364 DR PETTIGREW ON THE PHYSIOLOGY OF WINGS. 



with it the internal side of the base of the wings with which it is articulated, 

 from which ensues the depression of the external side of the wing ; and when 

 it approaches the sternal portion of the trunk, the contrary takes place. During 

 the depression of the wings the dorsum is curved from before backwards, or in 

 such a manner that its anterior extremity is brought nearer to its posterior, that 

 its middle is elevated, and its lateral portions removed further from each other. 

 The reverse takes place in the elevation of the wings ; the anterior extremity 

 of the dorsum being removed to a greater distance from the posterior, its 

 middle being depressed, and its sides brought nearer to each other. Thus its 

 bending in one direction produces a diminution of its curve in the direction 

 normally opposed to it ; and by the alternations of this motion, assisted by 

 other means, the body is alternately compressed and dilated, and the wings 

 are raised and depressed by turns." * 



Objections to Mechanical Theory of Insect Wing Movements specified. — The 

 objections to Chabrier's mechanical theory of the action of insects' wings 

 may be briefly stated : — 



First, The movements of the wings of insects are not necessarily absolutely 

 synchronous. On the contrary, insects have the power of moving their wings 

 independently. 



Second, Insects can twist or plait theirwings at the root— the butterfly having 

 the power of causing the one wing to overlap the other when required. 



Third, Insects can increase the convexity of their wings during the up stroke 

 and decrease it during the down stroke. 



Fourth, They can in some cases fold and diminish the area of the wing 

 during the up stroke and increase it during the down one. 



Fifth, In the dragon-flies we can without difficulty trace the muscles termi- 

 nating in the roots of the wings — a presumptive proof that in other insects there 

 is a direct connexion between the muscles of the thorax and the wings they 

 are destined to move. 



Sixth, All insects have the power of elevating their wings when dressing 

 them, so that the reaction of the air is not necessary to the up stroke, although 

 it certainly contributes to it in flight. They can, moreover, during the intervals 

 of rest, develope figure of 8 curves along the anterior and posterior margins 

 of the pinion independently of the air. 



Seventh, There are muscles in the dragon-fly, and I believe in other insects 

 also, delegated to elevate as well as depress the wing. 



Eighth, There are elastic ligaments which recover or flex and partly elevate 

 the wing when the organ is depressed artificially and not engaged in flight. In 



* " General Observations on the Anatomy of the Thorax in Insects, and on its Functions during 

 Flight." By E. T. Bennett, F.L.S., &c. (Extracted chiefly from the " Essai sur le vol des Insectes," par 

 J. Chabriee, Mem. du Museum d'Histoire Naturelle. Zool. Journal, vol. i. art. xlvi. 1825.) 



