218 THE MECHANICAL FUNCTIONS. 



shape, the same annular structure of the denser parts of Its 

 integument, the same arrangements of longitudinal and ob- 

 lique muscles connecting these rings, the same apparatus of 

 short feet, with claws, or bristles, or tufts of hairs, for faci- 

 litating progression; in short, all the circumstances most 

 characteristic of the vermiformx type are equally exemplified 

 in the different tribes of caterpillars, as in the proper An- 

 nelida. 



But these vermiform insects have this peculiarity, that 

 they contain in their interior the rudiments of all the or- 

 gans of the perfect insect. These organs, however, are con- 

 cealed from view by a great number of membranous cover- 

 ings, which successively invest one another, like the coats of 

 an onion, and are thrown off, one after another, as the inter- 

 nal parts are gradually developed. These external invest- 

 ments, 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 development is rendered necessary 

 in consequence of the greater compactness of the integu- 

 ments of insects, as compared with those of the annelida. In 

 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 di- 

 latation of the internal 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 underneath, 

 moulded on a larger model, and admitting of greater exten- 

 sion 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 development: un- 

 til, 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 



