MOUTH-PARTS, WINGS, AND LEGS 49 



movements are sufficient, at least for the simplest form 

 of flight. The anterior margin of an insect's wing is 

 strengthened by a more or less elaborate system of 

 thickened nervures, and is thus kept rigid, while the 

 membranous hind part yields readily to air pressure ; 

 so that during oscillation the plane of the wing constantly 

 changes its inclination with respect to the axis of the 

 insect's body. In the ascending stroke the upper surface 

 of the wing inclines backward, while in the downward 

 stroke the same surface inclines forward. This was de- 

 monstrated by gilding the tip of a wasp's wing, and 

 allowing the insect to fly in the sunlight. A brilliant 

 and continuous image of the wing in its successive posi- 

 tions was thus obtained ; and its tip was seen to describe 

 a very elongate figure 8. Now an inclined plane which 

 strikes the air has a tendency to move in the direction 

 of its own inclination ; so that whether the wings of an 

 insect are ascending or descending, forward action is still 

 maintained. Professor Marey constructed a mechanical 

 model of an insect's wing, and by moving this rapidly, in 

 a vertical plane, between two lighted candles, showed 

 that while the flame near to the thin edge of the wing was 

 strongly blown away by the current produced, that near 

 to the thick edge was equally strongly drawn inwards 

 — thus showing that the current of air is in the same 

 direction both in the upward and downward strokes of 

 the wing. By the movement of an insect's wings, there- 

 fore, " an effect is produced analogous with that which 

 takes place when an oar is used in the stern of a boat in 

 the action of sculling. Each stroke of the oar, which 

 presents an inclined plane to the resisting water, divides 

 the resistance into two forces; one acts in a motion 

 opposed to the motion of the oar, the other in a direction 

 perpendicular to that movement, and it is the latter which 



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