PHENOMENA OF FLIGHT IN THE ANIMAL KINGDOM. 237 



erences correspond, in virtue of the relations wliicli I Lave long since 

 explained to yon, to analogous differences in the rapidity with wh:3li 

 the elementary impulse traverses the muscular fiber of these different 

 animals. The muscular fiber of the insect will then be characterized, 

 physiologically, by the property which it possesses of furnishing a con- 

 siderable number of distinct impulses, as well as it is anatomically 

 characterized by its relative size and its striation. 



The graphic process which enables us to judge of the frequency of 

 the strokes, also permits us to show the perfect synchronism of the play 

 of the wings. For this purpose it is necessary to choose an insect 

 of which the amplitude of the wing- vibrations is large, so that in their 

 moment of greatest elevation they may nearly meet above the dorsal 

 region of the animal. If the insect is placed near enough to the regis- 

 tering cylinder, the dorsal region turned toward the blackened surface, 

 it is clear that at the moment when the wings approach each other they 

 will leave their traces on the paper, thus describing a series of loops and. 

 curves, of which the perfect correspondence proves the synchronism of 

 the motions from which they originate. 



Simultaneous tracings of the -wings of a wasp in short flight. The perfect syn- 

 chronism of the twoAviugs will be observed. 



Furthermore, we can convince ourselves that a sort of necessary con- 

 nection exists between the motions of the two wings. If we throw an 

 insect violently upon the ground, so that it is stunned and can no longer 

 execute voluntary motions, we observe that, by producing motions in 

 one of the wings, the other follows, to a certain extent, the movements 

 inflicted on its fellow. If one of the wings of an insect is depressed, the 

 other also bends down ; if one be raised the other elevates itself. Cer- 

 tain species, especially the wasp, lend, themselves very readily to this 

 experiment. According to Chabrier, the author of an extensive work 

 on the mechanism of the flight of insects, synchronism cannot fail to 

 exist. This author considers the depression of the wing as the only 

 effective portion of the stroke ; its elevation is a passive phenomenon due 

 to the action of physical forces. In fact, after the depression each dor- 

 sal arc of the thorax is deflected like a bent bow, and when the muscu- 

 lar contraction ceases the bow springs back in virtue of its elasticity, 

 and the wing is raised. Now, if the pressure did not act simultaneously 

 on the two extremities of the bow it could not be flexed as it is, and. the 

 mechanism which we suppose would be impossible. The reality of this 

 synchronism is, then, a strong proof in favor of this manner of under- 

 standing the motion of the wing. 



After having determined, in a general manner, the frequency of the 

 vibrations of the wing, we seek to know the variation produced in the 

 number of these vibrations by agents capable of influencing the activity 

 of the animal. In the first rank of such agents must be placed heat and 

 cold. We know that warm, dry weather is essential to insects, espe- 

 cially coleoptera, to enable them to fly well ; special observation has 

 confirmed this fact. We are able to state that, within certain limits, the 

 frequency of the strokes is augmented with an increase of the tempera- 

 ture, and that they become slower under a gradual increase of cold. 



