308 



MOVEMENT >IN LOWER ANIMALS 



covering, stretched upon a frame of hollovt' tubes of the 

 same material. They are moved by muscles lying in the 

 thorax, and these muscles are remarkable for their rapid 

 rate of contraction. The number of wings in the insects 

 varies from two to four, and they also vary in size from 

 the tiny wings of the gnats to the relatively enormous 

 surfaces of the butterfly wings. In some forms, such as 

 the beetles, the front pair is adapted solely for a protec- 

 tive covering, while the flying is done with the second 

 pair. The dragon fly is the most perfect flying machine 

 known, being able to fly forward, backward, or remain 

 poised in the air at will, while its movements are marked 

 with great speed and accuracy. 



In birds the arms are modified into wings by a special 

 covering of feathers. These feathers are so arranged as 

 to allow the air to pass through them as the wing is raised 

 and to prevent such passage on the down stroke. This 



Fig. 141 — Wings of the bat, formed by expansions of the skin over the length- 

 ened iinger bones. 



arrangement enables them to strike the air very strongly 

 at each down stroke, and thus to force the body upward 

 and forward. The muscles which operate these wings are 



