“The motionless insect maintains its wings in a position of vertical extension; the plane is therefore at right angles to the axis of the object-glass. The breadth of the wing can be seen in its entirety; the nervures can be counted, and the rounding off of the extremities of the wings is perfectly obvious. On the other hand, the flying insect moves its wings in a horizontal direction, and owing to the resistance of the air the expanse of the wings is obliquely disposed, and only the projection of its surface can be seen in the photograph. This is why the extremity of the wings appears as if it were pointed, while the other parts look much narrower than normal. The extent of the obliquity can be measured from the apparent alteration in width, for the projection of this plane with the vertical is the sine of the angle. From this it may be gathered that the right wing (Fig. 168, third image) was inclined at an angle of about 50° with the vertical, say 40° with the horizontal. This inclination necessarily varies at different points of the trajectory and must augment with the rapidity of movement; the obliquity reaching its maximum in those portions of the wings which move with the greatest velocity, namely, towards the extremities. The result is that the wing becomes twisted at certain periods of the movement.” (See the fourth image in Fig. 168.) The position of the balancers seems to vary according to that of the wings. (Marey’s Movement, pp. 253–257.) [Illustration: FIG. 168.—Tipula in the act of flying, showing the various attitudes of the wings and the position of the balancers. ] =Graber’s views as to the mechanism of the wings, flight, etc.=—Although in reality insects possess but four wings, nature, says Graber, evidently endeavors to make them dipteral. This end is attained in a twofold manner. In the butterflies, bees, and cicadas, the four wings never act independently of each other, as two individual pairs, but they are always joined to a single flying plate by means of peculiar hooks, rows of claws, grooved clamps, and similar contrivances proceeding from the modified edges of the wings; indeed, this connection is usually carried so far that the hind wings are entirely taken in tow by the front, and consequently possess a relatively weak mechanism of motion. The other mode of wing reduction consists in the fact that one pair is thrown entirely out of employment. We observe this for instance in bugs, beetles, grasshoppers, etc.