Thirdly, the sustentor ridges, which, as Riley states, may be more or less obsolete in some forms, in Paphia (Fig. 596, B) and Limenitis form “quite a deep notch, which doubtless assists in catching hold of the larval skin in the efforts to attach the cremaster.”
Fig. 597.—Pupation of butterflies: a, attachment of larva of Danais archippus; p, attachment of larva of Paphia glycerium; b, ideal larva soon after suspension; d, ideal larva a few hours later, the needle (n) separating the forming membrane from the sustainers; l, ideal larva just before splitting of larval skin, with retaining membrane loosened from the sustainers and showing its connection both with the larval and pupal rectum. In all the figures the joints of the body are numbered; the forming chrysalis is shaded in transverse lines; the intervening space between it and larval skin is dotted: h, is the hillock of silk; hl, hooks of hind legs; ap, anal plate; lr, larval rectum; pr, pupal rectum; mr, retaining membrane; c, cremaster; s, sustainers.—This and Figs. 593–596 after Riley.
“It is principally,” adds Riley, “by the leverage obtained by the hooking of the sustainers in the retaining membrane, which acts as a swimming fulcrum, that the chrysalis is prevented from falling after the cremaster is withdrawn from the larval skin. It is also principally by this same means that it is enabled to reach the silk with the cremastral hook-pads.”
“Dissected immediately after suspension, the last abdominal segment of the larva is found to be bathed, especially between the legs and around the rectum, in an abundance of translucent, membranous material.”
“An hour or more after suspension the end of the forming chrysalis begins to separate from the larval skin, except at the tip of the cremaster (Fig. 597, b). Gradually the skin of the legs and of the whole subjoint (10th segment) stretches, and with the stretching, the cremaster elongates, the rectal piece recedes more and more from the larval rectum, and the sustentor ridges diverge more and more from the cremaster, carrying with them, on the sustainers, a part of the soft membrane.” The rectal ligament will sustain at least 10 or 12 times the weight of the chrysalis. That of Apatura seems to rely almost entirely on the rectal ligament, assisted by the partial holding of the delicate larval skin.
FORMATION OF THE PUPA AND IMAGO IN THE HOLOMETABOLOUS INSECTS (THE DIPTERA EXCEPTED)
We have seen that in the incomplete metamorphosis, although there may be as many as five, and possibly seven moults, and in Chloëon as many as 20, and in Cicada septemdecim perhaps 25 or 30, there is but a slight change of form from one stage to another, and no period of inactivity. And this gradual outer transformation is so far as yet known paralleled by that of the internal organs, the slight successive changes of which do not differ from those observed in the growth of ametabolous insects. With the growth of the internal organs there probably goes on a series of gradual regenerative processes, and Korschelt and Heider state that we may venture to assume that each changed cell or group of cells which have become exhausted by the exercise of the functions of life are reabsorbed and become restored through the vital powers of the tissues, so that as the result there goes on a constant, gradual regeneration of the organs.
While the Hemiptera have only an incomplete metamorphosis, the males of the Coccidæ are, as shown by O. Schmidt, remarkable for passing through a complete or holometabolous development, with four stages, three of which are pupal and inactive. Hence, as Schmidt observes, there is here a hypermetamorphosis, like that of the Meloidæ, Stylopidæ, etc.
Shortly before the end of the larval stage of the male appear the imaginal buds of the eyes, legs, and wings. In the 2d or 1st pupal stage there is an atrophy of the antennæ and legs. On the other hand, at this stage the female completes its metamorphosis.