10) made by examining microscopically sections cut through them, shows
that the epidermis is pushed in to form a little pouch (_C, p_) and that
into this grows the actual wing-rudiment. Consequently the whitish disk
which seems to lie within the body-wall of the larva, is really a
double fold of the epidermis, the outer fold forming the pouch, the
inner the actual wing-bud. Into the cavity of the latter pass branches
from the air-tube system. In its earliest stage, the wing-bud is simply
an ingrowing mass of cells (fig. 10 _A_) which subsequently becomes an
inpushed pouch (_B_). Until the last stage of larval life the wing-bud
remains hidden in its pouch, and no cuticle is formed over it. When the
pupal stage draws near the bud grows out of its sheath, and projecting
from the general surface of the epidermis becomes covered with cuticle
to be revealed, as we have seen, after the last larval moult, as the
pupal wing. Thus all through the life of the humble, crawling
caterpillar, 'it doth not yet appear what it shall be,' but there are
being prepared, hidden and unseen, the wondrous organs of flight, which
in due time will equip the insect for the glorious aerial existence that
awaits it.

[Illustration: Fig. 10. A, B, C, Sections through epidermis and cuticle,
showing three stages in growth of the imaginal disc (_w_) of a wing in
the caterpillar of a White Butterfly (_Pieris_). _ep_, epidermis; _cu_,
cuticle; _t_, air-tube, whence branches pass into the developing wing.
In C, _cu'_ represents the new cuticle forming beneath the old one, and
(_p_) the pouch within which the wing-disc (_w_) lies. Highly magnified.
After Gonin, _Bull. Soc. Vaud._ XXX.]

As mentioned above, this hidden growth of the wing-rudiments, in
butterflies, beetles, flies, bees, and the great majority of the winged
insects, has been emphasised by Sharp (1899) as a character contrasting