MOTIONS OF INSECTS. 347 



shall consider their structure here. Every wing- con- 

 sists of two membranes, more or less transparent, ap- 

 plied to each other : the upper membrane being- very 

 strongly attached to tJie nervures (Neurce), and the 

 lower adhering more loosely, so as to be separable 

 from them. The nervures'* are a kind of hollow tube, 

 — above elastic, horny, and convex ; and flat and 

 nearly membranaceous below, — which take their origin 

 in the trunk, and keep diminishing- gradually, the mar- 

 ginal ones excepted, to their termination. The ves- 

 sels contained in the nervures consist of a spiral thread, 

 whence they appear to be air-vessels communicating 

 with the tracheae in the trunk. — The expansion of the 

 wing at the will of the insect is a problem that can 

 only be solved by supposing that a subtile fluid is intro- 

 duced into tliese vessels, which seem perfectly analo- 

 gous to those in tlie wings of birds ; and that thus an 

 impulse is communicated to every part of the organ, 

 sufficient to keep it in proper tension. We see by this 

 that a wing is supported in its flight like a sail by its 

 cordage''. It is remarkable that those insects which 

 keep the longest on the wing, the dragon-flies (LiOel- 

 I'ulidce), for instance, have their wings most covered 

 with nervures. The wings of insects in flying, you are 

 to observe, move vertically or up and down, and not 

 Iiorizontally. 



In considering the flight of insects, I shall treat of 

 that of eacli order separately, beginning with the Co- 



^ Frcncli natuialisfs use this term {nervure) for the veins of wings, 

 li*a,vc5, &c., restricting ncfi'fi {rierf) to the rainif'itaiio:i3 from the brain 

 Mini spinal marrow. W"e have adopted the term, which we express in 

 Latin I)y nnira, from the Gr;elv ycjoa. '' Jurine Hijmrnnpl . 19. ■ 



