Part 20

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].

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].

Though by far the greater number of the chrysalises of moths are of an uniform chestnut, brown, or black,--a few are of other colours; as that of _Geometra alniaria_, which is of a glaucous blue; of _Noctua sponsa_, lilac; and of _Noctua pacta_, of a lovely blue, caused by a kind of bloom, like that of a plum, spread upon a brown ground. A similar bloom is found on that of _Parnassius Apollo_, and on the anterior part of that of _Platypterix cultaria_ and _sicula_; in which last, Kliemann observed it to the be renewed when rubbed off[624] Many pupæ have the sheaths of the wings of a different colour from that of the rest of the body; a few are variegated with paler streaks or bands, as _Clostera Anastomosis_, which has two red longitudinal stripes down its dark-brown back; and that of the common gooseberry and currant moth, which may be found in every garden, has alternate rings of black and yellow[625].

A few pupæ vary in their colour, as the painted lady-butterfly (_Vanessa Cardui_), some of which are light-brown with gray streaks and golden dots, others wholly of a golden yellow or brown, others of a light green[626].

Almost all at their first assumption of the pupa state have a different colour from that which they take a few days afterwards. This last they retain until the disclosure of the perfect insect; except some that have transparent skins, which a few days previously to this period exhibit the colours of the included animal.

iv. There is as great variety in the length of the _age_ of Insects in their pupa as in their larva state. Some species continue in it only _two_ or _three days_ (_Aleyrodes Chelidonii_ Latr., _Tinea proletella_ L.); others, as many _weeks_, or _months_, or even _years_. Each, however, has in general a stated period, which in ordinary circumstances it neither much exceeds nor falls short of. The only general rule that can be laid down is--that _small_ pupæ continue in that state a shorter time than those of _larger_ bulk. Thus, amongst coleopterous genera, the more minute species of _Curculio_ L.; amongst the _Hymenoptera_, the _Ichneumones minuti_ L.; amongst the _Lepidoptera_, the subcutaneous tribes; and the majority of the _Diptera_,--remain as pupæ only a few days or weeks: while the larger species in all these orders commonly exist in the same state several months--many even upwards of _two_ years. There are, however, numerous exceptions to this rule; for some large pupæ are disclosed in a much shorter time than some others not a twentieth part of their bulk.

The reasons both of the rule and of the exceptions to it are sufficiently obvious. And first, as to the rule:--If you open a pupa soon after its assumption of that state, you will find its interior filled with a milky fluid, in the midst of which the rudiments of its future limbs and organs, themselves almost as fluid, swim. Now the end to be accomplished during the pupa's existence is, the gradual evaporation of the watery parts of this fluid, and the development of the organs of the inclosed animal by the absorption and assimilation of the residuum. Reaumur, by inclosing a pupa in a stopped glass tube, collected a quantity of clear and apparently of pure water, equal to eight or ten large drops, which had evaporated from it, and was condensed against the sides of the tube, and it was found to have lost an eighteenth part of its weight[627]. It is plain, therefore, that this necessary transpiration, other circumstances being alike, must take place sooner in a _small_ than in a _large_ pupa. Next, as to the exceptions:--Since the more speedy or more tardy evaporation of fluids depends upon their exposure to a greater or less degree of heat, we might _à priori_ conclude, that pupæ exposed to a high temperature would sooner attain maturity, even though larger in bulk, than others exposed to a low one:--and this is the fact. The pupa of a large moth, which has assumed that state in the early part of summer, will often disclose the perfect insect in twelve or fourteen days; while that of an Ichneumon, not one hundredth part of its size, that did not enter this state till late in autumn, will not appear as a fly for seven or eight months. But this is not the whole. The very same insect, according as it has become a pupa at an earlier or later period of the year, will at one time live but a few weeks, at another several months, in that state. Thus, if the caterpillar of _Papilio Machaon_, one of those which has annually a double brood, becomes a pupa in July, the butterfly will appear in _thirteen days_: if not until September, it will not make its appearance until the June following; that is, not in less than _nine_ or _ten months_: and the case is the same with the pupæ of _Noctua Psi_, and of a vast number of other insects. To put beyond all doubt the dependence of these remarkable variations on temperature merely, it was only necessary that they should be effected, as Lister long ago advised[628], by artificial means. This Reaumur accomplished. In the month of January he placed the chrysalises of several moths and butterflies, which would not naturally have been disclosed until the following May, in a hothouse: the result was, that the perfect insects made their appearance in less than a fortnight, in the very depth of winter; and by other numerous and varied experiments he ascertained, that in this heated atmosphere five or six _days_ hastened their maturity more than as many _weeks_ would have done in the open air. The disclosed insects were in every respect perfect, and the females, after pairing, laid their eggs, and then died, just as if they had not been thus prematurely forced into existence. The converse of this experiment equally succeeded:--by keeping pupæ the whole summer in an icehouse, Reaumur caused them to produce the fly one full year later than their ordinary period[629].