EXPANSION OF THE WINGS. 321 



a kind of circulation, or rather an injection of an 

 aqueous fluid into these organs, actually takes place, 

 may be ascertained by a very simple experiment. 

 If you clip the wings of a butterfly during the 

 process of expansion, you will see that the 

 nervures are not only hollow, but that, however 

 dry and empty they may subsequently be found, 

 they at that time actually contain such a fluid. 

 Swammerdam, who appears to have been the first 

 physiologist that paid attention to this subject, 

 was of opinion, that an aeriform as well as an 

 aquiform fluid contributes to produce the effect 

 we are considering. He had observed that if a 

 small portion be cut off from the wing of a bee, 

 a fluid of the latter kind exuded from its vessels in 

 the form of pellucid globules, becoming insensibly 

 drops, which he concluded proved the action of the 

 latter ; and he noticed also that the wings were 

 furnished with tracheae, which were at that time 

 distended by the injected air ; whence he justly 

 surmised, that the action of the air was also of 

 great importance to produce the expansion of the 

 wing*" But Swammerdam appears only twice 

 to have seen the expansion of the dragon-fly, in 

 which this phenomenon is best observed. Herold, 



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