precaution; but another large tribe of larvæ have recourse to further
manœuvres for their defence before they assume the pupa. Those of
the aphidivorous flies (_Syrphus_ F. &c.), of the various lady-birds
(_Coccinella_ L.), and tortoise-beetles (_Cassida_ L.), &c. fix
themselves by the anus with a gummy substance to the leaves or twigs
under which they propose to conceal themselves during their existence
in that state. Others previously suspend themselves by a silken thread
fixed to the tail, or passing round the body; by which also, when
become pupæ, they are afterwards pendent in a similar position; and
lastly, a very great number of larvæ wholly inclose themselves in cases
or cocoons, composed of silk and various other materials, by which
during their state of repose they are protected both from their enemies
and the action of the atmosphere. As these two last-mentioned processes
are extremely curious and interesting, I shall not fear tiring you by
entering into some further detail respecting them: explaining _first_
the mode by which larvæ _suspend_ themselves, both before and after
they are become pupæ, by silken threads; and _next_, the various
_cases_ or _cocoons_ in which others inclose themselves, and their
manner of operating in the formation of them.
1. The larvæ which suspend themselves and their pupæ, with the
exception of the tribe of _Alucitæ_, and some _Geometræ_ of the family
of _G. pendularia_, _punctaria_, &c. are almost all _butterflies_[493].
No others follow this mode. They may be divided into two great
classes--those which suspend themselves _perpendicularly_ by the
_tail_, and those which suspend themselves _horizontally_ by means of
a thread girthed round their _middle_. In both cases it should be
observed, that the suspension of the pupa is the object in view; but as
the process is the work of the larva, this seems the proper place for
explaining it. To begin with the _first_ case.
You are aware that all _lepidopterous_ larvæ have the faculty of
spinning silk threads from their mouths, and it will readily occur
to you that it is by means of these threads that they suspend
themselves. But how? How is a caterpillar to hang itself by the
tail to threads spun from the mouth? Even suppose this difficulty
overcome, others still greater remain. Suppose the caterpillar to be
suspended by its tail,--this is but a preparatory operation,--what
is required is, that the pupa shall hang in the same position:
now when you take into consideration that it is incased _within_
the skin of the larva, and without feet or other external organs;
that it has to extricate itself from this skin; to hang itself
in its place, and to detach the skin from the threads which hold
it--this will appear no trifling task. Indeed at first view it seems
impossible. Country-fellows for a prize sometimes amuse the assembled
inhabitants of a village by running races in sacks: take one of the
most active and adroit of these, bind him hand and foot, suspend him
by the bottom of his sack with his head downwards, to the branch
of a lofty tree; make an opening in one side of the sack, and set
him to extricate himself from it, to detach it from its hold, and
suspend himself by his feet in its place. Though endowed with the
suppleness of an Indian juggler, and promised his sack full of gold
for a reward, you would set him an absolute impossibility: yet this
is what our caterpillars, instructed by a beneficent Creator, easily
perform. Their manœuvres I shall now endeavour to explain.
When the caterpillar has selected the under-side of the leaf or
other object to which it purposes suspending itself, its first
process is to spin upon it a little hillock of silk consisting of
numerous loosely interwoven threads; it then bends its body so as to
insinuate the anal pair of prolegs amongst these threads, in which,
by a slight exertion, the little crochets which surround them[494]
become so strongly entangled as to support its weight with ease. It
now suffers the anterior part of the body to fall down, and it hangs
perpendicularly from its silken support with its head downwards. In
this position it remains often for twenty-four hours, at intervals
alternately contracting and dilating itself. At length the skin is
seen to split on the back near the head, and a portion of the pupa
appears, which by repeated swellings acts like a wedge, and rapidly
extends the slit towards the tail. By the continuance of these
alternate contractions and dilatations of the conical pupa, the skin
of the caterpillar is at last collected in folds near the tail, like
a stocking which we roll upon the ancle before withdrawing it from
the foot. But now comes the important operation. The pupa, being much
shorter than the caterpillar, is as yet at some distance from the
silken hillock on which it is to be fastened; it is supported merely
by the unsplit terminal portion of the latter's skin. How shall it
disengage itself from this remnant of its case, and be suspended in
the air while it climbs up to take its place? Without arms or legs
to support itself, the anxious spectator expects to see it fall to
the earth. His fears, however, are vain; the supple segments of the
pupa's abdomen serve in the place of arms. Between two of these,
as with a pair of pincers, it seizes on a portion of the skin; and
bending its body once more, entirely extricates its tail from it.
It is now wholly out of the skin, against one side of which it is
supported, but yet at some distance from the leaf. The next step it
must take is to climb up to the required height. For this purpose
it repeats the same ingenious manœuvre, making its cast-off skin
serve as a sort of ladder, it successively with different segments
seizes a higher and a higher portion, until in the end it reaches the
summit, where with its tail it feels for the silken threads that are
to support it. But how can the tail be fastened to them? you ask.
This difficulty has been provided against by Creative Wisdom. The
tail of the pupa is furnished with numerous little hooks pointing in
different directions[495], as well adapted to the end in view as the
crochets of the larva's prolegs, and some of these hooks are sure to
fasten themselves upon the silk the moment the tail is thrust amongst
it. Our pupa has now nearly completed its labours; it has withdrawn
its tail from the slough, climbed up it, and suspended itself to the
silken hillock--manœuvres so delicate and perilous, that we cannot
but admire that an insect which executes them but once in its life,
should execute them so well: nor could it, as Reaumur has well and
piously observed, had it not been instructed by a GREAT MASTER. One
more exertion remains: it seems to have as great an antipathy to
its cast-off skin, as one of us should, when newly clothed after
a long imprisonment, to the filthy prison garments we had put off.
It will not suffer this memento of its former state to remain near
it, and is no sooner suspended in security than it endeavours to
make it fall. For this end--it seizes, as it were with its tail, the
threads to which the skin is fastened, and then very rapidly whirls
itself round, often not fewer than twenty times. By this manœuvre
it generally succeeds in breaking them, and the skin falls down.
Sometimes, however, the first attempt fails: in that case, after a
moment's rest, it makes a second, twirling itself in an opposite
direction; and this is rarely unsuccessful. Yet now and then it is
forced to repeat its whirling, not less than four or five times:
and Reaumur has seen instances where the feet of the skin were so
firmly hooked, that after many fruitless efforts the pupa, as if in
despair, gave up the task and suffered it to remain[496]. After these
exertions, it hangs the remainder of its existence in this state
until the butterfly is disclosed.
We are now to consider the _second_ mode of suspension, in which
larvæ by means of a silken girth round their middle, fix themselves
_horizontally_ under leaves, &c. These follow the same process with
that of those last described, in spinning a small hillock of silk to
which they fasten their hind legs; and if the operation concerned
the larva state alone, this would be sufficient, as by means of this
support, and of their prolegs, they could easily retain themselves
in a horizontal position. But these larvæ act as if they foresaw
the assumption of a state in which they will be deprived of legs.
It is the suspension of the forthcoming pupa that is the object
in view; and though this can be hung by the tail in the same way
with those of the first class, yet it is plain that it cannot be
retained in a horizontal position, which for some unknown reason is
essential to it, without some support to its anterior extremity. It
is necessary for the larva, therefore, not only to fix its posterior
legs amongst a collection of silken fibres, but to spin a _girth_
of the same material round its body. This girth, though apparently
of a single thread, will be found on examination to be composed of
several, often as many as fifty or sixty; and is fastened on each
side of the body of the larva about the middle, to the surface under
which it is placed. Three different modes of fixing these girths
are adopted by the caterpillars of different butterflies. Some, as
those of the common cabbage-butterfly (_Pieris Brassicæ_), which have
remarkably pliable bodies, bend them almost double on one side, then
fix the thread and carry it over to the other in the same position,
repeating this operation as often as is necessary. Others, as that
of _Lycæna Argus_ and many more of the _Papiliones Rurales_ and
_Urbicolæ_ L., which have a short and more rigid body, after having
bent the head on one side so as to fix one end of the thread, bring
themselves into a straight position, and, by a manœuvre not easily
described, contrive to introduce the head under the thread, which
they then bend themselves to fasten on the other side, pushing it
to its proper situation by the successive tension and contraction
of their segments. But the most curious mode, though indeed that
which seems most natural, is adopted by the caterpillar of the
beautiful swallow-tail butterfly (_Papilio Machaon_) and others of
the same family. This first forms the loop which is to serve for
its girth, and then creeps under it. But the difficulty it has to
surmount is, to keep itself from being entangled in the fifty or
sixty fine distinct threads of which the girth is composed, and to
preserve them all extended so as to be able to introduce its body
beneath them. For this purpose it makes use of the two first pair of
its fore-legs, employing them as a woman does her hands in winding
a skein of cotton, to collect and keep all the threads of its card
unentangled and properly stretched; and it is often with great
difficulty, towards the end of the process, that it prevents them
from slipping off. When a sufficient number of threads is completed,
the animal bends its head between its legs, and insinuates it under
the collected loop, which by its annular contraction it easily pushes
to the middle of the body.
In about thirty hours after the larvæ which girth themselves have
finished their operations, the skin splits, and the pupa disengages
itself from it by those contractions and dilatations of its segments
which have been before described, pushing the exuviæ in folds to
the tail, by different motions of which it generally succeeds in
detaching them. One would have thought there would be considerable
difficulty in slipping the skin past the girth; but this, according
to Reaumur, seems to be easily effected[497].
If you are desirous of witnessing for yourself the manœuvres by
which these curious modes of suspension are effected, you may be
readily gratified. It is only necessary to collect and feed until
their metamorphosis the black spinous caterpillars of the common
peacock-butterfly (_Vanessa Io_), which in most places may be found
upon nettles, or those of the _Pieris Brassicæ_, which swarm in
cabbages or brocoli in every garden. The former will exhibit to you
a specimen of _vertical_, the latter of _horizontal_ suspension. It
should be observed, however, that to hit the precise moment when
these processes are going on, it is necessary to feed a considerable
number of the larvæ of each kind; some one of which, if you watch
them narrowly when they have attained their full growth, you will
scarcely fail to surprise in the act.
I must observe here, that although the vertical and horizontal are
the two principal positions in which caterpillars suspend themselves,
yet that others are inclined at various angles; and some are attached
with less art, appearing only to be fastened by some part of their
abdomen to the body upon which they are fixed[498].
2. The larvæ whose procedures I am in the next place to describe,
are those which, previously to assuming the pupa state, inclose
themselves in _cases_ or _cocoons_ of different materials. For the
sake of method, I shall divide these into two great classes: _First_,
those which form their cocoons entirely or principally of _silk_; and
_secondly_, those which form them chiefly of _other substances_.
To begin with the _first_. The larvæ which inclose themselves in
_silken_ cocoons are chiefly of the Lepidopterous tribes of _Bombycidæ_
and _Noctuidæ_; but a few _Geometræ_ (_G. papilioniaria_, _lactearia_,
&c.); most of the _Hymenoptera_; some _Coleoptera_, as certain of the
weevil tribe (_Hypera Arator_, _Rumicis_ Germ.), and those brilliant
beetles frequenting aquatic plants constituting the genus _Donacia_ F.;
the Neuropterous genera _Hemerobius_ and _Myrmeleon_; _Mycetophila_
and a few others in the _Diptera_; and _Pulex_ in the _Aphaniptera_
fabricate coverings of the same material. In all, with the exception
of _Myrmeleon_ and _Hemerobius_ (and perhaps _Hypera Rumicis_, &c.?)
which have their spinning apparatus at the extremity of the abdomen,
the silken thread employed in forming these coverings proceeds from
the middle part of the under-lip, as before explained; and is in fact
composed of two threads gummed together as they issue from the two
adjoining orifices of the spinner.
Of the larvæ which inclose themselves in _silk_, the most familiarly
known is the silk-worm: the cocoon of this consists exteriorly of a
thin, transparent, gauze-like coating, through the interstices of
which can be seen an inner, smaller, oval ball of a more close and
compact texture. The whole is in fact composed of one single thread,
but arranged in two distinct modes. To form the _exterior_ envelope,
which is merely the scaffolding by means of which the inner and
more solid covering is constructed, the caterpillar, after fixing
upon a space between two leaves or twigs or angles suitable for
its purpose, begins by glueing one end of its thread to one of the
adjoining surfaces. This thread it next conducts to another part and
then fastens, repeating this process and interlacing it in various
directions, until it has surrounded itself with a slight and loosely
spun netting. In the centre of this, when contracted into a space
sufficiently small, it lays the foundation of the _interior_ cocoon.
Fixing itself by its prolegs to some of the surrounding threads, it
bends its body, and by successive motions of its head from side to
side spins a layer of silk on the side opposite to it: when this
is of the requisite thickness, the larva shifts its position, and
repeats the same process in another quarter, covering each layer
in turn with a new one until the interior cavity is reduced to the
size desired. Thus, the silken thread which forms this new cocoon is
not, as might have been supposed, wound circularly as we wind the
thread of a ball of cotton; but backwards and forwards in a series
of zigzags, so as to compose a number of distinct layers. Malpighi
could distinguish six of these layers[499], and Reaumur suspects
there is often a greater number[500]. The former found the length of
the thread of silk composing them when wound off, without including
the exterior case, to be not less than 930 feet[501]; but others have
computed it at more than a thousand[502]: consequently the threads
of five cocoons united would be a mile in length. Estimating by the
weight,--the thread of a pound of cocoons, each of which weighs about
two grains and a half, would extend more than 600 miles[503], and
such is its tenuity, that the threads of five or six cocoons require
to be joined to form one of the thickness requisite in the silk
manufacture. It is the continuous thread of the inner cocoon which is
most valuable; the outer loose coating from its irregularity cannot
be wound off, and is known in commerce by the name of _floss silk_.
Manœuvres in their general principle similar to those of the
silk-worm are followed by most of those larvæ which inclose
themselves in silken cocoons. Many species, however, adopt variations
in the mode of procedure all of which it would be tedious to
particularize, but some of them are worth mentioning. The larvæ of
_Tortrix prasinana_, and other species of moths which form cocoons
resembling a reversed boat, arrange their threads in layers, so as to
construct two parallel walls gradually inclining towards the top and
ends, where they finally force them to approach each other by means
of an apparatus of silken cables[504]. And the larva of _Saturnia
Pavonia_, though it forms the base of its flask-shaped cocoon by
spinning like the silk-worm a number of interwoven zigzags, places
the threads which compose the interior funnel-like opening of the
apex nearly straight, parallel to each other, and converging towards
the same point in the centre[505].
These last, as well as almost all larvæ, constantly remain in the
_inside_ of the cocoon during its construction. But De Geer has given
us the history of a minute caterpillar of a species of moth (_Tinea_
L.) which feeds on the under side of the leaves of the _Rhamnus
Frangula_, or Black Alder, that actually weaves half of its cocoon
on the _outside_. This cocoon, which is very small, is beautifully
fluted, consisting of several longitudinal cords, with the intervals
filled by fine net-work, and shaped like a reversed boat[506]. The
animal begins by laying the foundations of one of the ends of
her cocoon, she adds new threads to this small beginning, and so
proceeds. As the work advances she retreats backwards, and her body
is situated nearly in the same line with the cocoon she has begun,
and quite _out_ of it; she only touches with her head and legs its
anterior margin. When half the cocoon, or rather of its exterior
layer, is finished, she suspends her operations for some moments. She
then for the first time introduces her head _into_ this demi-cocoon,
and turns herself in it by doubling her supple body, and passing
one part over the other, so that at last she manages to bring her
tail into the pointed end of the cocoon, the head and the anterior
half of her body remaining without. Thus situated, she commences her
operations afresh. At a distance from the margin of the demi-cocoon,
equal to its length, she begins to spin the pointed end of the other
moiety, the length of her body serving her as a measure that enables
her to begin at the proper distance from it. This new portion she
spins in the same manner as the other; but as she is prevented by the
demi-cocoon in which the posterior part of her body is lodged from
retreating backwards, she contracts her body more, which answers the
same purpose. When the new work is so advanced that she can no longer
contract her body, she bends the anterior part of it considerably,
and reverses her head. When the distance between the margin of the
two halves of the cocoon is very small, so as no longer to admit the
head between them, in order to unite them she is obliged to have
recourse to another manœuvre. Withdrawing her head, she extends
silken longitudinal threads between the two margins, and thus unites
them. This part is more clumsy, and not so regular as the rest
of the cocoon, so that the point of union is always discoverable.
These caterpillars do not always divide the cocoon into two _equal_
portions, for often they will finish three quarters of the cocoon
before they enter it, and begin at the other end[507].
The general rule is,--that each larva spins for itself a separate
cocoon; but amongst those of _Arctia chrysorhea_ and others which
live in society, two or three sometimes begin their operations so
close together that they are under the necessity of forming one
common cocoon, which serves for a covering to the whole number.
The same thing happens to silk-worms, the double or treble cocoons
of which are called _Dupions_ by the breeders. The larvæ of some
Ichneumons, besides forming each its separate cocoon, spin a joint
cottony covering for the whole[508], which is effected thus:--After
they leave the caterpillar they have devoured, they fix themselves
side by side at a little distance from it, and begin to spin each a
cocoon; and in order to defend its end and side that is not covered
by others, they spin further an envelope of loose silk, and thus this
exterior covering is formed.
The size, figure, colour, substance, and texture of silken cocoons
are extremely various. Their _size_ indeed is usually proportioned
to that of the included larva or pupa; yet it is by no means always
so. Some large caterpillars spin cocoons so small, that the observer
can hardly conceive how they can be contained in so narrow a compass:
_Eriogaster Catax_ is a moth of this description[509]. And others
smaller in size lodge themselves in apartments apparently much
more spacious than necessary. The transparent hammock-like cocoons
of _Hepialus Humuli_ and _Arctia villica_, two other moths, would
contain several of their pupæ. I possess one in which the pupa is
suspended in the centre, that is ten times its size, and not very
short in dimensions of that of _Attacus Paphia_, a giant silk-moth.
The largest cocoon I ever read or heard of, is that thus described by
Mr. Hobhouse in his _Travels_: "Depending," says he, "from the boughs
of the pines, near the Attic mountain Parnes, and stretching across
from tree to tree so as to obstruct our passage, were the pods,
_thrice as big as a turkey's egg!_ and the thick webs of a chrysalis,
whose moth must be far larger than any of those in our country."[510]
If this statement is correct, and I am not aware that there is any
reason for doubting it, the cocoon must be vastly larger than the
pupa, or the moth it produced would far exceed in size any yet known.
Perhaps, however, as this gentleman is probably no entomologist,
what he took for a cocoon might be a nidus, in which many larvæ were
associated, of the nature of those formerly described[511].
With regard to _figure_, the majority are like those of the
silk-worm, of a shape more or less oval or elliptic: some, however,
vary from this. That of _Lasiocampa Rubi_ is oblong. I have one from
New Holland somewhat resembling an acorn, fixed to the twigs of
some tree or shrub, of a very close silk, and covered by a circular
operculum, which the animal pushes off when it assumes the imago;
this is ovate or conico-ovate; others again are globose[512]; others
are conical[513], as that of _Gastropacha quercifolia_; others
almost fusiform[514] (_Odenesis potatoria_). Reaumur received
one from Arabia which was nearly cylindrical[515]. Those of _T.
prasinana_ before noticed, and many other _Tortrices_, are shaped
like a reversed boat[516]; that of _Saturnia Pavonia_ and others of
the same tribe, like a Florence flask with a wide and short neck. The
cocoon of _Lygæna Filipendulæ_ resembles a grain of barley. Another
cocoon in my cabinet, of very slight network, is shaped something
like an air-balloon. But the most remarkable one for its form and
characters, is one that I received from the rich cabinet above
quoted. This, which is of an unusually close texture, is suspended by
a thread more than two inches long from the point of a leaf; it then
swells into a perfect cone, at the base about four-fifths of an inch
in diameter and half an inch in length, and covered with scattered
setiform appendages: from the centre of the base projects a large
hemispherical protuberance, which terminates in a long stalk, much
thicker than the thread that suspends the cocoon. There is commonly
no difference between the shape of cocoons spun by larvæ which are
to give birth to different sexes of the same species. The silk-worm
cocoons, however, which will produce _male_ moths, have more silk at
the ends, and consequently are more round than those which are to
produce _females_: but the difference is not striking.