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].
This extraordinary fact leads us to a very singular and unexpected
conclusion--that we have the power of lengthening or shortening the
life of many insects at pleasure; that we can cause one individual to
live more than twice as long as another of the same species, and _vice
versâ_. Had Paracelsus made this discovery, it would have led him to
pursue his researches after the elixir of immortality with redoubled
confidence, and would have supplied him with an argument for the
possibility of prolonging the life of man beyond its usual term, which
his sceptical opponents would have found some difficulty in rebutting.
Even the logical Reaumur seems inclined to infer from it, that this
object of the alchemists was not so chimerical as we are wont to
conclude[630]. He confesses, however, if it were to be attained only
by the same process as effects the extension of an insect's life--by
prolonging its state of torpor and insensibility,--that few would
choose to enjoy it on such conditions. The man of pleasure, blunted
by excess of use to all modern stimuli, might perhaps not object to a
sleep of a hundred years, in the hope of finding something new under
the sun when he waked; and an ardent astronomer would probably commit
himself with scientific joy to a repose as long and as sound as that
of the seven sleepers, for the chance of viewing his predicted return
of a comet, on stepping out of his cave: but ordinary mortals would
consign themselves to the perils of so long a night with reluctance,
apprehending a fate no better than what befel the magician, who
ordered himself to be cut in small pieces and put in pickle, with the
expectation of becoming young again[631].