Fig. 10. A, B, C, Sections through epidermis and cuticle, showing three stages in growth of the imaginal disc (w) of a wing in the caterpillar of a White Butterfly (Pieris). ep, epidermis; cu, cuticle; t, air-tube, whence branches pass into the developing wing. In C, cu' represents the new cuticle forming beneath the old one, and (p) the pouch within which the wing-disc (w) lies. Highly magnified. After Gonin, Bull. Soc. Vaud. xxx.
As mentioned above, this hidden growth of the wing-rudiments, in butterflies, beetles, flies, bees, and the great majority of the winged insects, has been emphasised by [Sharp (1899)] as a character contrasting markedly with the outward and visible growth of the wing-rudiments in such insects as cockroaches, bugs, and dragon-flies. The divergence between the two modes of development is certainly very striking, and a conceivable method of transition from the one to the other is not easy to explain. Sharp has expressed the divergence by the terms Endopterygota, applied to all the orders of insects with hidden wing-rudiments (the 'Metabola' or 'Holometabola' of most classifications) and Exopterygota, including all those insects whose wing-rudiments are visible throughout growth ('Hemimetabola' and 'Ametabola'). Those curious lowly insects, belonging to the two orders of the Collembola and Thysanura, none of whose members ever develop wings at all, form a third sub-class, the Apterygota (see Classificatory Table, [p. 122]).
Not the wings only, but other structures of the imago, varying in extent in different orders, are formed from the imaginal discs. For example, de Réaumur and [G. Newport (1839)] found that if the thoracic leg of a late-stage caterpillar were cut off, the corresponding leg of the resulting butterfly would still be developed, although in a truncated condition. Gonin has shown that in the Cabbage White butterfly (Pieris brassicae) the legs of the imago are represented, through the greater part of larval life, only by small groups of cells situated within the bases of the larval legs. After the third moult these imaginal discs grow rapidly and the proximal portion of each, destined to develop into the thigh and shin of the butterfly's leg, sinks into a depression at the side of the thorax, while the tip of the shin and the five-segmented foot project into the cavity of the larval leg. Hence we understand that the amputation of the latter by the old naturalists truncated only and did not destroy the imaginal limb. In the blow-fly maggot, Weismann, [B. T. Lowne (1890)] and [J. Van Rees (1888)] have shown that the imaginal discs of the legs ([fig. 11]—1, 2, 3) grow out from deep dermal inpushings. Simple at first, these outgrowths by partial splitting, become differentiated into thigh and shin.
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 compound eyes, and its complex feelers, takes its rightful place at the front end of the insect.