HOW ANIMALS MOVE 



369 



(2) Locomotion in Air. The power of flight requires a 

 special modification of structure and an extraordinary 

 muscular effort, for air is 800 times lighter than water. 

 Nevertheless, the velocity attainable by certain birds is 

 greater than that of any fish or quadruped ; the hawk 

 being able to go at the rate of 1 50 miles an hour. The 

 bodies of insects and birds are made as light as possible 

 by the distribution of air sacs or air cavities. 136 



The wings of insects are generally four in number ; 

 sometimes only two, as in the fly. . They are moved by 

 muscles lying inside the thorax. They are simple ex- 

 pansions of the skin, or crust, being composed of two 

 delicate films of the epidermis stretched upon a network 

 of tubes. There are three main varieties : thin and 

 transparent, as in the dragon fly ; opaque, and covered 

 with minute colored scales, which are in reality flattened 

 hairs, as in the butterfly ; and hard and opaque, as the 

 first pair (called elytra) of the beetle. 



The wings of birds, on the other hand, are modified 

 fore limbs, consisting of three sets of feathers (called 



FIG. 322. Flamingoes taking Wing. 



primary, secondary, and tertiary), inserted on the hand, 

 forearm, and arm. The muscles which give the down- 

 ward stroke of the wing are fastened to the breastbone ; 

 DODGE'S GEN. ZOO'L. 24 



