PHENOMENA OF FLIGHT IN THE ANIMAL KINGDOM. 233 



The figures of eight are more or less widened or compressed according 

 to circumstances. Sometimes the point of the wing seems to move 

 almost in one plane. In the dragon lly (Libellida) a figure of eight is also 

 observed, but much more elongated ; the loops are narrow and laterally 

 compressed. "With the Macroglossa galium it sometimes seems as if the 

 preceding form had disappeared, and is replaced by a sort of ellipse. 

 However, in examining it closely it is soon perceived that this ellipse is 

 surmounted by a little loop, very slightly developed, relatively to the 

 curve which supports it. It seems that one of the loops is enlarged at 

 the expense of the other, but this last has not entirely disappeared, and 

 the vestige what remains testifies to the persistence of the figure of eight 

 which is encountered in most other cases, and which may serve as the 

 general type. 



Changes of the plane of the icing. — The luminous figure which the gilded 

 wing of an insect gives in its motions also shows that, during the alter- 

 nate motions of flight, the plane of the wing changes its position in 

 relation to the axis of the body of the insect. During the period of 

 elevation the upper face of the wing is directed backward, while it turns 

 a little forward during its descent. In fact, if we gild a large extent of 

 the upper face of the wing of a wasp, taking care that the gilding shall 

 be limited to this face, it is seen that the insect, placed in a ray of 

 light, gives the figure of eight with a very unequal intensity on the two 

 sides of the image, as is seen in the preceding figure. It is evident that 

 the cause of this phenomenon is found in a change of the i^lane of the 

 wing, a change in consequence of which the angle of incidence of the 

 solar rays, while lavorable during the ascent of the wing, is unfavorable 

 during the descent. If the animal is turned so that the luminous figure 

 is observed inversely, the figure of eight presents, in an inverse position, 

 the striking inequality of its two halves, catching the light in a portion 

 which was just before without it, and losing it where it had previously 

 shone. We further find, in the employment of the graphic method, 

 new proofs of the changes of plane in the wings of insects during flight. 

 This change of i)lane is of great importance, for in this rests, as we 

 shall see, the immediate cause of the propulsion of the body of the an- 

 imal by the application of the motive force. 



Method of contact. — Does the extremity of the wing really describe 

 this double loop which we perceive, or is this form the result of an opti- 

 cal illusion — a play of light ? Though such an objection is hardly 

 probable, it is necessary to refute it. To assure myself more entirely 

 of the reality of the displacement of the wing than the optic method 

 rendered perceptible, I have introduced, while the wing was in motion, 

 the extremity of a little bodkin into the interior of the loops of the fig- 

 ure of eight, and I have established that in the interior of these curves 

 free spaces really exist, of a funnel shape, in which the bodkiu pene- 

 trated without encountering the wing, while if I attempted to touch 

 the intersection where the lines cross the wing immediately struck 

 against the bodkin, and fight was interrupted. Still greater precision 

 can be brought to bear on the appreciation of these motions, and, know- 

 ing that the wing describes a double loop, it may also be known in what 

 manner it transverses the branches. It is sufiQcient to bring near to 

 the wing in motion a leaf of paper blackened on both sides; the wing, 

 in pursuing its course, strikes against one of the sides of the paper, and 

 the trace which it leaves testifies to the manner in which the motion is 

 accomplished. 



Graphic method. — This method is not applicable to our problem with- 

 out important modifications. We have just seen that it is difficult to 



