1 86 DARWINISM AND THE PROBLEMS OF LIFE 



All the articulates, tracheates as well as crustaceans, 

 can only grow periodically, because .they are clothed 

 with a hard coat of mail. This armour completely 

 encloses the animal, and the muscles are attached to 

 it, and find in it the necessary resistance for the pull 

 on the bones that they effect. 



The coat is too rigid and solid to allow any expansion 

 of the body it encloses. The insect can only increase 

 in size, therefore, by bursting the shell ; and this it 

 does at certain places and times. When the armour 

 is thus broken, the soft - skinned animal creeps out 

 of it. 



Underneath the shell of every insect there is a layer 

 of skin which has the function of secreting the material 

 of the coat, much as our salivary glands secrete saliva. 

 This skin now forms a new shell while the old one still 

 covers it, so that when the insect emerges from the 

 broken one, the new coat becomes visible. It is, how- 

 ever, soft at first, and the insect can expand and grow 

 in it. But it soon stiffens in the air, and then becomes 

 a dead mass. Underneath it a new coat is secreted, 

 and this will replace it in turn when the time comes. 



The growth of the insect at each cast is accompanied 

 by other changes. In many insects the wings make 

 their appearance ; they were wanting in the larvae, 

 were visible as short stumps at the first cast, and 

 increased with each succeeding one until they reached 

 their full size at the last; in these cases the insect is 

 thus turned into the imago, or adult and mature 

 organism. This gradual growth is found in the 

 dragon-flies, moths, locusts, and others. 



