evidently wise to begin a general survey of the subject with some of
those simpler cases in which the differences between the young and
adult insect are comparatively slight. We shall then be in a position to
understand better the meaning of the more puzzling and complex cases in
which the differences between the stages are profound.

In the first place it is necessary to realise that the changes which any
insect passes through during its life-story are essentially
accompaniments of its growth. The limits of this little book allow only
slight reference to features of internal structure; we must be content,
in the main, to deal with the outward form. But there is an important
relation between this outward form and the underlying living tissues
which must be clearly understood. Throughout the great race of
animals--the Arthropoda--of which insects form a class, the body is
covered outwardly by a _cuticle_ or secretion of the underlying layer of
living cells which form the outer skin or _epidermis_[3] (see fig. 10
_ep_, _cu_, p. 39). This cuticle has regions which are hard and firm,
forming an _exoskeleton_, and, between these, areas which are relatively
soft and flexible. The firm regions are commonly segmental in their
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