g. Mechanism of flight
Marey’s views on the flight of insects.—As we owe more to Marey than to any one else for what exact knowledge we have of the theory of flight of insects, the following account is condensed from his work entitled “Movement.” The exceedingly complicated movements of the wings would lead us, he says, to suppose that there exists in insects a very complex set of muscles of flight, but in reality, he claims, there are only the two elevator and depressor muscles of each wing.[[34]] And Marey says that when we examine more closely the mechanical conditions of the flight of insects, we see that an upward and downward motion given by the muscles is sufficient to produce all these successive acts, so well coordinated with each other; the resistance of the air effecting all the other movements. He also refers to the experiments of Giraud which prove that the insect needs for flight a rigid main-rib and a flexible membrane.
Fig. 163.—The two upper lines are produced by the contacts of a drone’s wing on a smoked cylinder. In the middle are recorded the vibrations of a tuning-fork (250 vibrations per second) for comparison with the frequency of the wing movements. Below are seen the movements of the wing of a bee.—After Marey.
If we take off the wing of an insect, and holding it by the small joint which connects it with the thorax, expose it to a current of air, we see that the plane of the wing is inclined more and more as it is subjected to a more powerful impulse of the wind. The anterior nervure resists, but the membranous portion which is prolonged behind bends on account of its greater pliancy.
The wings of insects may be regarded simply as vibrating wires, and hence the frequency of their movements can be calculated by the note produced. Their movements can be recorded directly on a revolving cylinder, previously blackened with smoke, the slightest touch of the tip of the wing removing the black and exposing the white paper beneath; Fig. 163 was obtained in this way. By this method it was calculated that in the common fly the wings made 330 strokes per second, the bee 190, the Macroglossus 72, the dragon-fly 28, and the butterfly (Pieris rapæ) 9. Thus the smaller the species, the more rapid are the movements of the wings.
Fig. 164.—Appearance of a wasp flying in the sun: the extremity of the wing is gilded.—After Marey.
The path or trajectory made by the tip of the wing is like a figure 8. Marey obtained this by fastening a spangle of gold-leaf to the extremity of a wasp’s wing. The insect was then seized with a pair of forceps and held in the sun in front of a dark background, the luminous trajectory shaping itself in the form of a lemniscate (Fig. 164).
To determine with accuracy the direction taken by the wing at different stages of the trajectory, a small piece of capillary glass tubing was blackened in the smoke of a candle, so that the slightest touch on the glass was sufficient to remove the black coating and show the direction of movement in each limb of the lemniscate. This experiment was arranged as shown in Fig. 165. Different points on the path of movement were tested by the smoked rod, and from the track along which the black had been removed the direction of movement was deduced. This direction is represented in the figure by means of arrows.