The exterior integument or skin of pupæ, which is usually lined with a very thin white pellicle, is of different consistence in different orders. In the Coleoptera and Hymenoptera it is, with a few exceptions, of a soft and membranous texture; in the Lepidoptera (especially those that are not defended by cocoons), and Diptera, it is more rigid and harder, being either coriaceous or corneous. Lepidopterous pupæ, however, are not excluded from the last skin of the larvæ with this hard covering. At the moment of this change the envelope is nearly as soft and membranous as in the order first mentioned. But they are besides covered with a viscous fluid, which appears to ooze out, chiefly from under the wings, and which very soon drying, forms the exterior hard shell[574]. At first the antennæ, wings, and legs, like those of Coleoptera and Hymenoptera, can be each separated from the body; and it is only after these parts have been glued together by the fluid just mentioned, which takes place in less than twenty-four hours[575], that they are immoveably attached to the body of the pupa, as we usually see them. In fact, the essential difference between incomplete and obtected pupæ seems to be, that in the former the limbs and body are only covered each with a single membranous integument, whereas in the latter they are besides glued together by a substance which forms an additional and harder envelope. It is not easy to explain the alteration that takes place in the texture of the skin of such dipterous pupæ as retain the skin of the larva. In the latter this is generally a transparent and very fine membrane: yet the very same integument becomes to the pupa an opaque and rigid case.
The surface of the skin of the greater number of pupæ is smooth, but in those of many Papilionidæ it is rugose and warty: this you may see, particularly in that of Papilio Machaon. In many of the hawkmoths (Sphinx L.) it is covered with impressed puncta. In Attacus Io the upper side of the channels that separate the intermediate segments of the abdomen are curiously striated with transverse striæ, formed of very minute granula, the lower side being transversely sulcated. In some few instances, as in Arctia Salicis, Laria pudibunda and fascelina, the skin of the pupa is clothed with hair[576]: as is also that of Hesperia Bixæ, according to Madame Merian[577]. De Geer has described a little beetle under the name of Tenebrio lardarius (Latridius Latr., Corticaria Marsh.), the pupa of which is beset with very fine hairs, terminating in a spherical or oval button[578].
ii. I shall include under the same head both the figure or shape, and parts of pupæ, as the latter in most kinds are either the same or nearly the same as those of the larva, or merely incasing those of the imago, so as not to require that detailed notice that I judged necessary when treating of the parts of larvæ.
With regard to incomplete pupæ, nothing further can be said of their extremely various figure, than that it has a general resemblance to that of the perfect insect. The head, trunk, abdomen, and their respective external organs, are alike visible in both; but in the pupæ, the latter, instead of occupying their natural situation, are all closely folded under the breast and abdomen: or, as in the case of the long ovipositors of some Ichneumons, laid along the back. In a specimen of some coleopterous insect now before me, the following is the order of the arrangement of the parts:—The head is inflexed; the mandibulæ are open; between them are seen the labium and labial palpi; these appear to cover and conceal the maxillæ, and the maxillary palpi extend on each side beyond them; the antennæ pass above the thighs of the two anterior pair of legs, and then turning down over the breast between them and the posterior legs, repose upon the base of the wings; which also are turned down between the intermediate and posterior pair of legs, and rest upon the latter; the tibiæ are bent in and folded upon the thigh, and the tarsi turn outwards[579]. In another coleopterous species, the wings and elytra are placed under the hind-legs. In Hymenopterous pupæ the antennæ appear usually to lie between the legs[580]. In many Tipulæ the long legs are bent into three folds in the pupæ; but the tarsi are extended, and lie close to each other, the anterior pair being the shortest[581]. In a specimen belonging to this tribe in my cabinet, which I think contained Ctenocera pectinicornis, the six leg-cases are of the same length, exactly parallel and adjacent, and being annulated wear the appearance of tracheæ[582]. These parts have each their separate case, so that a pin may be introduced between them and the body: which cases, as well as the general envelope, are usually formed of a fine soft transparent membrane; but sometimes, as in the lady-bird (Coccinella), the tortoise-beetle (Cassida), the crane-fly (Tipula), &c. it is harder and more opaque, so that though it is usually easy for a practised Entomologist from an examination of the pupa, particularly in the Hymenoptera, to predict to what genus the insect to be disclosed from them will belong, yet in these cases the organs being not so conspicuous, a less experienced examiner might be perplexed, and unable to come to a conclusion.
Although hymenopterous pupæ have usually no parts but what are afterwards seen in the perfect insect, this is not the case with several coleopterous and dipterous ones, which are furnished with various temporary appendages, indispensable to them to bring about their final change, or for other purposes. Thus, the pupa of the male of Lucanus Cervus has two short, jointed anal processes[583]. That of Hydrophilus caraboides has a pedunculated lunulate one; and moreover, the sides of the abdominal segments, and the top of the thorax, are beset with hairs, which are not seen in the perfect insect[584]. The abdomen of many, also, is armed with spines. That, the arrangement of whose organs I lately described, has a quadruple series in the back of this part; viz. on each of the first five segments, 3, 2, 2, 3. The five first ventral segments also have on each side three spines; the inner are incurved, the intermediate nearly upright, and the outer one recurved. These spines, except those of the innermost ventral series, terminate in a bristle. In another coleopterous species the back part of the head is armed with a pair of lateral spines, and that of the thorax with three processes, the external ones armed with a single spine, and the intermediate one with a pair. De Geer has figured the pupa of an Asilus, the head of which is armed with eight spines—two robust ones in front, and three smaller ones, connected at the base on each side. The abdominal segments, also, are fringed with spines[585]. The abdomen of the pupa of Ctenocera pectinicornis is armed with several strong conical spines, pointing mostly towards the tail, which is likewise the case with that of Tipula lunata[586]. As the above pupæ are usually subterranean or subcortical, the spines assist in pushing them out of the ground, &c. The respiratory horns that proceed from the thorax of the pupæ of many of the aquatic gnats will be noticed in another place. Those of Corethra culiciformis and of some other aquatic gnat-like Diptera, have their anus furnished with a pair of oars, or natatory laminæ, by which they rise to the surface[587].
The figure of obtected pupæ, or chrysalises, is more uniform. They are commonly obtuse at the anterior extremity, and gradually contracted to a point at the posterior, or tail. The outline usually inclines to a long oval or an ellipse; but in some, as Attacus Io and Luna, the pupa is shorter and more spherical. In Geometra sambucaria it represents an elongated cone, and in Hepialus it is nearly cylindrical. In the butterfly tribe (Papilio L.) the outline is frequently rendered angular by various protuberances.
In all these pupæ may be distinguished the following parts:—first, the Head-case (Cephalo-theca), or anterior extremity; secondly, the Trunk-case (Cyto-theca), or intermediate part; and thirdly, the Abdomen-case (Gastro-theca).
1. The Head-case covers and protects the head of the inclosed imago. From its sides behind proceed the antennæ-cases (Cera-theca); and before from the middle, the tongue-case (Glosso-theca). Just below the base of the antennæ-case you may discern the eye-cases (Ophthalmo-theca), surrounded on their inner side by a crescent-shaped lævigated piece, which may perhaps transmit some light to the inclosed prisoner.
2. The Trunk-case, divided into the thorax, or upper surface, extending from the head to the dorsal segments of the abdomen, and consisting of three pieces, answering to the prothorax, mesothorax, and metathorax of the perfect insect: the first answering to the prothorax small, the second covering the mesothorax very large, and the two next representing the metathorax, at first appearing to belong to the abdomen, but having no spiracle; and the breast (pectus) or under-surface reaching from the head to the ventral abdominal segments, from which proceed the wing-cases (Ptero-theca) and leg-cases (Podo-theca), which organs, with the antenna-cases and tongue-case, entirely cover, or rather form, the breast. The arrangement of the whole is as follows:—The wing-cases, which are more or less triangular, and exhibit the larger nervures of the wings, are a lateral continuation of the mesothorax, which turn downwards from the sides of the breast, and cover, or replace, the three first ventral segments of the abdomen. The antenna-cases, united to the anterior portion of the head just behind the eye-cases, repose immediately next to those of the wings running parallel with their inner margin. Then follow the legs, the tibiæ forming an angle with the thigh, and the case of the anterior pair being innermost, and representing the breast-bone in the pupa. The tongue lies over the forelegs, except in the case of some sphinxes, which I shall notice afterwards: so that the glosso-theca covers both them and it.
3. The abdomen-case consists of nine segments when viewed on the back, and of only six when viewed below; so that it might be said to have ten dorsal and six ventral segments: but the fact is, that the place of the three anterior ventral segments, or rather ventral portions of the segments, (for they form complete rings without any lateral suture,) are replaced by the wings and other organs: in consequence of this, the fourth segment, which is less covered than the three first, at its posterior margin forms an annulus or ring. In counting the abdominal segments of a pupa, you must be careful not to include the piece that represents the metathorax, which looks as if it belonged to the abdomen[588]. In the pupæ of butterflies you will discover evident traces of ten dorsal segments; but in many moths, and some hawk-moths, you will perceive at first only eight, or even seven, but a closer examination will enable you to discover the line that marks out the others; and if you divide the puparium longitudinally, and inspect its internal surface, you will see very visible sutures between them. The intermediate segments are sometimes separated from each other and the preceding and subsequent ones by deep channels. In the pupa of Papilio Machaon there is one such channel between the third and fourth segments. In Bombyx regalis the channel is between the sixth and seventh, and in B. imperatoria there are three, namely, a channel between the third and fourth, and fourth and fifth, and fifth and sixth segments. The way in which insects with an exserted sting fold it in the pupa seems not to have been noticed; but from an observation of De Geer upon one species of Ichneumon, it appears to be turned up over the back of the abdomen[589].