to just one-half the weight of a fly. If the area covered by the tenent hairs be added, an increase of pressure is gained, equal to about one-fourth the weight of a fly. This leaves one-fourth to be accounted for by slight viscidity of the fluid, by the action I have so often alluded to, which may be called 'grasping,' by molecular attraction, and, doubtless, by other agents still more subtle, with which we have at present scarcely any acquaintance." _How Insects Fly._ Who of us, as remarked by an eminent ornithologist, can even now explain the long sustained, peculiar flight of the hawk, or turkey buzzard, as it sails in the air without changing the position of its wings? and, we would add, the somewhat similar flight of a butterfly? It is the poetry of motion, and a marvellous exhibition of grace and ease, combined with a wonderful underlying strength and lightness of the parts concerned in flight.