Pieris brassicae,' and finds that the future wing is then indicated by a thickening and bagging inwards of the hypodermis, and by some embryonic cells and a trachea in close relation with this mass (Fig. 168, A). The structure grows so as to form a sac projecting to the interior of the body, connected with the bodywall by a pedicel, and penetrated by a trachea forming branches consisting of rolled and contorted small tracheae (Fig. 168, B). If the body-wall be dissected off the caterpillar immediately before pupation the wings appear in crumpled form, as shown in Fig. 169. This fact was known to the older entomologists, and gave rise to the idea that the butterfly could be detected in a caterpillar by merely stripping off the integument.

st I

a

FIG. 169.-Anterior parts of a cater

The exact mode by which the wings become external at the time of appearance of the chrysalis is not ascertained; but it would appear from Gonin's observations that it is not by a process of evagination, but by destruction of the hypodermis lying outside the wing. However this may be, it is well known that, when the caterpillar's skin is finally shed and the chrysalis appears, the wings are free, external appendages, and soon become fastened down to the body by an exudation that hardens so as to form the shell of the chrysalis.

pillar of P. brassicae, the bodywall having been dissected off, immediately before pupation. a, a', Anterior and posterior wings; st I, first spiracle; p, p', second and third legs. (After Gonin.)

Scales and nervures. Before tracing the further development it will be well to discuss the structure of the scales and nervures that form such important features in the Lepidopterous wing.

If a section be made of the perfect wing of a Lepidopteron, it is found that the two layers or walls of the wing are firmly held together by material irregularly arranged, in a somewhat columnar manner. The thickness of the wing is much greater where the section cuts through a nervure (Fig. 170, A). The nervures apparently differ as to the structures found in them. Spuler observed in a nervure of Triphaena pronuba, a body having in section a considerable diameter, that he considered to be a

1 Bull. Soc. Vaudoise, xxx. 1894, No. 115.

trachea, and also a "wing-rib" and blood-cells. He remarks that even in nervures, perfectly formed as to their chitinous parts, either wing-rib or trachea or both may be absent.1 Schäffer was unable to find any tracheae in the completed wings he examined, and he states that the

matrix of the tracheae and even their inner linings disappear. The wing-ribs were, however, found by him to be present (Fig. 170, A and B).

The scales that form so conspicuous a feature in Lepidoptera exist in surprising profusion, and

FIG. 170.-Structure of wing of imago. A, Transverse section of basal portion of wing [of Vanessa?] containing a nervure; c, cuticle; fr, wing-rib; g, wall of nervure ("Grundmembran "); h, hypodermis; p, connecting columns: 7, lumen of nervure; B, section of a rib; b, one of the chitinous projections; str, central rod. (After Schäffer.)

FIG. 171.-Scales of male Lepidoptera. A, Scale from upper surface of Everes comyntas; B, from upper surface of Pieris rapae; C, from inner side of fold of inner margin of hind wing of Laertias philenor; D, one of the cover-scales from the costal androconium of Eudamus proteus; E, F, G, scales from androconium of Thorybes pylades. (After Scudder).

are of the most varied forms. They may be briefly described. as delicate, chitinous bags; in the completed state these bags are flattened, so as to bring the sides quite, or very nearly, together. Their colour is due to contained pigments, or to striation of the exposed surface of the scale; the latter condition. 1 Zeitschr. wiss. Zool. liii. 1892, p. 623. 2 Zool. Jahrb. Anat. iii. 1889, p. 646.

giving rise to metallic "interference-colours."

A

The walls of the

B

scales are themselves, in some cases, tinted with pigment. It is said that some of the scales contain air, and that the glistening whiteness of certain scales is due to this. The exposed surface of the scale usually differs from the surface that is pressed down on the wing in being delicately and regularly striated; the colours of the upper and under surfaces of a scale may also be quite different. Scales are essentially of the nature of hairs, and all the transitions between hairs and true scales may be found on the wings of certain Lepidoptera that bear both hairs and scales, e.g. Ithomia. It has been calculated that there are a million and a half of scales on the wings of an individual of the genus Morpho. The scales are arranged on the wing in an overlapping manner, somewhat like slates on the roof of a house. Each scale has a short stalk, and is maintained in position by the stalk fitting into a cavity in a projection of the wing-membrane (Fig. 172).

1

FIG.

172. Insertion of scales. A, Socket holding the stalk in Galleria mellonella; B, insertion of the scale of Polyommatus phloeas. b, Base of scale; 7, holding-ring; w, surface of wing. (After Spuler.)

Androconia. -The males of numerous butterflies possess scales peculiar in kind and various in arrangement. They may be either irregularly scattered over the wing, or they may form very complex definite structures (Fig. 173). They were formerly called "plumules," but Scudder has replaced this name by the better one, "androconia." The function of the androconia is still obscure. An odour is believed to be connected with them. Thomas supposes that these scales are hollow tubes in connection with glands at their bases, and that matter secreted by the glands passes through the scales and becomes diffused. In nearly all Lepidoptera it is the male that seeks the female; if therefore odorous scales were present in one sex only we should have supposed that this would have been the female rather than the male. As, however, the reverse is the case, the function of the androconia is supposed to be that of charming the female. Scudder considers that the covering part of the androconial

1 Amer. Natural., xxvii. 1893, p. 1018.

structures is sometimes ornamental. As a rule, however, the "brands" of male Lepidoptera detract from their beauty to our eyes.

FIG. 173.-A, section of part of wing showing the complex androconia of Thanaos tages, a Skipper butterfly. The turning over of the costal margin of the wing is in this case part of the arrangement. a. Upper covering-scales attached to the costal portion of the under surface of the wing; b, edge of costal margin of the wing; c, costal nervure with its scales; d, field of the wing next the costal nervure, bearing stunted scales; e, the androconia proper, or male scales; f, posterior covering scales; g, lumen of the costal nervure: B, a portion of the costal area flattened out and seen from above lettering as before: C, section of androconium on the second nervure of Argynnis paphia. (After Aurivillius.)

Resuming our consideration of the development of the wings, we may remark that the history of the changes during the pupal state is still imperfect. By the changes of relative size of the thoracic segments the hind wing is brought to lie under the anterior one (i.e. between it and the body), so that in the newly formed pupa the arrangement is that shown in Fig. 174. The wings are two sacs filled with material surrounding peritracheal spaces in which run tracheae. The subsequent history of the tracheae is very obscure, and contrary opinions have been expressed as to their growth and disappearance. We have alluded to the fact that in some nervures tracheae are present, while in others they are absent; so that it is quite possible that

the histories of the formation of the nervures and of their relation to tracheae are different in various Lepidoptera. This conclusion is rendered more probable by the statement of Comstock and Needham,' that in some Insects the "peritracheal spaces" that mark out the position of the future nervures are destitute of tracheae. Gonin thinks the nervures are derived from the sheaths of the peritracheal spaces, and a review of all the facts suggests that the tracheae have only a secondary relation to the nervures, and that the view that a study of the pupal tracheae may be looked on as a study of the preliminary state of the nervures is not sufficiently exact. It is, however, probable that in Lepidoptera the pupal tracheae play an important though not a primary part in the

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.)

this may be by setting up changes in the cells near them by means of the air they supply. Semper long ago discovered hypodermal cylinders traversed by a string (Fig. 170, B), placed near the tracheae in the pupa.2 It appears probable that the "wing-ribs" found in the nervures (Fig. 170, A fr and B) are the final state of these cylinders, but the origin and import of the cylinders are still

unknown.

The formation of the scales of the wing commences very early-apparently soon after the casting of the larval skinthough 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 1 Amer. Natural., xxxii. 1898, p. 256. 2 Zeitschr. wiss. Zool. viii. 1857, p. 326.