274 THE MECHANICAL FUNCTIONS. 



are thrown off, one after anotlier, as the internal 

 parts are gradually developed. 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 rendered 

 necessary in consequence of the greater compact- 

 ness of the integuments of insects, as compared 

 with those of the annelida. Tn proportion as they 

 acquire density, they are less capable of being 

 farther stretched, and at length arrive at the limit 

 of their possible growth. Then it is that they 

 obstruct the dilatation of the internal organs, and 

 must be thrown off to make way for the further 

 growth of the insect. In the mean time, a new 

 skin has been preparing underneath, 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 farther 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. 



These successive peelings of the skin are but so 

 many steps in preparation for a more important 

 change. A time comes when the whole of the 

 coverings of the body are at once cast off, and the 

 insect assumes the form of a pupa, or chrysalis; 

 being wrapt as in a shroud, presenting no appear- 

 ance of external members, and retaining but feeble 



