heir 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
relatively far-separated ganglia into the shortened, condensed
nerve-cord of the Tortoise-shell butterfly (_Vanessa urticae_) with
several of the ganglia coalesced. In many Diptera, on the other hand,
the nervous system of the larva is more concentrated than that of the
imago.

The tubular heart also of a winged insect is the directly modified
survival of the larval heart.

Similarly the reproductive organs undergo a gradual, continuous
development throughout an insect's life-story. Their rudiments appear in
the embryo, often at a very early stage; they are recognisable in the
larva, and the matured structures in the imago are the result of their
slow process of growth, the details of which must be reckoned beyond the
scope of this book. For a full summary of the subject the reader is
referred to L.F. Henneguy's work (1904) containing references to much
important modern literature, which cannot be mentioned here.

On the other hand, the digestive system of insects that undergo a
metamorphosis, passes through a profound crisis of dissolution and
rebuilding. This is not surprising when we remember that there is often
a great difference between larva and imago in the nature of the food.
The digestive canal of a caterpillar runs a fairly straight course
thro