PTERODACTYLE. WINGS OF INSECTS. 509 



This is accomplished by the enormous extension of the bones 

 of the hand, especially the metacarpal me, which are here 

 separate ; and the membrane is further sustained by the legs 

 and tail. The thumb po is not included in the wing, but 

 serves as a hook by which the animal can suspend itself. 

 The only true flying Eeptile is (or rather was) the Pterodac- 

 tyle,, a kind of winged lizard, which does not now exist, but 

 of whose character the skeletons that are found imbedded in 

 the earth afford most convincing proof. The structure of its 

 wing differed from that of either Birds or Bats ; for it appears, 

 from the conformation of its anterior member (fig. 252), that 

 the animal could have used it for resting or walking, the 

 framework of the wing being formed by the enormous elonga- 

 tion of one finger only. 



Fig. 253. DRAGON FLY. 



670. The wings of Insects (fig. 253) have no correspon- 

 dence whatever with those of Vertebrata, except in serving 

 for the like use, and in being composed of an expanded sur- 

 face of membrane, stretched upon a firm framework. This 

 framework is not composed of solid pieces jointed together, 

 but is merely an extension of the air- tubes and vessels within 

 the body, which are strengthened by a continuation of its 

 hard envelope. Their only action is a hinge-like movement 

 at the point where they are united to the body ; and this 

 is accomplished by powerful muscles contained within the 

 thorax. 



671. In all instances, the action of the wings must be such, 

 that the air is struck with less force during the up-stroke than 



