niTRODUCTION. 



surface of the body ; in the latter the respiration is 

 effected by means of extremely beautiful and delicate 

 air-tubes, which convey the oxygenating element to all 

 parts of the body. The Sub-Kingdom Arthropoda may 

 be represented in a tabular form thus : — 



Arthropoda. 



Divisions i 



Tracheata r Class l.Insecta. 



(breathing by tracheas) | 



(aerial) i Class 2. Myriapoda. 



L ClassS.Arachnida. 



Branchiata 



(breathing by bran- 



chise) (aquatic) 



L 



Class 4. Crustacea. 



At present we are only concerned with Class I., the 

 Insecta, which it will be necessary to define, in order to 

 distinguish it from the two other air-breathing classes 

 of the same sub-kingdom. Insects are air-breathing 

 animals having three well-marked divisions of the body, 

 the head, thorax, and abdomen, always distinct one 

 from the other; the thorax, or middle portion, is 

 composed of three segments, the prothorax, to which is 

 attached the first pair of legs, the mesothorax with the 

 second pair of legs and first pair of wings, and the 

 metathorax with the third pair of legs and the second 

 pair of wings, where these last organs are present. The 

 head always carries two antennae. Insects are generally 

 furnished with wings, and undergo a series of trans- 

 formations {^netamorphosis) before arriving at their adult 



