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 Reaumur 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.

[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.