HOW ANIMALS MOVE. 157 
water up and down, instead of laterally, with a fin-like 
horizontal tail. Many air-breathing animals swim with 
facility on the surface, as the Water-birds, having webbed 
toes, and most of the Reptiles and Quadrupeds. 
(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 150 miles an hour. The bodies of In- 
sects and Birds are made as light as possible by the dis- 
tribution of air-cavities.” 
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 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 humerus. The muscles which give the down- 
ward stroke of the wing are fastened to the breast-bone ; 
and their power, in proportion to the weight of the Bird, 
is as 10,000 to 1. Yet the Insect is even superior in vigor 
and velocity of flight.” In ascending, the Bird slightly 
rotates the wing, striking downward and a little back- 
ward; while the tail acts asa rudder. <A short, rounded, 
concave wing, as in the common Fowl, is not so well fit- 
ted for high and prolonged flight as the long, broad, point- 
ed, and flat wing of the Eagle. The wing is folded by 
