The extraordinary powers of flight which insects possess are due to the conjoined influences of the two conditions that have been named--the lightness of their air-filled bodies, and the strength of their chest-packed muscles. Where light air is circulated instead of heavy blood, great vascularity serves only to make existence more ethereal. Plethora probably takes the insect nearer to the skies, instead of dragging it towards the dust. The hawk-moth, with its burly body, may often be seen hovering gracefully, on quivering wings, over some favourite flower, as if it were hung there on cords, while it rifles it of its store of accumulated sweets by means of its long unfolded tongue. The common house-fly makes 600 strokes every second in its ordinary flight, and gets through five feet of space by means of them; but when alarmed, it can increase the velocity of its wing-strokes some five or six fold, and move through thirty-five feet in the second. Kirby believed, that if the house-fly were made equal to the horse in size, and had its muscular power increased in the same proportion, it would be able to traverse the globe with the rapidity of lightning. The dragon-fly often remains on the wing in pursuit of its prey for hours at a stretch, and yet will sometimes baffle the swallow by its speed, although that bird is calculated to be able to move at the rate of a mile in a minute. But the dexterity of this insect is even more surprising than its swiftness, for it is able to do what no bird can: it is able to stop instantaneously in the midst of its most rapid course, and change the direction of its flight, going sideways or backwards, without altering the position of its body. As a general rule, insect wings that are intended for employment in flight are transparent membranes, with the course of the air-tubes marked out upon them as opaque nervures. These air-tubes, it will be remembered, are lined by spires of dense cartilage; and hence it is that they become nervures so well adapted to act like tent-lines in keeping the expanded membranes stretched. In the dragon-flies, the nervures are minutely netted for the sake of increased strength; in the bees, the nervures are simply parallel. Most insects have two pairs of these transparent membraneous wings; but in such as burrow, one pair is converted into a dense leather-like case, under which the other pair are folded away. In the flies, only one pair of wings can be found at all, the other pair being changed into two little club-shaped bodies, called balancers.