The antennæ.—These appendages also have the same relation with those of the caterpillar as in the case of the legs, the larval appendages being only the point of departure of the imaginal growth. Weismann has observed in Corethra how at the approach of each moult an invagination like the finger of a glove allows the antenna to elongate from its base. The process, says Gonin, is identical in the caterpillar of Pieris. At the last moult the invagination is so pronounced that it is not effaced with the renewal of the chitinous integument. Several days later it again begins to grow larger. As the imaginal bud gradually sinks into the cavity of the head, it presses back the hypodermic wall and thus forms an envelope around it. Its base, widely opened, gives admission to the nerves, besides capillaries and sometimes a large trachea.
Fig. 610.—Larva in same stage as Fig. 613; side view of head and thorax: a, a′, wings, with the folds on the surface, and the sinuous track of the tracheal bundles; st I, prothoracic stigma; p, p′, ends of the legs.
As soon as it reaches the posterior region of the head, the antenna in lengthening becomes folded and describes the great curves which led Réaumur to compare it to a ram’s horn (Fig. 613). The leaf of the envelope thickens in the interior and all around the base of the organ. Its ultimate rôle is closely like that of the two other hypodermic formations. It is at the outset this layer of cells which in the larva supports the ocelli. This layer, hidden on each side under the parietal region, thickens and regenerates, forming a circular pad which becomes more prominent and finally assumes the form of the compound eye of the imago.
Fig. 611.—Head of the larva just before pupation: between the two mandibles (m) is seen the relief of the tongue or maxillæ (m′); f, spinneret; l, labrum; a, antenna.
Fig. 612.—Same stage as in Fig. 611, but after the removal of the larval skin, and including the lateral scale: A, side, B, front, view; c, “cimier” (the dotted line shows the position it takes in the pupa); a, antenna; o, eye; t, tongue.—This and Figs. 608–611, after Gonin.
Finally, this layer gives rise to a conical prolongation (Fig. 612, c), which after exuviation appears as a tuft of long hairs, and is called by Gonin the crest (cimier, Fig. 612), which is characteristic of the pupæ of Pieridæ. It is only differentiated towards the end of the 4th larval stage in a median depression of the vertex. It is an imaginal bud in the most general sense of the word.
