COMPARATIVE ZOOLOGY. 
160 
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 
