304 THE MECHANICAL FUNCTIONS. 



the comparative expansion of different parts. 

 The enlargement of any one part, however, ex- 

 ercises a certain influence on all the neighbour- 

 ing parts, and thus are the foundations laid of 

 all the endless diversities which characterize the 

 several species belonging to each tribe and 

 family. 



In the progress of developement, we may re- 

 cognize two principles, which, though apparently 

 opposite to each other, concur and harmonize in 

 their operation : these are expansion and concen- 

 tration. Thus while those segments of body 

 which follow the head are greatly enlarged, in 

 order to support the more recently developed 

 organs of progressive motion, they are also more 

 consolidated, and rendered stronger by the union 

 of several pieces which were before separate. 

 The hinder segments, having no such appen- 

 dages to support, are less dilated, and the whole 

 body is much shortened by the approximation of 

 the segments, which in this way compose the 

 abdomen, or hinder division of the insect. 



The progress of the metamorphoses of insects 

 is most strikingly displayed in the history of the 

 Lepidopterous, or butterfly and moth tribe.* 



* The four periods of the existence of the Bombyx mori, or 

 the moth of the silk-worm, are shown in the annexed engravings, 

 Fig. 145 are the eggs; Fig. 146, the Larva, or caterpillar ; Fig. 

 147, the Pupa, or chrysalis; and Fig. 148, the Imago, or per- 

 fect insect. 



