These little animals, thus swathed and banded, exhibit no unapt representation of an Egyptian mummy; though Lamarck applies the term Mumia to incomplete pupæ[590], to which it seems less happily applicable.
Chrysalises, as to the modifications of their general figure, may be conveniently divided into two great classes: first, those that have no angular projections, the anal mucro of some excepted, on different parts of their body; and secondly, those which have such projections. Each of these classes affords variations in its peculiar characters which require to be noticed.
1. The first of these are called angular pupæ[591], and are confined to the Butterfly or diurnal tribes. In some the head projects into one short conical protuberance: this you may see in the chrysalis of the common cabbage butterfly (Pieris Brassicæ), and others of the same genus[592]; in the brimstone-butterfly (Colias Rhamni[593]), and in the beautiful purple emperor or high-flier (Apatura Iris F.[594]): though in this last it is not conspicuous. But the most remarkable instance of a single eminence from the head is exhibited by the pupa of a tropical butterfly (Morpho Idomeneus Latr.), figured by Madame Merian. In this the head projects into a long incurved obtuse horn[595]. In others the head is armed with two mucros, or conical eminences. This is the case with the common butterfly of the nettle (Vanessa Urticæ F.[596]), and with that of the beautiful Papilio Machaon[597]. In these the prominences are trigonal. These processes, which in some, as in the peacock-butterfly (Vanessa Io), stand upright[598], and in others diverge (Papilio Machaon), form the eye-cases of the included imago; and in their outer base is planted the crescent-shaped piece I lately mentioned, which seems intended to convey light into it. In many the prothorax, besides a lateral angular projection, has in the middle another triangular or trigonal one, somewhat resembling a Roman nose; on each side of which is a smaller elevated black point: so that it requires no great stretch of imagination to find out in it a sort of resemblance to the human face, which, though not quite so striking as honest Goedart figures it[599], is however very considerable. In the pupa of Morpho Menelaus, figured by Madame Merian[600], this nasiform prominence of the prothorax is extended into a long arched horn, reaching to the middle of the abdomen. The pupa of the silver-washed fritillary (Argynnis Paphia F.), and others of the same genus, exhibit beneath this nasiform prominence a very deep depression, itself beset with one or more series of smaller angular elevations. The back of the abdomen is often furnished with two rows of protuberances, in some species larger, in others smaller[601]; sometimes sharp and conical, and sometimes flat, and in some instances resembling the fins of fishes[602]. These bosses usually decrease in size towards the tail.
2. The second kind of chrysalises are denominated conical[603]. These, which include the crepuscular and nocturnal Lepidoptera, and the butterflies with onisciform larvæ, have no protuberances, and are less variable in their form—their anterior extremity being almost constantly oval and rounded, and their posterior conical and acute. An exception to this form is met with in the pupa of a moth long celebrated (Lasiocampa Pithyocampa)[604], which has the head acute and the tail obtuse, and armed with two points[605]. Another occurs in that of the Cossus, which has two points on the head, by which it makes an opening in its cocoon: when it assumes the imago, one of these is placed below the other[606]. And some few have the anterior end nearly flat instead of rounded. The pupa of the orange-tip butterfly (Pieris Cardamines) seems intermediate between the angular and conical kinds: it is somewhat boat-shaped, and distinguished by a fusiform process from the head and tail[607]. Other modifications of the usual figure are met with, but are for the most part so slight as not to require notice. One or two, however, should not be passed over. The pupæ of many hawk-moths (Sphinx L.) have the anterior piece of the head-case elongated into a sort of cylindrical proboscis, which is incurved beneath the breast: you will find this formation in S. Convolvuli and Ligustri[608]. In some, as in a species figured by Madame Merian, that feeds upon the Annona squamosa, it is rolled up like a serpent in many folds[609]. In Noctua Linariæ the tongue-case turns upwards, and is prominent laterally beyond the body[610]. This singular appendage is one of those beautiful instances of compensating contrivances, as Dr. Paley calls them, which perpetually occur in the insect tribes. The tongue of these hawk-moths is of very great length, often three inches, while the pupa itself is scarcely two; it could not possibly, therefore, have been extended at length, as it is in common cases, but is coiled up within the above protuberance. When the tongue is but a little longer than the breast, the ordinary plan is adhered to, but the apex of the breast projects a little over the abdomen into a sort of nose, in which the end of the tongue is contained. This conformation may be seen in the pupa of Noctua Gamma, Verbasci, and many other species. Sometimes, as in N. Linariæ F., this projection is recurved into a short horn.
I have before adverted to the adminicula or short spines looking towards the anus, with which the dorsal segments of the abdomen of some pupæ are armed; and by which, when the time for their exclusion is arrived, they are enabled to push themselves upwards or outwards from their several places of confinement[611]: you will find these in the pupa of the great goat-moth (Cossus ligniperda); and in the cylindrical pupa of the moth called the ghost (Hepialus Humuli F.) there are two rows of sharp triangular spines on the back of each segment. These are not laid flat, but, as they do also in the Cossus, form an acute angle with the body; which gives them greater power of resistance. Those that constitute the row nearest the base of the segment are longer than the anterior row, the middle spines than the lateral ones. The first and last segment are without them, and the last segment but one has a sharp ventral transverse ridge, armed with many sharp teeth[612]. The abdominal spines lately mentioned, of semicomplete pupæ, are also adminicula.
The tail of this description of pupæ is in many instances armed with a mucro, or sharp point, emerging from its upper side. You will see this in most hawk-moths. In the pupa of Hesperia Proteus the mucro is truncate at the apex; in that of Bombyx imperatoria it is long, and terminates in two diverging points. In the majority of chrysalises of both descriptions the tail is acute, and usually furnished with hooks of different kinds. These are so various in shape and number, &c. that they would probably afford good characters for discriminating many allied species. In some there are but two or three, in others five or six, in others they are more numerous[613]. Sometimes they are quite straight[614], but most commonly recurved, so as to form a hook. The hawk-moths, and a few others, as Bombyx Pini, Cerura Vinula, &c., have no anal hooks whatever. Under this head I shall observe, that in many conical pupæ below the anal angle or mucro, is the appearance of a vertical foramen or passage: this is particularly conspicuous in Hepialus, in which it is surmounted by a bifid ridge, and has under it a pair of minute black tubercles.
A pretty accurate judgement of the division to which the perfect insect when disclosed will belong, may usually be formed from the figure of its chrysalis. All the angular ones, with scarcely any exception, inclose butterflies. The converse, however, does not hold; for some that are not angular, as those of Parnassius Apollo and Mnemosyne, and most of the Linnean Plebeii urbicolæ, also inclose flies of that description. With these exceptions, all conical chrysalises give birth to moths or hawkmoths. An idea even of the family or genus under which the perfect insect will arrange, may be generally formed from the figure of the chrysalis; less distinctly, however, in the conical or rounded, than in the angular kinds, in which the prominences of the head and trunk, as before explained, usually vary in different families. Even the sex of some moths may be judged from the pupæ: those of females being thicker; and those also of the females that have no wings, or only the rudiments of them, will of course vary somewhat from the ordinary form: but there is a still more striking difference in that of Callimorpha? vestita F., and others of the singular tribe before noticed[615], called by the Germans Sacktrager (sack-bearers), from the sack-like cases in which the larva resides. The females of these having not only no wings, but no antennæ, and legs not longer than those of the larva, their pupa more resembles that of a dipterous than of a lepidopterous insect, it being not easy to determine which is the head and which the tail[616].
In these too we can often learn from the outline of the wing-cases, whether the inhabitant of the chrysalis has these organs indented or intire. If the former, the margins of these cases are sinuate, as in that of Vanessa C. album; if the latter, they are intire, as in Pieris Brassicæ. Even in conical pupæ,—the size, the shape of the antennæ, which may be distinguished through the skin that covers them, and slight modifications of the ordinary form,—give indications of the genus of the included insect sufficiently conclusive to a practised eye.
The true figure of coarctate pupæ when they are mature, the parts of the future fly being very visible, and each being included in a separate case[617], is that of those that belong to the incomplete division; but as this is a character not cognizable without dissection, it is customary, in speaking of pupæ of this description, to refer solely to the shape of the exterior covering, which is in fact a cocoon formed of the dried skin of the larva moulded into a different form. In this sense the figure of coarctate pupæ is extremely various. The majority of them are more or less oval or elliptical, without any distinct parts, were it not that they usually retain traces of the segments which composed the larva's body[618]. Of this figure are the pupæ of the common cheese-maggot[619], and many other flies. Others (Sepedon Latr.) have the pupa shaped like a boat. That of Scæva Pyrastri F. assumes the figure of a flask; or, according to Reaumur's more accurate comparison, of a tear[620]. The tail of many of these pupæ, particularly of aquatic species, is elongated into a sort of beak, either simple or forked, or is beset with spines variously arranged. The pupa of Stratyomis Chamæleon, and other allied species, differs from all the rest of this subdivision in retaining the exact form of the larva[621]; and hence constitutes an exception to the general character of our second great Division.
iii. There is much less variety in the colour of pupæ than in that of larvæ. The majority of coleopterous and hymenopterous pupæ are white, or whitish; of lepidopterous and dipterous, brown of various shades, often verging on black in the former and on red in the latter. The angular lepidopterous ones, however, are more gaily decorated. Some, Pieris Brassicæ, are of a greenish yellow, marked with spots of black; others are of a uniform green, Apatura Iris, Pieris Cardamines; others, reddish, Vanessa C. album; others again red with black spots, Urania Leilus[622]. A still greater number shine as though gilded with burnished gold—either applied in partial streaks, Vanessa Cardui; or covering the entire surface, Vanessa Urticæ. It was from this gilded appearance in some obtected pupæ that the terms Chrysalis and Aurelia were applied to the whole. The alchemists mistook this for real gold; and referred to the case as an argument in favour of the transmutation of metals. But Reaumur has satisfactorily shown, that in this instance the old proverb is strictly applicable—"All is not gold that glitters." He found that this appearance is owing to the shining white membrane immediately below the outer skin, which being of a transparent yellow gives a golden tinge to the former; in the same way that tinfoil, when covered with a yellow varnish, assumes the metallic appearance which we see in gilt leather. He mentions, too, that for the production of this effect—it is essential that the inner membrane be moist: whence may be explained the disappearance of the gilding as soon as the butterfly is ready to escape from the pupa. The shade of colour in these gilded chrysalises is various: some are of a rich yellow, like pure gold; others much paler; and some nearly as white as silver. That of Hipparchia Cassiæ F. is red with silver spots[623].