CHAPTER II. 



STRUCTURES IMPLICATED IN METAMORPHOSIS. 



The external covering of insects varies greatly in its consistence. 

 It is generally a tough and flexible skin. The integument of some 

 full-grown insects is almost leathery, but in the majority all the 

 outside tissues are very thick, solid, and hard. They look like 

 horn, but their intimate composition is very different. Horn dis- 

 solves away, but the integument of insects carbonises and retains 

 its form when exposed to great heat, and their chemical com- 

 position is different. This integument, whether it is as thin and 

 flexible as the skin of a silkworm, or as hard and dense as the 

 envelope of a beetle, is always composed of a particular substance, 

 called cJiitine. 



The skin is formed of two layers, one deep, soft, and not made 

 up of cJiitine, and the other external and constituted mainly of 

 this substance, to which are added, according to the advanced or 

 retarded condition of the development of the insect, more or less 

 colouring matter, fat, and calcareous salts. The deep layer is the 

 true skin, and the superficial is the epidermis or scarf skin. It is 

 the epidermis which is detached and moulted off during the progress 

 of growth and development, for instance, when silkworms change 

 their skins, during their caterpillar state. Both layers are inti- 

 mately connected, and the true skin has small glands in it whose 

 tiny ducts traverse the epidermis, and even enter the hairs. The 

 epidermis is composed of an assemblage of very regularly-shaped 

 cells containing colouring matter. If the skin of a caterpillar, 

 a chrysalis, and a butterfly is examined, the marvellously beautiful 

 cells and hairs of the perfect insect can be seen to be modified 



