PTERODACTYLS!.—WINGS OF INSECTS. 
509 
This is accomplished by the enormous extension of the hones 
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 vms) 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 
