298 INSECTA. 



and the arrangement of its internal viscera, when arrived at maturity, 

 has been already described. We have yet, however, to mention 

 the series of phenomena observable during the progress of its 

 growth, and the mode of its expansion from the minute size that 

 it exhibits on leaving the egg to the full dimensions which it ulti- 

 mately acquires. In order fully to understand the circumstances 

 connected with this part of our subject, it is necessary to premise 

 that the outer integument of most larvae is of a dense corneous tex- 

 ture, coriaceous in some parts, but quite hard and horny in others. 

 In the second place, it is but very slightly extensible ; and more- 

 over, as is always the case with epidermic structures, is not per- 

 meated by any vascular apparatus, and consequently is absolutely 

 incapable of growth when once formed. This epidermis encases 

 every portion of the larva; the body, the legs, the antennae, the 

 jaws, and all external organs are closely invested with a cuticular en- 

 velope, such as, from its want of extensibility, would form an insu- 

 perable obstacle to developement was there not some extraordinary 

 provision made to meet the necessity of the case. The plan adopt- 

 ed is to cast off at intervals the old cuticle by a process termed 

 moulting ; an operation which is repeated several times during the 

 life of the insect in its larva condition, and is accomplished in the 

 following manner : The caterpillar becomes for a few days slug- 

 gish and inactive, leaves off eating, and endeavours to conceal it- 

 self from observation. The skin, or more properly the cuticle, 

 becomes loosened from the subjacent tissues, and soon a rent ap- 

 pears upon the back of the animal, which gradually enlarges in a 

 longitudinal direction, and the imprisoned insect, after a long series 

 of efforts, at length succeeds in extricating itself from its old cover- 

 ing, and appears in a new skin of larger dimensions than the one it 

 replaces, which however in all other particulars it closely resembles. 

 With the old epidermis the larva throws off all external appendages 

 to the cuticle : the horny coverings of the jaws, the cornese of the 

 eyes, the cases of the claws are all removed ; and many writers have 

 even found attached to the exuviae an epidermic pellicle that had 

 formed a lining to the rectum, and delicate prolongations of the 

 cuticle derived from the interior of the larger ramifications of the 

 air-tubes. Absurd, indeed, have been the explanations given by 

 various writers of the nature of the process under consideration. 

 Swammerdam and Bonnet, nay, even our own illustrious entomo- 

 logists Kirby and Spence, believed that even at the birth of the ca- 

 terpillar all these skins existed ready formed one beneath the other, 



