arrangement, and the intervening flexible connections render possible
accurate motions of the exoskeletal parts in relation to each other,
the motions being due to the contraction of muscles which are attached
within the exoskeleton.

[3] The term 'hypodermis' frequently applied to this layer is
misleading. The layer is the true outer skin--ectoderm or epidermis.

Now this jointed exoskeleton--an admirably formed suit of armour though
it often is--has one drawback: it is not part of the insect's living
tissues. It is a cuticle formed by the solidifying of a fluid secreted
by the epidermal cells, therefore without life, without the power of
growth, and with only a limited capacity for stretching. It follows,
therefore, that at least during the period through which the insect
continues to grow, the cuticle must be periodically shed. Thus in the
life-story of an insect or other arthropod, such as a lobster, a spider,
or a centipede, there must be a succession of cuticle-castings--'moults'
or _ecdyses_ as they are often called.

When such a moult is about to take place the cuticle separates from the
underlying epidermis, and a fluid collects beneath. A delicate new
cuticle (see fig. 10 _cu'_) is then formed in contact with the
epidermis, and the old cuticle opens, usually with a slit lengthwise
along the back, to allow the insect in its new coat to emerge. At first
this new coat is thin and flabby, but after a period of exposure to the
air it hardens and darkens, becoming a worthy and larger successor to
that which has been cast. The cuticle moreover is by no means wholly
external. The greater part of the digestive canal and the whole
air-tube system are formed by inpushings of the outer skin (ectoderm)
and are consequently lined with an extension of the chitinous cuticle