io6 



ANIMALS OF LAND AND SEA 



weight there is a wing area of 4 square yards, 6 square feet 

 and 105 square inches; in a butterfly of average size each 

 pound of body weight represents a wing area of 3 square yards, 



8 square feet, and 

 87 square inches; in 

 the swallow this is 

 reduced to only 4 

 square feet and 18 

 square inches, in the 

 pigeon to i square 

 foot and 14 square 

 inches, and in the 

 stork to only 122 

 square inches. 



Not only do the 

 small birds have 

 larger wings than big- 

 ger ones, but they 

 move them much 

 more rapidly. The 

 wings of the smaller 

 humming-birds vi- 

 brate so fast that it 

 is difficult for the eye 

 to follow them. The 

 wing beats of the 

 sparrow are 780 per 

 minute, of the duck 

 540, of the pigeon 

 480, and of the crow 

 about 120. 



It was the necessity 

 of finding answers to these questions, why do small birds have 

 to have larger wings than big ones, and why do they have to 

 move their larger w ings more rapidly, that made the develop- 

 ment of the flying-machine so diflicult. The wings of many of 



Figs. 194-201. 



Some destructive, and some pre- 

 daceous Beetles. 



For explanations of the figures see p. xvii. 



