PKOGUESSION IN OR THROUGH THE AIR. 



173 



where no folding is requisite, they form an exquisitely reti- 

 culated structure. The nervures, it may be remarked, are 

 strongest in the beetles, where the body is heavy and the 

 wing small. They decrease in thickness as those conditions 

 are reversed, and entirely disappear in the minute chalcis and 

 psilus.^ The function of the nervures is not ascertained ; but 

 as they contain spiral vessels which apparently communicate 

 with the tracheae of the trunk, some have regarded them as 

 being connected with the respiratory system; whilst others 

 have looked upon them as the receptacles of a subtle fluid, 

 which the insect can introduce and withdraw at pleasure to 

 obtain the requi'site degree of expansion and tension in the 

 wing. Neither hypothesis is satisfactory, as respiration and 

 flight can be performed in their absence. They appear to 

 me, when present, rather to act as mechanical stays or 

 stretchers, in virtue of their rigidity and elasticity alone, — 

 their arrangement being such that they admit of the wing 

 being folded in various directions, if necessary, during flexion, 

 and give it the requisite degree of firmness during extension. 

 They are, therefore, in every respect analogous to the skeleton 

 of the wing in the bat and bird. In those wings which, 

 during the period of repose, are folded up beneath the elytra, 

 the mere extension of the wing in the dead insect, where no 

 injection of fluid can occur, causes the nervures to fall into 

 position, and the membranous portions of the wing to unfurl 

 or roll out precisely as in the living insect, and as happens in 

 the bat and bird. This result is obtained by the spiral arrange- 

 ment of the nervures at the root of the wing ; the anterior ner- 

 vure occupying a higher position than that further back, as in 

 the leaves of a fan. The spiral arrangement occurring at 

 the root extends also to the margins, so that wings which fold 

 up or close, as well as those which do not, are twisted upon 

 themselves, and present a certain degree of convexity on their 

 superior or upper surface, and a corresponding concavity on 

 their inferior or under surface; their free edges supplying 

 those fine curves which act with such efficacy upon the air, 

 in obtaining the maximum of resistance and the minimum 

 of displacement ; or what is the same thing, the maximum 

 of support with the minimum of slip (figs. 92 and 93). 

 1 Kirby and Spence, vol. ii. 5th eel., p. 352. 



