body a relative shortening is to be noticed, and the imago of any insect
with complete transformation is reduced in length as compared with the
full-fed larva. Now these wings and other structures characteristic of
the imago, appear in the pupa which is revealed by the shedding of the
last larval cuticle. From these facts we infer that the wing-rudiments
must be present in the larva, hidden beneath the cuticle; and until the
last larval instar, not beneath the cuticle only, but growing in
such-wise that they are hidden by the epidermis. For if they were
growing outwardly the new cuticle would be formed over them, so that
they would be apparent after the next moult. But it is clear that only
in the pupa, forming beneath the cuticle of the last larval instar, can
they grow outwards.

Anatomical study of the caterpillar at various stages verifies the
conclusions 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.