INSECTS AND OTHER ANIMALS 5 



No antennae are present and the eight legs are all attached to the first- 

 named section. They breathe either by air tubes somewhat similar to 

 those of the other groups; by sacs containing many thin plates resembling 

 leaves of a book, whence these structures take the name of book-lungs; 

 or, in the smallest forms, directly through the body surface. In the 

 mites there is no evident division of the body into sections. Though 

 most of the group are land forms, a few are aquatic. 



In the Hexapoda or Insects (Fig. 9) the segments of the body are 

 grouped in three distinct sections; the head, thorax and abdomen. A 

 pair of antennae is (with rare exceptions) present on the head; the six 

 legs are attached to the thorax as are the four wings usually present; 

 the animals breathe by air tubes; and while living under a great diversity 

 of conditions, the group as a whole is emphatically a terrestrial one, 

 though in many cases their early life is spent in water. 



