470 MOTIONS OF INSECTS. 



transparent, applied to each other : the upper memhrane being very strongly 

 attached to the nervures (neur<s), and the lower adhering more loosely, so 

 as to be separable from them. The nervures 1 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 gradu- 

 ally, the marginal ones 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 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 introduced into these vessels 2 , 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. 3 It is remarkable that those insects which keep the longest 

 on the wing, the dragon-flies (Libcllulind) 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 in- 

 struments of flight are their wing-cases (elytra), and in one instance winglets 

 (alul(B). 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 concavo-convex scales, of a stiff 

 membranaceous substance, generally fringed at their extremity. I know at 

 present of only one coleopterous insect that has them (Dytiscus mnrginalis) . 

 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 Molorchus, Atractocerus, Necydalis, and some 

 other genera they are folded transversely under the elytra, generally near 

 the middle, with a lateral longitudinal fold, but occasionally near the ex- 

 tremity. When they prepare for flight, their antennae being set out, the 

 elytra are opened so as to form an angle with the body and admit the free 

 play of the wings ; and they then fly off, striking the air by the vertical 

 motion of these organs, the elytra all the while remaining immoveable. 

 The Cetonitf, however, as noticed by M. Audouin, differ from most if not 

 all other coleopterous insects in keeping their elytra closed during their 

 flight. 4 During their flight the bodies of insects of this order, as far as I 

 have observed them, are always in a position nearly vertical, which gives 

 to the larger sorts, the stag-beetle for instance, a very singular appearance. 

 Olivier, probably having some of the larger and heavier beetles in his eye, 



1 French naturalists use this term (nervure) for the veins of wings, leaves, &c., re- 

 stricting nerve (nerf} to the ramifications from the brain and spinal marrow. We 

 have adopted the term, which \ve express in Latin by neura, from the Greek :> 



8 Recent observations by several distinguished microscopical naturalists fully con- 

 firm this opinion. 



3 Jurine, Hymenopt. 19. 



4 Ann. Soc. JEnt. de France, viii. p. xlviii. 



