172 



ANIMAL LOCOMOTION. 



The wings of insects present different degrees of opacity — • 

 those of the moths and butterflies being non-transparent; 

 those of the dragon-flies, bees, and common flies presenting a 

 delicate, filmy, gossamer-like appearance. The wings in every 

 case are composed of a duplicature of the integument or in- 



Fia. 91.— The Dragon-fly [Petalura gigantea). In this insect the wings are 

 finely curved and delicately transparent, the nervures being most strongly 

 developed at the roots of the wings and along the anterior margins (e e, ff), 

 and least so at the tips (&&), and along the posterior margins {a a). The 

 anterior pair (e e) are analogous in every respect to the posterior (//). Both 

 make a certain angle with the horizon, the anterior pair(ee), which are prin- 

 cipally used as elevators, making a smaller angle than the posterior pair 

 iff)-, which are used as drivers. The wings of the dragon-fly make the proper 

 angles for flight even in repose, so that the insect can take to wing instantly. 

 The insect flies with astonishing Original. 



vesting membrane, and are strengthened in various directions 

 by a system of hollow, horny tubes, known to entomologists 

 as the neurae or nervures. The nervures taper towards the 

 extremity of the wing, and are strongest towards its root and 

 anterior margin, where they supply the place of the arm in 

 bats and birds. They are variously arranged. In the beetles 

 they pursue a somewhat longitudinal course, and are jointed to 

 admit of the wing being folded up transversely beneath the 

 elytra.^ In the locusts the nervures diverge from a common 

 centre, after the manner of a fan, so that by their aid the wing 

 is crushed up or expanded as required ; whilst in the dragon-fly, 



1 The wings of the May-fly are folded longitudinally and transversely, so 

 that they are crumpled up into little squares. 



