50 THE STUDY OF INSECTS. 



the term to the Hexapoda, and it is in this sense that we 

 use it. 



The name Hexapoda is from two Greek words : hex, six ; 

 and/(?//i-, foot. It refers to the fact that the members of 

 this order differ from other Arthropods in the possession of 

 only six feet. 



Insects breath by means of a system of air-tubes (tra- 

 cheae) which extends through the body. This is true even 

 in the case of those that live in water and are supplied with 

 gill-like organs (the tracheal gills ; see p. 75). The head is 

 distinct from the thorax, and bears a single pair of antennae ; 

 in these respects they are closely allied to the Myriapods. 

 But they can be easily distinguished by the number of their 

 feet, and, usually, also by the presence of wings. 



The Metamorphoses of Insects. 



Nearly all insects in the course of their lives undergo re- 

 markable changes in form. Thus the butterfly, which de- 

 lights us with its airy flight, was at one time a caterpillar ; 

 the bee, which goes so busily from flower to flower, lived first 

 the life of a clumsy, footless grub ; and the graceful fly was 

 developed from a maggot. 



In the following pages considerable attention will be 

 given to descriptions of the changes through which various 

 insects pass. It is our wish in this place merely to define 

 certain terms which are used in describing these changes. 



Development without Metamorphosis. — In one of the orders 

 of insects, the Thysanura, the young insect just hatched 

 from the ^^% is of the same form as the adult insect. These 

 insects merely grow larger, without any more marked change 

 in form than takes place in our own bodies during our life. 

 They are said, therefore, to develop without metamorphosis. 



Incomplete Metamorphosis, — There are many insects which 

 undergo a striking change of form during their life, although 

 the young greatly resembles the adult. Thus a young locust 

 just out from the egg can be easily recognized as a locust. 



