208 



ANIMAL MECHANISM. 



tory to fliglit. We speak not only of the spreading of the 

 wings, which in the coleoptera precedes flight, a movement 

 which is sometimes so slow as to be easily observed, nor of 

 the unfolding of the first pair of wings, as wasps do before 

 they fly. Other insects, the diptera, turn their wings as on 

 a pivot around its main-rib in a very remarkable manner, 

 at the moment when the wings which were previously ex- 

 tended on the back in the attitude of repose start outwards 

 and forwards before they begin to fly. Flies, tipulse and 

 many other kinds, show this preparatory movement very clearly 

 when the insect, being exhausted, has no longer any energy 

 in its flight. We see the main-rib of the wing remain sen- 

 sibly immovable, and around it turns the membranous portion 

 whose free border is directed downwards. This position 

 having been obtained, the insect has only to cause its wing 

 to oscillate in an almost horizontal direction from backwards 

 forwards, and from forwards backwards. If this motion as on 

 a pivot did not exist, the wing would cut the air with its edge, 

 and would be utterly incapable of producing flight. In other 

 species, as in the agrion, a small dragon-fly, for instance, the 

 four wings, during repose, are laid back to back one against 

 the other above the abdomen of the animal. Their main- 

 ribs are upwards, and keep their position when the wings pass 

 downwards and forwards ; here no preparation for flight is 

 necessary. In these insects, as in butterflies, the wing has 

 only to set itself in motion when the creature flies. 



It is interesting to follow throughout the series of insects 

 the variations presented by the mechanism of flight. 



The confirmation of the theory just propounded is found in 

 the experiments which certain naturalists have made by 

 means of vivisection. For the most interesting of these we 

 are indebted to Professor M. Giraud. All these experiments 

 prove that the insect needs for the due function of flight a 

 rigid main-rib and a flexible membrane. If we cover the 

 flexible part of the wing with a coating which hardens as it 

 dries, flight is prevented. We hinder it also by destroying 

 the rigidity of the anterior nervure. 



If we only cut ofi*, on the contrary, a portion of the flexible 

 membrane, parallel to its hinder edge, the power of flight 



