VI ARTHROPODA 213 



history^ there is under normal conditions no possibility of increase in 

 the volume of the body, ensheathed as it is in inextensible armour. And 

 change of form is similarly impossible. These difficulties are met by the 

 developing Arthropod undergoing periodical moults or ecdyses, at which 

 it sheds- its complete suit of cuticle. A new cuticle is developed under- 

 neath the old and this remains for a brief period soft and extensible, so 

 that before it becomes hard and rigid the animal is able to undergo a 

 rapid increase in bulk. The Arthropod therefore grows in a succession 

 of spurts, one at each ecdysis, instead of by a continuous process. 



So also with the change of form. At each ecdysis a slight change 

 takes place and the sum of these comparatively slight changes makes up 

 the, frequently great, total change in form from the young up to the 

 adult condition. 



Metamorphosis 



In some of the most highly evolved Arthropods — e.g. those included 

 in certain orders of Insects — there has come about an interesting further 

 development, in that the two processes — increase in size and change in 

 form — instead of keeping step with one another have become concen- 

 trated in different parts of the life-history, practically the whole of the 

 change in form taking place at the last two ecdyses, while the increase 

 in size takes place in the preceding part of the life-history. In such 

 cases the change in form at the last two moults is so striking in amount 

 that it is given the special name metamorphosis. 



An excellent example of metamorphosis is afforded by any ordinary 

 Butterfly or Moth (Fig. 97). Here the earlier parts of the life-history 

 are passed as a Caterpillar larva (Fig. 97, A), of elongated worm-like 

 form, which feeds voraciously and grows actively ^ without shovying any 

 conspicuous change in form. On the completion of the penultimate 

 moult, however, the creature is found to have undergone an extra- 

 ordinary change in form, having become a resting pupa or chrysalis 

 (Fig. 97, B). In this stage the creature is somewhat spindle-shaped : 

 there are no legs or other appendages projecting beyond the general 

 surface, although careful inspection reveals appendages plastered down 

 to the body and absolutely immovable and useless : there is no mouth 

 or anus. There is, in fact, no obvious sign of life except it may be an 

 occasional slight bending of the body. After a more or less prolonged 

 pupal period the last ecdysis takes place, and again an extraordinary 

 change in form is to be seen — the creature becoming now an imago or 



1 The skin has reverted to the soft extensible condition like that of a worm, 

 so that the growth of the caterpillar is no longer restricted to sudden spurts. 



