216 THE MECHANICAL FUNCTIONS. 



jected to a long series of modifications, and passing through 

 many intermediate stages of development. The power of 

 flight is never conferred upon the insect in the earlier peri- 

 ods of its existence: for before its structure can obtain the 

 lightness which fits it for rising in the air, and before it can 

 acquire instruments capable of acting upon so light an ele- 

 ment, it has to go through several preparatory changes, some 

 of which are so considerable as to justify the term oi meta- 

 Tnoiyhoses, which has been generally given to them."^ But 

 transient is the state of perfection in every thing that re- 

 lates to animal existence. When the insect has by a slow 

 development reached this ultimate elaboration of its organs, 

 its life is hastening to a close; and the period of its perfect 

 state is generally the shortest of its whole existence. 



The history of the successive stages of development of in- 

 sects opens a highly interesting field of philosophical inqui- 

 ry. For a certain period of the early life of these animals, 

 the growth of all the parts appears to proceed equably and 

 uniformly: but at subsequent epochs, some parts acquire a 

 great and sudden increase of size, and others that were in a 

 rudimental condition become highly developed, and consti- 

 tute what appear to be new forms of organs, although their 

 elements were in existence from a much earlier period. The 

 modifications which the harder and more solid structures of 

 insects exhibit in the progress of these changes, are particu- 

 larly remarkable, as illustrating the principles on which the 

 development is conducted. The researches of modern en- 

 tomologists have led to the conclusion that the frame-work, 

 or skeleton of insects, is always formed by the union of a 

 certain determinate number of parts, or elements, originally 

 distinct from one another, but v/hich are variously joined 

 and soldered together in the progress of growth: frequently 

 exhibiting a great disproportion in the comparative expan- 

 sion of different parts. The enlargement of any one part, 

 however, exercises a certain influence on all the neighbour- 



* Transformations quite as remarkable occur in several tribes of animals 

 belonging to other classes: such as those of the Frog among reptiles, and of 

 the Lemsea among parasitic worms. 



