[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 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