Fig. 174—Transverse section of part of the newly disclosed chrysalis of Pieris brassicae, showing the position and structure of the wings, hanging from one side of the body. aa, Anterior wing; ap, posterior wing; e, e, peritracheal spaces; t, t, tracheae. (After Gonin.)
The formation of the scales of the wing commences very early—apparently soon after the casting of the larval skin—though the completion of the scales and their pigmentation is delayed to a late period of the pupal life. The scales are formed by special cells of the hypodermis that are placed deeper in the interior of the wing than the other hypodermal cells. Each scale is formed by one cell, and protrudes through the overlying hypodermis; the membrane into which the scales are inserted is a subsequently developed structure, and the beautiful articulation of the scale with the wing takes place by a division of the stalk of the scale where it is encompassed by the membrane. Semper was not able to show that the scale-forming cells are certainly hypodermal cells, but this has since been demonstrated by Schäffer, who also shows that each of the cells contains an excretory vesicle.
Fig. 175.—Early condition of scales and nervures. (After Semper.) A, Section of portion of wing of pupa of Sphinx pinastri; a, basal membrane with trachea beneath it; c, scale-forming cell; d, early state of a scale; e, e, more advanced stages; f, hypodermal cells. B, part of a cellular cylinder that excretes the nervure [or more probably the rib or "Rippe" of Schäffer; cf. Fig. 170, B]; b, epithelial [hypodermal] cells; a, central string [supposed by Semper to be a nerve].
Very little is positively known as to the development of the colour in the wing-scales. It has been pointed out by Hopkins[[199]] that in some cases the colours are of the nature of urates; that is, of excretory matter of the kind that usually passes from the body by direct channels, and in the case of Lepidoptera, by the Malpighian tubes. Miss Newbigin suggests that the organic pigments used in scale-coloration will be found to be of two kinds, urates and melanins, the urates being derivatives from nitrogenous, the melanins from carbonaceous, matters.[[200]] Marchal, who has devoted a great deal of attention to the study of the Malpighian tubes, informs us that the subdermal pigments of caterpillars are frequently in large part deposits of urates, and he is of opinion that, the function of the Malpighian tubes being arrested at certain periods of the metamorphosis, elimination of the matter they separate when functionally active then takes place in a variety of other ways.[[201]] A similar condition as to the melanin-pigments and the respiratory functions appears also probable. The scales when first formed are pallid, and the physiology of their pigmentation is not fully ascertained; it is, however, known that when the scales are pallid the hypodermis is either pigmented or in close contact with pigmentary matter, and that as the scales become coloured this pigmentation of the hypodermis diminishes; so that it is clear that the colour of the scales is obtained from matter in the interior of the developing wing, and probably by the agency of the hypodermis.
The pattern on the wings of Lepidoptera is formed before the emergence from the pupa. In the Tortoiseshell butterfly, according to Schäffer, it commences to appear about the ninth day of the pupal life, and the pattern is completed about the eleventh or twelfth day. He also states that the process varies in its rapidity, and this, he thinks, may depend on the previous condition of the larva. According to Buckell the pupa of Nemeobius lucina is sufficiently transparent to allow the development of the colour of the imago to be watched. He says that the coloration occurred first in front; that its entire production occupied less than twenty-four hours, and only commenced about forty-eight hours before the imago emerged.[[202]] When the butterfly leaves the pupal skin the wings are soft, crumpled sacs, of comparatively small size, but, as everyone knows, they rapidly expand and become rigid; the physiology of this process is apparently still unknown.
A great deal of evidence, both direct and indirect, has accumulated showing that the organisation of many Lepidoptera is excessively sensitive, so that slight changes of condition produce remarkable results; and it has also been shown that in the early part of the life this sensitiveness is especially great at the period of ecdysis. Numerous butterflies produce more than one generation a year, and sometimes the generations are so different that they have passed current with entomologists as distinct species. The phenomena of this character are styled "seasonal variations" or "seasonal dimorphism." It has, however, been shown that, by careful management, the eggs of a generation (say form a) may be made to produce form a, whereas in the usual course of nature they would produce form b. A very remarkable condition is exhibited by the North American Papilio ajax. There are three forms of the species, known as P. ajax, P. telamonides, and P. marcellus. It is uncertain how many generations there may be in one year of this species, as the length of the life-cycle varies greatly according to circumstances. But in West Virginia all the butterflies of this species that emerge from the chrysalis before the middle of April are the form marcellus; those produced between the middle of April and the end of May are telamonides; while those that appear after this are ajax. P. telamonides is not, however, the offspring of marcellus, for both forms emerge from pupae that have passed through the winter (and are the offspring of ajax), those that emerge early being marcellus, those that appear later telamonides.
In various parts of Asia and Africa the butterflies produced during the wet season differ more or less markedly from those of the same species produced during the dry season. These are called "wet" and "dry season" forms. Their aetiology has not been investigated, this discovery being comparatively recent.
Turning to the early life we find that some larvae vary in colour, and that this variation is sometimes of a definite character, the larva being one of two different colours—green or brown. In other cases the variation of the species is less definitely dimorphic, a considerable range of variation being exhibited by the species. In tracing the life-histories of Lepidopterous larvae it is not rare to find species in which the larva abruptly changes its form and colour in the middle of its life, and so completely that no one would believe the identity of the individual in the two successive conditions had it not been shown by direct observation; in these cases the change in appearance is usually associated with a change in habits, the larva being, perhaps, a miner in leaves in its first stages, and an external feeder subsequently. In the case of the larval variation we have alluded to above, it is understood that there is no marked change of habits. Poulton has shown[[203]] that it is not infrequent for some of these latter kinds of variable larvae to change colour during life, and he considers that light or conditions of illumination, that he speaks of as "phytoscopic," are the inducing causes. Great difference is, however, exhibited according to species, some variable species not being so amenable to these influences as others are. In dimorphic forms the change was observed to take place at a moult, the larva changing its skin and appearing of another colour. In some cases the result of the change was to bring the colour of the larva into harmony with its surroundings, but in others it was not so. During the final stage many larvae are susceptible, the result being made evident only when the pupa is disclosed. Variably coloured pupae of certain species of butterflies have long been known, and it has been shown that some of the varieties can be induced by changing the surroundings. The result of the changes is in certain cases correspondence between the colour of the individual and its surroundings. In the case of other species having pupae of variable colour, the colour of the pupa is without relation to, or harmony with, the surroundings.