CHAPTER II

GROWTH AND CHANGE


The caterpillar differs markedly from the butterfly. As we pursue our
studies of insect growth and transformation we shall find that in some
cases the difference between young and adult is much greater--as for
example between the maggot and the house-fly, in others far less--as
between the young and full-grown grasshopper or plant-bug. It is
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