68 ON THE ORIGIN AND [CHAP. 



different from the softness and suppleness of our 

 own skin. The chitine, or horny substance, of which 

 the outside of an insect consists, is formed by a layer 

 of cells lying beneath it, and, once secreted, cannot be 

 altered. From this the result is, that without a change 

 of skin, a change of form is impossible. In some 

 cases, as for instance in CkloPon, each change of skin 

 is accompanied by a change of form, and thus the 

 perfect insect is gradually evolved. In others, as 

 in caterpillars, several changes of skin take place 

 without any material alteration of form, and the 

 change, instead of being spread over many, is confined 

 to the last two moults. 



One explanation of this difference between the 

 larvae which change their form with every change of 

 skin, and those which do not, is, I believe, to be found 

 in the structure of the mouth. That of the cater- 

 pillar is provided with a pair of strong jaws, fitted 

 to eat leaves ; and the digestive organs are adapted 

 for this kind of food. On the contrary, the mouth of 

 the butterfly is suctorial ; it has a long proboscis, 

 beautifully adapted to suck the nectar from flowers, 

 but which would be quite useless, and indeed only 

 an embarrassment to the larva. The digestive organs 

 also of the butterfly are adapted for the assimilation, 

 not of leaves, but of honey. Now it is evident that 

 if the mouth-parts of the larva were slowly meta- 

 morphosed into those of the perfect insect, through 

 a number of small changes, the insect would in the 

 meantime be unable to feed, and liable to perish of 

 starvation in the midst of plenty. In the Orthoptera, 

 and among those insects in which the changes are 



