300 THE MECHANICAL FUNCTIONS. 



by a great number of membraneous coverings, 

 which successively invest one another, like the 

 coats of an onion, and are thrown off, one after 

 another, as the internal parts are gradually de- 

 veloped. These external investments, which 

 hide the real form of the future animal, have 

 been compared to a mask ; so that the insect, 

 while wearing this disguise, has been termed 

 larva, which is the Latin name for a mask. 



This operose mode of developement is ren- 

 dered necessary in consequence of the greater 

 compactness of the integuments of insects, as 

 compared with those of the annelida. In pro- 

 portion as they acquire density, they are Jess 

 capable of being further stretched, and at length 

 arrive at the limit of their possible growth. Then 

 it is that they obstruct the dilatation of the in- 

 ternal organs, and must be thrown off to make 

 way for the farther growth of the insect. In the 

 mean time a new skin has been preparing under- 

 neath, moulded on a larger model, and admitting 

 of greater extension than the one which preceded 

 it. This new skin, at first, readily yields to the 

 distending force from within, and a new impulse 

 is given to the powers of developement : until, 

 becoming itself too rigid to be further stretched, 

 it must, in its turn, be cast off in order to give 

 place to another skin. Such is the process 

 which is repeated periodically, for a great number 

 of times, before the larva has attained its full size. 



