DR PETTIGREW ON THE PHYSIOLOGY OF WINGS. 



357 



Mechanical Theory of the Action of the Insect's Wing as stated by Chabrier. — 

 In one instance only, according to Chabrier,* are the muscles of flight in 

 insects inserted directly into the root of the wing. This solitary example is 



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Fig 24. 



the dragon-fly. Chabrier regards the action of the insect's wing as purely 

 mechanical. His argument may be stated in a few words. He observes, 

 that whereas the muscles which propel the wings of the insect are, with one 

 exception (the dragon-fly), confined to the interior of the thorax, that there- 

 fore they exert no direct influence upon the wings. He further gives it as 

 his opinion, that the wings are actuated by the muscles only during the down 

 stroke, and that the up stroke is entirely due to the reaction of the air — in 

 fact, that if the wings only be depressed rythmically, the air will do the 

 remainder of the work. Unfortunately for this theory there is no time to 

 wait for the reaction of the air, the wings being driven with such velocity as 

 necessitates their being partly elevated either by elastic ligaments or elevator 

 muscles, in addition to the reaction of the air (vide page 345). Chabrier, as will 

 be seen, delegates to the air the task of reversing the planes of the wing, and 

 of conferring upon it those peculiar curves which, overlooked by him, I have 



* Memoires du Museum d'Histoire Nat urelle. Tome septieme. Paris, 1821. Essai sur le vol des 

 Insectes, par I. Chabrier, p. 297. Plates x. xi. and xii. 



