THE THORACIC EXO-SKELETON. 207 



from the action of the air on its surfaces, and made a model of 

 a wing with a stiff anterior border, which he states rotates like 

 the wing of an insect. It may be remarked that a screw 

 movement which results from the resistance of the air would 

 not assist in the support of the insect or in its forward move- 

 ment, although it might resemble the movements of flight ; the 

 screw of a steamer merely turned by a current of water does not 

 urge the steamer on, yet its movement resembles that produced 

 by the engine which drives the vessel. 



Marey, moreover, knew nothing of the structure of the wing- 

 joint. He says : ' The exceedingly complicated movements of 

 the wing would induce us to suppose that a very complex 

 muscular apparatus exists, but the anatomical investigation 

 of the parts does not reveal any muscles capable of giving 

 rise to all the wing movements; we scarcely find any but 

 elevators and depressors of the wing.' These observations 

 show how profoundly ignorant this distinguished physiologist 

 was of the complex character of the wing-joint, and of the 

 numerous muscles by which it is moved. 



The number of vibrations made by the wings was demon- 

 strated graphically on a revolving smoked cylinder by Marey. 

 He found that in the house-fly the wing makes 330 complete 

 vibrations in a second, and gives the following as the number 

 of vibrations per second in several insects. The wings of the 

 drone-fly make 240 vibrations ; of the bee, 190 ; of the wasp, 

 no; of the humming-bird hawk - moth (Macroglossa 

 stellatarum), 72 ; of a dragon-fly, 28 ; and of a butterfly, 9. 

 Marey and Landois* state that it needs 330 to 340 electric 

 shocks per second to produce tetanus in the wing muscles 

 of an insect, hence it may be concluded that 300 separate con- 

 tractions may occur. Such rapidity of action appears almost 

 incredible, but it must be remembered that the wings represent 

 the long arms of levers which are moved by very minute 

 changes in the convexity of the dorsum of the thorax. 



Lendenfeld's memoir [93] on the flight of the Dragon-flies is 



* Landois, ' Physiology,' translated by Stirling, vol. ii., p. 7 15. 



