DR. PETTIGREW ON THE MECHANISM OF FLIGHT. 223 



are frequently employed as sustainers or gliders in flight, the posterior or true wings 

 acting more particularly as elevators and propellers. In such cases the elytra are twisted 

 upon themselves after the manner of wings (Plate XV. fig. 73), the anterior margin 

 of the wing-case towards its root, or where it joins the body, bein bent downwards 

 and. forwards, while towards its extremity it is bent upwards and back/cards. The ante- 



margin consequently presents two 



xity of the curve nearest the 



body being directed downwards and forwards, the convexity of that furthest away from 

 the body being, on the contrary, directed upward* and backwards. Similar but reverse 

 curves are found on the posterior margin ; so that the air which is caught by the concavity 

 occurring towards the extremity of the wing-ease anteriorly is prevented from escaping 

 towards the root of the wing-case in a forward direction by the elytra in this situa- 

 tion, as has been explained, being bent downwards and forwards. It is therefore 

 compelled to pursue a spiral, oblique, and backward course along the under surface 

 of the wing-case, until it arrives at the root of the wing-case posteriorly, where it is 



liberated. 



■ 



The curves observed on the anterior and posterior margins of the winy-case are essen- 

 tially the same as those met with in corresponding positions in the wing of the bird. 



(See pp. 242, 243, and 244.) 



The Inferior or Membranous Wings — their twisted Form, Nervures, Function, SfC. 

 The true win«-s of insects (and by this I mean the inferior or under membranous ones) 

 present different degrees of opacity— those of the moths and butterflies being non-trans- 

 parent (Plate XIII. figs. 19 and 27), those of the dragonflies (Plate XIII. fig. 24), bees, 

 and common flies (Plate XIII. fig. 20) presenting a delicate, filmy, gossamer-like ap- 

 pearance. Both pairs of wings have this feature in common, and it is fundamental : 

 they are composed of a duplicature of the integument or investing membrane, and 



strengthened in various directions by a system of hollow, horny tubes, known to 



are „„™ D 



entomologists as the neurse or nervures (Plate XIII. fig. 17 d, e,f, nnn; Plate XV. 

 no-. 65 d, e,f, c, a, b). These 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 (Plate XIII. fig. 15 r, d, e,f) and birds (Plate XIV. fig. 29 d',e'f). They are variously 

 arranged. In the beetles they pursue a somewhat longitudinal course, and are jointed 

 (Plate XV fi<*. 65 e) to admit of the wing being folded up transversely beneath the 

 elytra*. In the locusts (Plate XIII. fig. 23 a, b) they 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 dragonfly, where no folding is requisite, they form an exquisitely reticulated 

 structure (Plate XIII. fig- 24 e,f). The neure, it may be remarked, are strongest in the 

 beetles (Plate XIII. fig. 17 d, e,f), where the body is heavy and the wing small. They de- 

 crease in thickness as those conditions are reversed (Plate XIII. figs. 24 and 27), and 

 entirely disappear in the minute Chalcis and Psilusf. The function of the neune is not 

 ascertained; but as they contain spiral vessels which apparently communicate with the 



* The wings of the May-fly are folded transversely and longitudinally as well, so that they are crumpled up into 



little squares. 



t Kirby and Spence, vol. ii. 5th ed. p. 352. 



2 H 2 



