MOTIONS OF INSECTS. 34^ 



below, — which take their origin in the trunk, and keep 

 diminishing gradually, the marginal ones excepted, to 

 their termination. The vessels contained in the nervures 

 consist of a spiral thiead, 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 onl}' be solved by supposing that 

 a subtile fluid is introduced into these vessels, which 

 seem perfectly analogous to those in the 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 

 [Libellulina) for instance, have their wings most covered 

 with nervures. The wings of insects in flying, like those 

 of other flying animals, you are to observe, move verti- 

 cally, or up and down. 



In considering the flight of insects, I shall treat of 

 that of each order separately, beginning with the Coleo- 

 ptera or beetles. Their subsidiary instruments of flight 

 are their wing-cases [Elytra), and in one instance, wing- 

 lets [Alula;). The former ^, which in some are of a 

 hard horny substance, and in others are softer and more 

 like leather, though they are kept immoveable in flight, 

 are probably, by their resistance to the air, not without 

 their use on this occasion. The winglets are small con- 

 cavo-convex scales, of a stiff membranaceous substance, 

 generally fringed at their extremity'^. I know at pre- 



* Jurine Hymenopl. 19. ** Plate X. P'ig. I. 



' Plate XXHI. Fig. (5. a. 



