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 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 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.