160 COMPARATIVE ZOOLOGY. 



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

 muscles lying inside the thorax. They are simple expan- 

 sions of the skin, or crust, being composed of two delicate 

 films of the epidermis stretched upon a net-work of tubes. 

 There are three main varieties : thin and transparent, as 

 in the Dragon-fly ; opaque, and covered with minute col- 

 ored 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 pri- 

 mary, secondary, and tertiary), inserted on the hand, fore- 

 arm, and arm. The muscles which give the downward 

 stroke of the wing are fastened to the breast-bone ; and 

 their power, in proportion to the weight of the Bird, is 

 very great. Yet the Insect is even superior in vigor and 

 velocity of flight. 83 In ascending, the Bird slightly rotates 

 the wing, striking downward and a little backward ; while 

 the tail acts as a rudder. A short, rounded, concave wing, 

 as in the common Fowl, is not so well fitted for high and 

 prolonged flight as the long, broad, pointed, and flat wing 



. 



FIG. 125. Flamingoes taking Wing. 



of the Eagle. The wing is folded by means of an elastic 

 skin and muscle connecting the shoulder and wrist. Be- 

 sides Insects and Birds, a few other animals have the power 



