244 PHENOMENA OF FLIGHT IN THE ANIMAL KINGDOM. 



conuectecl witli aerial locomotion. The dragon-fly, {Libellula,) wliicli pro- 

 pels itself so rapidly, exhibits none of these lateral movements; its wings 

 move exclusively in a vertical i)laue, as if they turned on a hinge, lint 

 we have seen, in the optic method, that the course of the wing in 

 space can be followed by gilding its extremity, and placing it in a ray 

 of sunlight. Now this arrangement furnishes us with a figure of eight, 

 and we furtlier know that during each complete vibration the wing- 

 changes its inclination twice. These movements are not controlled di- 

 rectly by the muscles. They are the mechanical effects of the resistance 

 of the air acting alternately on the u])per and lower surfaces of the 

 wing in its alternate movements. When the wing leaves the ujtper 

 limit of its position, it inclines neither to one side nor to tbe other, its 

 plane being parallel to the length of the animal. But when the impulse 

 of the air is exercised, or as soon as the wing begins to be depressed, 

 the rigid portion, the anterior nervation resists flexure, while the flex- 

 ible membrane which follows it gives way; drawn down by tbe nerva- 

 tion which lowers it, elevated by the air wiiich uplifts it, this membrane 

 takes an intermediate position ; it inclines about 45°, more or less, accord- 

 ing to circumstances. The wing continues its downward motion thus 

 inclined toward the horizon. Thus the reaction of the air, which com- 

 bines its effect and acts perpendicularly u])onthe surface which it strikes, 

 can be decomposed into two forces, a vertical and a horizontal force; one 

 serving to elevate, and the second to propel the animal. After this 

 flrst period the wing membrane will have arrived at the end of its 

 course; the direction of its motion is changed, its action is reversed. 

 A moment of repose, infinitely short, separates these two ])hases, during 

 which the wing resumes its normal position parallel to the axis of the 

 body. The nervure draws it up again, the air resists as before, and 

 from this conflict results a position between the horizontal and the ver- 

 tical — an inclination of 45°. This second period contributes as did the 

 first, to locomotion. How remarkable is the simplicity of apparatus by 

 which the desired end is attained ! 



The horizontal force which is generated by the inclination of the plane 

 of the wing is transmitted to the body of the animal and helps to push 

 it forward. But as the body of the insect does not instantaneously take 

 up the motion which is imparted to it, a part of this force is expended 

 in curving the nervure of the wing which, at the same time that it is 

 lowered, is pushed forward. Here is an artificial wing of large size con- 

 structed in accordance with the type M'hich we have described ; an an- 

 terior nervation represented by a stiff rod, with a membrane behind 

 formed of paper pasted npou its edge. Try to strike down an object 

 immediately before you, and you will not succeed. If you strike at an 

 object before you with a downward blow the wing will be resisted by the 

 air, and it will deviate greatly from the point at which you are aim- 

 ing. From this deviating motion of the wing from the change of plane 

 which it effects, the looped figure which it describes evidently results. 

 It is the combination of these motions which generates the figure of 

 eight previously described. We can now safely say that the two exper- 

 imental facts are now interpreted by our theory. 



A very slight difference has been observed between the two sides of 

 the wing in certain insects; the lower surface is less polished than the 

 upper; it is furnished with rugosities, hairs, or points, which, according to 

 Chabrier, give more hold on the air and reduce the loss of force by slid- 

 ing. This disposition may contribute to insure the predominance of the 

 useful effect of the lowering over the elevating motion. Furthermore, 

 this predominance of the depressing action of the wing does not exist 



