42 THE CHANGE OF SKIN. 



possess no power of receiving addition. When the 

 period of repletion arrives^ therefore^ the larva has no 

 alternative but to get rid of his old coat_, which he 

 does with great ease, although before taking this 

 measure for his relief, his dulness and loss of appetite 

 indicate the inconvenience that he experiences from 

 his strait-jacket, and from the operations going 

 forward for his emancipation. In fact, two or three 

 days before the actual change of skin, it is found that 

 the under layers of the skin separate from the epi- 

 dermis in all parts of the body, and the secretion of a 

 new layer of epidermis then commences*. When this 

 has proceeded to the necessary length, the old skin 

 splits along a part of the back, and through this 

 opening the larva first works its head, and afterwards 

 draws the whole of its body. On examining the cast 

 skin we find that the creature has not only got rid of 

 its outer coat, but that the organs of the mouth have 

 been left behind ; whilst even the delicate membranes 

 lining the main stems and principal branches of the 

 tracheae or air-tubes, and in some cases, at any rate, 

 the lining meml)rane of the extremity of the intes- 

 tinal canal, are also attached to the cast-off garment. 

 The larva itself does not at first appear to be much 

 benefited by the change, although its size is so much 



* It is scarcely necessary to mention the opinion put forward 

 by Swammerdam and Bonnet, and supported by Kir by and 

 Spence (apparently because they feared that the vis formatrix of 

 Herold was an invention of the evil one), that all the skins of the 

 larva, or in fact of the Insect, from its first integument to that 

 which is to envelope it when it arrives at maturity, exist within 

 it in the form of germs from the moment of its quitting the egg. 

 This assumption was made entirely for the purpose of supporting 

 a favourite theory, is totally incapable of proof, and in the 

 present state of science is quite unnecessary. 



