depends upon its bulk, upon the temperature to which it is exposed,
and upon a combination of these two circumstances. This experiment
appears very simple. We seem to ourselves to have accomplished what
is so often undertaken in vain--to have found an entrance into the
cabinet of Nature, and to have made ourselves masters of the contents
of one of the pages of her sealed and secret book. We deceive,
ourselves, however: this book, when it seems most legible, is often
interlined with _sympathetic inks_, if I may so speak, which require
tests unknown to us for their detection. If you lay up a considerable
number of the pupæ of a moth now called _Eriogaster lanestris_,
the larva of which is not uncommon in June on the black-thorn,
selected precisely of the same size, and exposed to exactly the same
temperature, the greater number of them will disclose the perfect
insect in the February following; some not till the February of the
year ensuing, and the remainder not before the same month in the
third year[632]. Mr. Jones of Chelsea, a most acute lepidopterist,
in one of his excursions captured a female of _Arctia mendica_,
another moth, which laid a number of eggs, thirty-six of which
produced caterpillars: all these fed, spun their cocoons, and went
into the pupa state in the usual manner, but at the proper season
only twelve produced the fly. As this was no uncommon circumstance,
he concluded that the rest were dead: to his great astonishment,
however, in the next season twelve more made their appearance; and
the following year the remainder burst into life, equally perfect
with the foregoing[633]. In this extraordinary result, which also
occasionally has been observed to take place in the emperor-moth
(_Saturnia pavonia_), the privet-hawkmoth (_Sphinx Ligustri_), and
that of the spurge (_S. Euphorbiæ_)[634], and other species,--it is
clear that something besides mere size and temperature is concerned:
for, these circumstances being precisely alike, one pupa arrives
at maturity in six months, and another of the same brood requires
between two and three years. We can guess, that the end which the
All-wise Creator has in view, in causing this remarkable difference,
is the prevention of all possibility of the destruction of the
species. _Eriogaster lanestris_ and _Arctia mendica_, &c., for
instance, are doomed, for some reason unknown to us[635], to be
disclosed from the pupa in the cold and stormy months of February
and March, almost every day of which in certain years is so ungenial
that few insects could then survive exposure, much less deposit their
eggs and ensure the succession of a progeny. Now, were all these to
make their appearance in the perfect state in the _same_ year, it
might happen that the whole race in a particular district would be
destroyed. But this possibility is effectually guarded against by the
beautiful provision under consideration, it being very improbable
that three successive seasons should be throughout unfavourable;
and without such occurrence, it is clear that some of the race of
this moth will be preserved. In the case of other moths, whose pupæ
though disclosed in the summer are governed by the same rule, the
prevention of the extinction of the species, by any extraordinary
increase in a particular year of their natural enemies, seems the
object in view[636]. But though the intention be thus obvious, the
means by which it is effected are impenetrably concealed. What
physiologist would not be puzzled with the eggs of a bird, of which
one-third should require for their hatching to be sat upon only a
fortnight, another third a month, and the remainder six weeks? Yet
this would be an anomaly exactly analogous to that observed by Mr.
Jones with respect to the pupæ of _A. mendica_. Reaumur found that
when the skin of pupæ was varnished, so as to prevent absorption,
the appearance of the fly happened nearly two months later than in
ordinary circumstances. Are we to conjecture that those of the moth
just mentioned, or of _E. lanestris_, that are latest matured, from
a greater degree of viscidity in the fluid that forms them[637],
have thicker and more impervious skins than those disclosed at an
earlier period? Or are we to refer the difference to some unknown
peculiarity of organization? On any supposition, the fact remains
equally wonderful; and I know of none the illustration of which is
more worthy of the patient investigation of the physiologist.
As the period of maturity of the perfect insect is thus in some cases
not fixed even to years, and as in many it seems dependent upon such
variable causes; nothing appears more improbable than that it should
ever be so strictly determined, that even the week in which the fly
will leave its pupa-case can be pretty accurately predicted. Such,
however, is the fact with regard to the _Ephemera_ so interestingly
described by Reaumur, the myriads of which that issue from the banks
of the Seine all appear in two or three days, somewhere between the
10th and 18th of the month of August[638] in every year; at which
time the fishermen regularly expect them. A like regularity attends
the appearance of those described by Swammerdam, which every year,
for three days about the feast of St. John, issue in clouds from the
Rhine[639]--Not only is the week fixed, but in several instances
even the hour. The Ephemeræ observed by Reaumur appear at no other
time than between _eight_ and _ten_ o'clock in the _evening_; and so
unalterably is their exclusion fixed, that neither cold nor rain can
retard it. Between these hours, in the evenings on which they appear,
you may see them fill the air, but an hour before or after, you will
in vain look for one[640]. So also the silkworm-moth and the hawkmoth
of the evening primrose (_Sphinx Œnotheræ_) constantly break forth
from the pupa at _sunrise_: and the hawkmoth of the lime (_Smerinthus
Tiliæ_) as certainly at _noon_[641]. Schroeter states, that of
sixteen specimens of the death's-head-hawkmoth (_S. Atropos_) which
he bred, every one was disclosed between _four_ and _seven_ o'clock
in the _afternoon_[642].
Before I conclude this head, I must observe, that after a caterpillar
or gnat has spun its cocoon, it sometimes remains for a considerable
period before it incloses itself in the pupa-case, and casts off the
form of a larva. Thus the little parasite (_Ichneumon glomeratus_
L.) that destroys the caterpillar of the common cabbage-butterfly,
remains a larva in its cocoon for many months, but it becomes a
perfect insect a few days after it has put on its puparium[643]; and
the caterpillars of the great goat-moth (_Cossus ligniperda_), if
they spin their cocoon in the autumn, remain in it through the winter
in the larva state; whereas, if they inclose themselves in the month
of June, they assume the pupa, so as to appear as flies in three or
four weeks[644]. It is not therefore easy to state precisely the age
of those pupæ which are produced from larvæ that spin cocoons.
v. I have not much to say with regard to the _sex_ of pupæ. The male
is probably to be distinguished from the female by being smaller; but
in the first great division of pupæ, those which resemble the larvæ,
and are locomotive, the female in numerous cases may be known by the
Ovipositor, or instrument for depositing her eggs in their proper
station: and the male also has his anal instruments. Sometimes in
this state the animal is so matured, as to be capable of continuing
its kind. I have found the pupæ both of a _Gryllus_ L. and of a
_Cimex_ L. _in coitu_.
vi. Though the pupæ of the second great division are usually not
locomotive, yet I must not omit some notice of their _motions_. As the
legs of insects in this state are folded within a common or partial
integument, of course none of the pupæ now under consideration,
with the exception of those of the _Trichoptera_ order, can walk:
_coarctate_ ones are even incapable of the slightest motion, and
exhibit no symptom whatever of animation. Some of those that are
termed _incomplete_, however, and most chrysalises, have the power of
communicating to their bodies a slight movement, extending more or
less in different species, which is effected by the abdominal segments
solely. The latter, during the first twelve hours of being pupæ, when
their skin is soft, frequently turn themselves, that the side on which
they lie may not be flattened; afterwards by far the majority merely
wriggle or twist their abdomen when touched, or in any way incommoded
or disturbed. We learn from De Geer, that the pupa of the ghost-moth
(_Hepialus Humuli_), the cocoon of which is more than twice the
length of the chrysalis, moves in it from one end to the other[645].
Bonnet observed one of a moth (perhaps _Lasiocampa Quercus_), which
alternately fixed itself at the top and bottom of its spacious and
obliquely-fixed cocoon; descending slowly, but ascending as quickly,
and almost in the same manner, as a chimney-sweeper in a chimney[646].
The pupa of the weevil of the water-hemlock (_Lixus paraplecticus_)
will move from one end of the interior of a branch to another by
means of its _adminicula_, aided by the motion of its abdominal
segments[647]. But the most locomotive of pupæ of the second division
are those of gnats, and many Tipulidans, which pass this state in the
water. These will move from the bottom to the surface, and back again,
with great facility and velocity. I have before mentioned several other
motions of pupæ[648], which I shall not repeat here, by which they
extricate themselves from their several places of intermediate repose,
before they leave the puparium: if the imago were to be disclosed in
the interior of a tree, or in the earth, its wings would be materially
injured in forcing its way out. The object of several of the above
motions may be to alarm insects that might attack these defenceless
beings. The twirling motion in particular, formerly noticed[649], in
some species, by causing a rustling against the sides of the cocoon,
makes a considerable noise--so singular in that of a red underwing-moth
(_Noctua pacta_), that Rösel tells us, (who by the by was more timid
than becomes a philosopher,) that the first time he heard it, he had
nearly thrown away the box that contained it, in his fright[650].
vii. We are next to consider _The extrication of the perfect insect
from the puparium, or pupa-case, and from the cocoon_. The period
when the pupa has attained maturity, and the inclosed insect is ready
to burst the walls of its prison, may be often ascertained. Just at
this time the colour frequently undergoes an alteration, the golden
or silver tint of the gilded chrysalises vanishes; and those which
are transparent, usually permit the form and colours of the insect
within and the motions of their limbs to be distinctly seen through
them. In the _Libellulina_ the eyes become more brilliant[651]. The
mature pupæ of the moth lately mentioned (_Eriogaster lanestris_)
have a particular swell of the abdominal segments, not apparent in
those that are to continue till another season, or longer[652]. Those
of the case-worms (_Trichoptera_) push off the grates from the cases
which they have hitherto inhabited, and swim about[653]. Other signs
and motions doubtless predict the approach of this great change in
other species, which have not been recorded.
The mode in which insects make their way out of the _puparium_
differs in different orders. In _obtected_ pupæ, the struggles of
the included butterfly or moth first effect a longitudinal slit down
the middle of the thorax, where there is usually a suture for the
purpose. The slit rapidly extends along the head, and down the parts
which compose the breast, and the insect gradually withdraws itself
from its case. It is not, however, from the outer skin merely that it
has to disengage itself, but also from a series of inner membranous
cases, which separately inclose the antennæ, proboscis, feet, &c., as
a glove does the fingers; and similar cases inclose the parts of the
perfect insect in pupæ of all the other orders. This is sometimes a
work of difficulty, but ordinarily it is effected with ease.
_Incomplete_ and _semicomplete_ pupæ undergo nearly the same process,
save that in them the body is not swathed up in a common case; and
therefore they have only to liberate themselves from the partial
cases that envelop the several parts of their body.
In _coarctate_ pupæ, as those of _Muscidæ_, _Syrphidæ_, _Œstridæ_, &c.,
the process is different. Their outer-case is ordinarily more rigid
and destitute of the sutures, which in the former tribes so easily
yield to a slight effort. Yet in these, at the anterior end under
which the head of the fly lies, and from which it always issues, there
is commonly a sort of lid, joined by a very indistinct suture to the
rest, which can be pushed off, leaving a sufficient opening for the
egress of the insect. In the pupæ of many of this tribe this lid is
composed of two semicircular pieces, which can be separately removed.
Many species seem to be able to force off the lid of their puparium, by
merely pushing against it with their heads: but the common flesh-fly
and many other _Muscidæ_, which are perhaps too feeble to effect this,
or whose puparia are stronger than ordinary, are furnished with a very
remarkable apparatus for this express and apparently sole purpose.
They are gifted with the power of introducing _air_ under the middle
part of the head, to which the antennæ are fixed, and of inflating
that part into a sort of membranous vesicle as big as the head itself;
by the action of which against the end of the pupa-case, the lid is
soon forced off. So powerful is this singular lever, that it is even
sufficient to rupture the fibrous galls in which the pupæ of the
gay-winged _Tephritis Cardui_[654] are inclosed. That it is designed
by Creative Wisdom to answer this sole purpose seems proved, from its
disappearing soon after the disclosure of the fly, whose head shortly
becomes all alike hard. Reaumur suspects that it may also be intended
to promote the circulation of the insect's fluids; but to me his
reasons appear not conclusive[655]. In one instance a mode still more
unexpected obtains. The illustrious naturalist just named found that
the fly which proceeded from one of the rat-tailed grubs (_Elophilus_
Latr.) had actually the power of completely reversing its situation in
its narrow case; and that it then employed its _tail_ in pushing off
the lid, which other species remove by means of their _heads_[656].
The extrication of insects whose pupæ are above ground, like those
of butterflies, many beetles, flies, &c., is comparatively a simple
operation. But what, you will ask, becomes of those species whose pupæ
are concealed deep in the earth, or in the heart of the trees on which
their larvæ have fed? Of this you shall be informed.--_Coleopterous_
insects disclosed from pupæ thus circumstanced, wait until their
organs have acquired strength, and their elytra are sufficiently
hardened to protect their filmy wings from damage in forcing their way
through the earth or wood which covers them. Thus _Oryctes nasicornis_,
a rhinoceros beetle common on the Continent, is a full _month_ before
it reaches the surface of the earth, after quitting its puparium. But
it is evident that no delay would enable _lepidopterous_ or _dipterous_
insects, which are without elytra, to make their way out of such
situations, without irreparable injury to their delicate wings. Many
of these, therefore, while still within the hard case of the pupa,
have the precaution, a few days previously to their exclusion, to
force themselves up to the surface of the earth, or, when they reside
in the interior of trees, to the entrance of their hole. This is
effected by a successive wriggling of the abdominal segments, which
in several species, of the _Coleoptera_, _Lepidoptera_, and _Diptera_
orders, for this purpose, as has been more than once observed[657], are
furnished with sharp points (_adminicula_), admitting a progressive,
but not a retrograde motion. The puparia of the great goat-moth
(_Cossus ligniperda_) may be often seen projecting from orifices in
willow-trees; and those of the common crane-fly (_Tipula oleracea_)
from the surface of the earth, to which they have thus made their way
from a depth of several inches.
In all the preceding instances the exclusion of the perfect insect
is complete, as soon as it has withdrawn itself from the puparium.
But to a very large number, even after this is effected, the arduous
task still remains of piercing the cocoons of leaves, of thick
silk, of tough gum, or even of wood, in which the pupæ are incased.
We can readily conceive how the strong jaws of _coleopterous_ and
_hymenopterous_ species may be employed to release them from their
confinement. But what instruments can be used for this purpose by
_moths_ in a state of great debility, whose mouth has nothing like
jaws--merely a soft membranous proboscis? How shall the silkworm-moth
(_B. Mori_) force its way through the close texture of a silken
ball, through which the finger could not be easily pushed? Or the
puss-moth (_Cerura Vinula_) pierce the walls of its house of glue
and wood, which scarcely yield to the knife? You will not doubt
that these difficulties have been foreseen by INFINITE WISDOM, and
provided against by INFINITE POWER. The egress of moths from their
cocoons is secured in two ways;--either by some peculiarity in the
first construction of the cocoon by the caterpillar, or by some
process which the pupa or perfect insect is instructed to perform. As
examples of each, several curious instances may be cited.
The larva of the moth which about 1760 made such havoc in the
province of Angoumois in France, becomes a pupa in the interior of
the grain of wheat which it has excavated; but the opening by which
it first entered is not bigger than a pin's point, and is quite
insufficient for the egress of the moth. How, then, is the latter to
force its way through the tough skin which surrounds it? The larva,
previously to assuming the pupa state, gnaws out a little circular
piece at that end of the grain where the head of the future moth
would lie, taking care not to detach it entirely. At this little
door, which is sufficient to protect it from intruders, the moth
has but to push, when it falls down, and leaves a free passage for
its exit. A contrivance almost similar is adopted by a caterpillar
which feeds in the interior of the heads of a species of teazel
(_Dipsacus_ L.), for a minute and interesting history of which we are
indebted to Bonnet. This caterpillar previously to its metamorphosis
actually cuts a circular opening in the head, sufficiently large
for the egress of the future moth; but to secure this sally-port
during its long sleep, it artfully closes it with fibres of the
teazel, closely but not strongly glued together[658]. Another small
caterpillar described by the same author, resides in the leaf of
an ash curiously rolled up into a cone, and then assumes the pupa,
which is inclosed in a silken cocoon, ingeniously suspended by two
threads like a hammock in the middle of its habitation, and of so
slight a texture that it presents no obstacle to the extrication of
the moth. It is the closely-joined sides of its leafy dwelling that
form a barrier, which, were it not for the precaution of the larva,
would be impenetrable to so small and weak an animal. The little
provident creature, before its change to a pupa, gnaws in the leaf a
round opening, taking care not to cut through the exterior epidermis.
This door is to serve the moth for its exit, like that formed by
the wheat-caterpillar. But in proportion to its bulk its verdant
apartment is of considerable size. How then shall the moth know the
exact place where its outlet has been traced? How, without a clue,
shall it discover in its dark abode the precise circle which requires
only a push to throw it down? Even this is foreseen and provided
against. Out of twenty positions in which its hammock might have
been slung, the caterpillar has been directed so to place it, that
the silken cord that suspends the head is fastened close to the side
of the door which it has previously constructed; and the moth, guided
by this _filum ariadneum_, at once makes its way out of an apartment
which, but for this contrivance, might have been to it a labyrinth as
inextricable as that of Minos[659].
The mode in which other caterpillars provide for their extrication,
when become moths, from their silken cocoons, is not less ingenious.
Those of _Eriogaster lanestris_ (of which I have lately said so much,)
and others, form oblong cocoons, which, viewed externally, you would
at the first glance assert were of one solid piece: but on examining
them more narrowly, you perceive one end of them to be a distinct
lid, of a size large enough to permit the moth to issue out; and that
it is kept in its place by a few slight threads, easily broken by
pressure from within[660]. A few pages back[661] I mentioned a cocoon
formed by the larva of _Tortrix prasinana_, of the shape of a boat
reversed, composed of two inclined walls fastened together at the
top and ends. In constructing this cocoon, it firmly glues to each
other the top and one end, so as to form an impermeable suture; but
the other end, at which the moth is to issue, though externally it
seems as strong as the rest, is merely drawn close by a slender thread
or two fastened on the inside, and easily broken from within. And,
what is particularly singular in the construction of this ingenious
habitation, the sides forming the end last mentioned, though originally
requiring force to draw them into their required position, become
so elastic as to close again when the moth has passed between them
and made her escape; the cocoon preserving its usual shape, even when
deprived of its inhabitant[662]. A similar cocoon is constructed by
another leaf-rolling caterpillar, that of _Tortrix chlorana_[663]. Many
similar proofs of contrivance in the construction of silken cocoons
might be adduced, but I shall confine myself to one more only--I mean
that furnished by the flask-shaped brown one of _Saturnia Pavonia_,
and some other moths. If you examine one of these cocoons, which are
common enough in some places on the pear-tree or the willow, you will
perceive that it is generally of a solid tissue of layers of silk
almost of the texture of parchment; but at the narrow end, or that
which may be compared to the neck of the flask, that it is composed of
a series of loosely-attached longitudinal threads, converging, like so
many bristles, to a blunt point, in the middle of which is a circular
opening[664]. It is through this opening that the moth escapes. The
silk of its cocoon is of so strong a texture and so closely gummed,
that had both ends been similarly closed, its egress would have been
impracticable; it finds, however, no difficulty in forcing its way
through the aperture of a sort of reversed funnel, formed of converging
threads that readily yield to pressure from within. But an objection
will here probably strike you. You will ask, Is not this facility of
egress purchased at too dear a rate? Must not a chrysalis in an open
cocoon be exposed to the attacks of those ichneumons of which you
have said so much, and of numerous other enemies, which will find
admittance through this vaunted door? Our caterpillar would seem to
have foreseen your dilemma; at least, under heavenly guidance, she
has guarded against the danger as effectually as if she had. If you
cut open the cocoon longitudinally, you will see that within the
exterior funnel-shaped end, at some distance she has framed a second
funnel, composed of a similar circular series of stiff threads, which,
proceeding from the sides of the cocoon, converge also to a point, and
form a sort of cone exactly like the closed peristome of a moss; or,
to use a more humble though not less apt illustration, like the wires
of certain mousetraps[665]. In this dome not the slightest opening is
left, and from its arched structure it is impenetrable to the most
violent efforts of any marauders from without; whilst it yields to
the slightest pressure from within, and allows the egress of the moth
with the utmost facility. When she has passed through it, the elastic
threads resume their former position, and the empty cocoon presents
just the same appearance as one still inhabited. Rösel relates with
amusing naïvété how this circumstance puzzled him the first time he
witnessed it: he could scarcely help thinking that there was something
supernatural in the appearance of one of these fine moths in a box
in which he had put a cocoon of this kind, but in which he could not
discover the slightest appearance of any insect having escaped from
it, until he slit it longitudinally[666]. But from an observation of
Meinecken, it appears that these converging threads serve a double
purpose; being necessary to compress the abdomen of the moth as it
emerges from the cocoon, which forces the fluids to enter the nervures
of the wings, and give them their proper expansion. For he found, that
when the pupa is taken out of the cocoon, the moth is disclosed at the
proper time, but remains always crippled in its wings; which never
expand properly, unless the abdomen be compressed with the finger and
thumb, so as to imitate the natural operation[667].