CATERPILLARS 55 



and it seems that they can digest only the fluid parts of 

 their food, which sheds some light on the enormous quantity 

 eaten. 



The extraordinary voracity of caterpillars is associated 

 with rapid growth, and this with periodic " moulting." 

 As in the other jointed-footed or arthropod animals (such 

 as centipedes, spiders, and crustaceans), the body is covered 

 with a chitinous cuticle a layer not in itself living or 

 cellular, made by the underlying living skin. It is really 

 of the nature of a secreted shell ; it cannot grow, and it 

 has little expansibility. Therefore, as the caterpillar grows, 

 it is continually becoming too large for its protective 

 cuticle, and that has to be moulted. Five moults very 

 frequently occur. Every moult is extraordinarily thorough- 

 going, involving all the many intuckings of the outer layer, 

 and the caterpillar is emphatically out of sorts at every 

 moult. There seem to be serious respiratory difficulties, 

 and there is considerable inflammation. The moult is a 

 critical event in the life-history, and the process often 

 ends fatally. 



After the caterpillar has reached its normal limit 

 of growth it passes, as every one knows, into a resting 

 phase, which is often prolonged for many months. It 

 becomes a pupa, nymph, or chrysalis, and undergoes 

 metamorphosis. In many butterflies and some small 

 moths, the larva fastens itself by its tail to a twig ; in many 

 other cases it suspends itself by a silken thread ; some hide 

 between two leaves fastened together by silk ; many 

 burrow beneath the ground ; most moths make some sort 

 of cocoon or shelter, which may be of pure silk neatly 

 wound, or of silk mixed with hair and all manner of 

 external things such as pieces of leaf, bark, moss, and 

 lichen, and even grains of earth. These cocoons are 

 usually constructed in sheltered corners, and are often very 



