238 PHENOMENA OF FLIGHT IN THE ANIMAL KINGDOM. 



II. — Form of the motions of the wings. 



After having studied the frequency of the vibrations of the wings, 

 it is necessary to study their form. For the end which we desire to 

 obtain — that is, to arrive at a theory of the flight of insects — the most 

 important element to comprehend is that which we now proceed to in- 

 vestigate, namely : The form of the trajectory which the wing describes 

 ill space instead of the rapidity with which this trajectory is described. 

 In order to arrive at this determination we shall have recourse to two 

 processes which will reciprocally correct each other — the optic method 

 and the ordinary graphic method. 



Optic determination of the movements of the icing. — When a brilliant 

 body moves with rapidity it leaves upon the retina a kind of luminous 

 train which acquaints us with the trajectory through which the body 

 has passed. Children sometimes amuse themselves in producing the 

 most varied figures by brandishing in the air a stick having one end on 

 fire. It is on tliis principle that the apparatus known in physics under 

 the name of Wheatsto7ie\s calidrophone is founded. This is a rod, fast- 

 ened upright on a heavy foot, to which complex vibrations may be given, 

 and to the ends of which a brilliant metallic bead has been aflBxed. If 

 tbe rod is put into vibration the brilliant bead describes in space lumin- 

 ous figures which vary with the different combinations of the vibratory 

 motions. If a brilliant spangle can be attached to the extremity of the 

 wing of an insect, this spangle, traversing without cessation the same 

 points in space, leaves a continuous luminous figure, exempt from the 

 imi^erfection which is caused by friction in the case of the graphic cyl- 

 inder. Tbe extremity of an insect's wing can thus be rendered brilliant 

 without mutilating it in any way ; it is sufficient to place upon it a drop 

 of varnish, to which a small piece of gold-leaf is applied. The varnish 

 dries so rapidly that the insect cannot throw off this little reflector of 

 light, and nothing more is necessary than to hold the animal in a fixed 

 position to observe the play of light upon the small brilliant surface. 

 Under these conditions the bee and the wasp furnish a well-marked 

 " figure of eight." 



Fitr. 4. 



Aspect of a wasp, the extremity of whose primary wings has been gilded. The animal 

 is supposed to be placed in a ray of light. 



