just drawn from superficial observation. A hundred and fifty
years ago P. Lyonet in his monumental work (1762) on the caterpillar of
the Goat Moth (Cossus) detected, in the second and third thoracic
segments, four little white masses buried in the fat-body, and, while
doubtful as to their real meaning, he suggested that their number and
position might well give rise to the suspicion that they were rudiments
of the wings of the moth. But it was a century later that A. Weismann in
his classical studies (1864) on the development of common flies, showed
the presence in the maggot of definite rudiments of wings, and other
organs of the adult--rudiments to which he gave the name of _imaginal
discs_. We will recur later to these transformations of the Diptera. For
the present, we pursue our survey of changes in the life-history of the
Lepidoptera and can take to guide us the excellent researches of J.
Gonin (1894).

Careful study of the imaginal discs of the wings in a caterpillar (fig.
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