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 their connection with the skin only by means
of a thread of cells. As the larva is legless and headless its outer
form is not affected by the discs and it is not surprising to learn that
they appear early. It has indeed been suggested that the pharyngeal
region of the larva, in connection with which the imaginal head-discs
are developed, should be regarded, though it lies in the thorax, as an
inpushed anterior section of the larval head. In any case this region is
pushed out during the formation of the pupa within the final larval
cuticle, so that the imaginal head with its contained brain, its
compound eyes, and its complex feelers, takes its rightful place at the
front end of the insect.

The mention of the brain suggests a few brief remarks on the changes in
the internal organs during insect transformation. There are no imaginal
discs for the nervous system; the brain, nerve-cords and ganglia of the
butterfly or bluebottle are the direct outcome of those of the
caterpillar or maggot. More than seventy years ago, Newport (1839)
traced the rapid but continuous changes, which, during the early pupal
period, convert the elongate nerve-cord of the caterpillar with its