at first, these outgrowths by partial
splitting, become differentiated into thigh and shin.

[Illustration: Fig. 11. Front region of Maggot of Blow-fly
(_Calliphora_) showing diagrammatically the imaginal discs, which are
shaded. _e_, eye; _f_, feeler; _W_, fore-wing; _w_, hind-wing; 1, 2, 3,
legs. _H_ is the 'cephalic vesicle,' which becomes everted at the close
of the metamorphosis, so as to bring the feelers and eyes to the front,
the brain (_B_) moving forwards at the same time. After Van Rees, _Zool.
Jahrb._ 1894, and Lowne's _Blow-fly_.]

Similarly the feelers and jaws of the butterfly are developed from
imaginal discs, and this fact explains how it comes to pass that they
differ so widely from the corresponding structures in the caterpillar.
The larval feelers (fig. 3 _At_) are short and stumpy, those of the
butterfly long and many-jointed. The maxilla of the larva (fig. 3 _Mx_)
consists of a base carrying two short jointed processes; in the
butterfly a certain portion of the maxilla, the hood or galea, is
modified into a long, flexible grooved process, capable of forming with
its fellow the trunk through which the insect sucks its liquid food
(fig. 2). Nothing but some such provision as that of the imaginal discs
could render possible the wonderful replacement of the caterpillar's
jaws, biting solid food, into those of the butterfly sipping nectar from
flowers.

A curious segmental displacement of the imaginal discs with regard to
the larva is noticeable in some Diptera. In the larva of the
harlequin-midge (Chironomus) as described by Miall and Hammond (1900)
the brain is situated in the thorax, and the imaginal discs for the
head, eyes, and feelers of the adult lie in close association with it,
though they arise from inpushings of the larval head. These rudiments do
not appear until the last larval stage has been reached. In the gnats
Culex and Corethra, on the other hand, the imaginal discs for the
head-appendages retain their normal position within the larval head, and
appear in an early stage of larval life. Among the flies of the
bluebottle group (Muscidae) the brain (fig. 11 _B_) is situated, as in
Chironomus, in the thoracic region of the legless maggot, which is the
larva of an insect of this family, and the imaginal discs for eyes and
feelers (fig. 11 _e_, _f_) lie just in front of it. Here, the imaginal
buds of the legs (fig. 11--1, 2, 3) and wings (fig. 11 _W_, _w_) are
deeply inpushed, retaining t