Fig. 165.—Experiment to test the direction of movement of an insect’s wing: a, a′, b, b′, different positions of the smoked rod.
Theory of insect flight.—“The theory of insect flight,” says Marey, “may be completely explained from the preceding experiments. The wing, in its to-and-fro movement, is bent in various directions by the resistance of the air. Its action is always that of an inclined plane striking against a fluid and utilizing that part of the resistance which is favorable to its onward progression.
“This mechanism is the same as that of a waterman’s scull, which as it moves backwards and forwards is obliquely inclined in opposite directions, each time communicating an impulse to the boat.”
The mechanism in the case of the insect’s wing is far simpler, however, than in the process of sculling, since “the flexible membrane which constitutes the anterior part of the wing presents a rigid border, which enables the wing to incline itself at the most favorable angle.”
“The muscles only maintain the to-and-fro movement, the resistance of the air does the rest, namely, effects those changes in surface obliquity which determine the formation of an 8–shaped trajectory by the extremity of the wing.”
Fig. 166.—Bee flying about in the chamber of the apparatus.—After Marey.
Lendenfeld has applied photography to determine the position of the wings of a dragon-fly, and Marey has carried chronophotography farther to indicate the normal trajectory of the wing, and to show the position in flight. Fig. 166 shows a bee in various phases of flight. “The insect sometimes assumes almost a horizontal position, in which case the lower part of its body is much nearer the object-glass than is its head, and yet both extremities are equally well defined in the photograph. The successive images are separated by an interval of 1
20 of a second (a long time when compared to the total time occupied by a complete wing movement, i.e. 1
190 of a second). And hence it is useless to attempt to gain a knowledge of the successive phases of movement by examining the successive photographs of a consecutive series representing an insect in flight. Nevertheless an examination of isolated images affords information of extreme interest with regard to the mechanism of flight.
“We have seen that owing to the resistance of the air the expanse of wing is distorted in various directions by atmospheric resistance. Now, as the oscillations during flight are executed in a horizontal plane, the obliquity of the wing-surface ought to diminish the apparent breadth of the wing. This appearance can be seen in Fig. 167. There is here a comparison between two Tipulæ: the one in the act of flight, the other perfectly motionless and resting against the glass window.