CHAPTER <LE 
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 chitine. 
The skin is formed of two layers, one deep, soft, and not made 
up of chitine, 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 
