Fig. 613.—Larva of Pieris brassicæ stripped of its skin some minutes before pupation: the antennæ (a) have been displaced, and the tongue cut off, to show the palpi (p); c, cimier: o, eye; m, vestige of a mandible; t, insertion of the tongue (see Fig. 612); aa, fore, ap, hind, wings; g, knee of a foot of the 3d pair.
On each side the base of the antenna comes in contact with the germ of the crest. The envelopes approach each other, and their thickened part constitutes with the ocular disks a new cephalic wall. The head of the butterfly thus marked off is triangular; all the larval parts remaining out of this area then disappear. The muscles and the nerves are resorbed by histolysis, then the external part of the imaginal envelopes and the old parietal hypodermis, reduced very thin and degenerated, is detached in shreds. The antenna becomes external throughout its whole extent. The transformation is in this case, then, almost as complete as in the thorax of Diptera or Hymenoptera. It is necessitated by the change of form and of volume of the head. The region of the ocelli persists unchanged almost alone from the larva to the imago also. The limit is not well marked between the portion which is the replacement or direct renovation of the epithelium.
Maxilla and labial palpi.—The development of the tongue (1st maxillæ) is so like that of the antennæ that it scarcely needs description. Beginning at the last moult, the hypodermic contents of the maxillæ is withdrawn in the cephalic cavity under the form of a hollow bud whose base is turned inward. The invagination remains less distinct than in the antennæ; it does not even reach to the anterior part of the œsophagus. The two symmetrical halves of the tongue approach each other and are thrice folded. When the caterpillar stops feeding, they each curve in in the form of an S, remaining lodged under the floor of the mouth (Fig. 613, t).
Underneath are to be seen two other buds, which by an identical process become the labial palpi (Figs. 614, 615, p).
At the anterior part of the head, where the organs are very close together, the envelopes form several folds without any further use (Fig. 615, r). The two leaves then fuse together and decay as at the surface of the tarsus.
Finally, in the mandibles and the labrum, there is only a cellular thickening without any invagination.
Fig. 614.—Section through the anterior region of the head of Pieris larva, four days after the 3d moult: o, œsophagus; m, m, 1st maxillæ containing the two imaginal buds of the tongue; p, p, labial palpi; Tr, trachea.
Fig. 615.—Section through the same place as in Fig. 614, 10 days after the 3d moult, the imaginal appendages having grown in size: r, r, caducous folds of the old hypodermis and of the envelopes. Other letters as in Fig. 614.—This and Figs. 613–614, after Gonin.