MOTIONS OF INSECTS. 



279 



excepted, to their termination. The vessels contained in the 

 nervures consist of a spiral thread, whence they appear to be 

 air-vessels communicating with the trachea? in the trunk. 

 The expansion of the wing at the will of the insect is a pro- 

 blem that can only be solved by supposing that a subtile fluid is 

 introduced into these vessels 1 , 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 sup- 

 ported in its flight like a sail by its cordage. 2 It is remark- 

 able 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 

 vertically, or up and down. 



In considering the flight of insects, I shall treat of that of 

 each order separately, beginning with the Coleoptera or beetles. 

 Their subsidiary instruments of flight are their wing-cases 

 (elytra), and in one instance winglets (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 im- 

 moveable in flight, are probably, by their resistance to the air, 

 not without their use on this occasion. The winglets are 

 small concavo-convex scales, of a stiff membranaceous sub- 

 stance, generally fringed at their extremity. I know at pre- 

 sent of only one coleopterous insect that has them (Dytiscus 

 marginalis). They are placed under the elytra at their base. 

 Their use is unknown ; but it may probably be connected 

 with their flight. The wings of beetles are usually very 

 ample, often of a substance between parchment and membrane. 

 The nervures that traverse and extend them, though not 

 numerous, are stronger and larger than those in the wings of 

 insects of the other orders, and are so dispersed as to give 

 perfect tension to the organ. When at rest — except in Mo- 

 lorchus, Atractocerus, Necydalis, and some other genera — they 

 are folded transversely under the elytra, generally near the 



1 Recent observations by several distinguished microscopical naturalists fully 

 confirm this opinion. 



2 Jurine, Hymenopt. 19. 



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