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
which is shed and renewed at every moult.

In all insects these successive moults tend to be associated with change
of form, sometimes slight, sometimes very great. The new cuticle is
rarely an exact reproduction of the old one, it exhibits some new
features, which are often indications of the insect's approach towards
maturity. Even in some of those interesting and primitive insects the
Bristle-tails (Thysanura) and Spring-tails (Collembola), in which wings
are never developed, perceptible differences in the form and arrangement
of the abdominal limbs can be traced through the successive stages, as
R. Heymons (1906) and K.W. Verhoeff (1911) have shown for Machilis. But
the changes undergone by such insects are comparatively so slight, that
the creatures are often known as 'Ametabola' or insects without
transformation in the life-history. Now there are a considerable number
of winged insects--cockroaches and grasshoppers for example--in which
the observable changes are also comparatively slight. We will sketch
briefly the main features of the life-story of such an insect.

[Illustration: Fig. 4. Common Cockroach (_Blatta orientalis_). _a_,
female; _b_, male; _c_, side view of female; _d_, young. After Marlatt,
_Entom. Bull._ 4, _U.S. Dept. Agric._]

The young creature is hatched from the egg in a form closely resembling,
on the whole, that of its parent, so that the term 'miniature adult'
sometimes applied to it, is not inappropriate. The baby cockroach (fig.
4 _d_) is known by its flattened body, rounded prothorax, and stiff,
jointed tail-feelers or cercopods; the baby grasshopper by its strong,
elongate hind-legs, adapted, like those of the adult, for vigorous
leaping. During the growth of the insect to the adult state there may be
four or five moults, each preceded and succeeded by a characteristic
instar[4]. The first instar differs, however, from