272 THE MECHANICAL FUNCTIONS. 



ducted. The researches of modern entomologists 

 have led to the conclusion that the framework, or 

 skeleton of insects, is always formed by the union 

 of a certain determinate number of parts, or ele- 

 ments, originally distinct from one another, but 

 which are variously joined and soldered together 

 in the progress of growth : frequently exhibiting a 

 great disproportion in the comparative expansion 

 of different parts. The enlargement of any one 

 part, however, exercises a certain influence on all 

 the neighbouring parts, and thus are the foun- 

 dations laid of all the endless diversities which 

 characterize the several species belonging to each 

 tribe and family. 



In the progress of developement, we may recog- 

 nize 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 

 compose the thorax are greatly enlarged, in order 

 to support the more recently developed organs of 

 progressive motion, they are also more consoli- 

 dated, and rendered stronger by the approximation 

 of several pieces which were before less closely 

 united. The posterior segments, having no such 

 appendages 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 

 Lepidoptera, or butterfly and moth tribes.* The 



* The four periods of the existence of the Bom by x mori, or 

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



