or the intermediate shades. The whites are very pure in the general
envelope of some species of _Ichneumonidæ_, and yellows often very
brilliant. But besides these more general colours, some cocoons are
black[517], some few blue or green, and others red[518]. Sometimes
the same cocoon is of two different colours. Those of certain
parasites of the tribe of _Ichneumones minuti_ L. the motions of one
of which I noticed on a former occasion[519], are alternately banded
with black or brown and white, or have only a pale or white belt in
the middle, which gives them a singular appearance. In both cases
the difference in colour depends upon the different tints with which
the silky gum is imbued in the reservoirs: the first portion of it
is white, and with this the larva first sketches the outline of its
cocoon, and then thickens the layers of silk considerably in those
parts where the white bands appear: when these are finished, its
stock of white silk is exhausted, and the remainder of the interior
of the cocoon is composed of brown silk[520]. The circular operculum
above mentioned as covering an acorn-shaped cocoon, is paler than
the latter, and also ornamented by a zone within the margin of deep
brown. The pale cocoon also of _Attacus Paphia_ is veined with silk
of a deep red.
I have very little to say on the _substance_ of the silk of cocoons.
Though that of the silk-worm is composed of such a slender thread, that
of many others is still finer, scarcely yielding in tenuity to the
spider's web. On the other hand, the silk of the cocoons of _Saturnia
Pavonia_ and of several foreign species is as thick as a hair.
With regard to the _texture_ of their cocoons--in some, as in that
of the silk-worm, the threads are so slightly glued to each other,
as to separate with facility; but in that of the emperor-moth just
mentioned they are intimately connected by a gummy matter, furnished,
as Reaumur conjectures, from the anus[521], with which the whole
interior of the cocoon is often plastered. Some, as that of the
silk-worm, are composed of an exterior loose envelope, and an inner
compact ball; others have no exterior covering, the whole cocoon
being of an uniform and thick texture. The larva of _Cossus Robiniæ_
Peck, in spinning its cocoon, makes the end next the opening to the
air, by which the imago is to emerge, of a slighter texture than
the rest of it[522]. The exterior case is sometimes, as in _Laria
pudibunda_, very closely woven, so as to resemble a real cocoon[523]:
its form is usually adapted to that of the inner one; but in some
which fix them under flat surfaces (_Laria fascelina_, _Callimorpha
Caja_,) it resembles a hammock[524]. Cocoons of a close texture have
generally no orifice in any part; but that of _Eriogaster lanestris_
is spun with openings, as if bored from without, the use of which,
however, does not seem to have been ascertained[525].
Many silken cocoons are of so close a fabric, as, when finished,
entirely to conceal the included insect; but a very considerable number
are of a more open texture, composed of a much smaller quantity of
silk, and that woven so loosely, that the larva or pupa may always
be discovered through it. Of this description are the cocoons of
_Hypogymna dispar_, _Arctia Salicis_, &c., which consist only of a few
slight meshes. Those of some others resemble gauze or lace[526]. Of
the first description is one in my cabinet before alluded to, shaped
somewhat like an air-balloon; the meshes are large and perfectly
square. The pupa hangs in the centre, fixed by some few slight threads
which diverge from it to all parts of the cocoon--so that it looks as
if it was suspended in the air, like Mahomet's coffin, without support.
Of the second description is a black one with very fine and nearly
circular meshes: the threads that form these are thick, and seem to be
agglutinated. In our own country, the cocoons of some beetles, as of
_Hypera Arator_, _Galeruca Tanaceti_, and of some little _Tineæ_, also
resemble gauze. Many of the larvæ, however, which spin these cocoons,
whose thinness is probably attributable to the smallness of their stock
of silk, seem anxious for a more complete concealment; and therefore
commonly either hide them between leaves tied together, in some with
a certain regularity, in others without art[527]; or thicken their
texture, and render it opaque, by the addition of grains of earth[528],
or of other materials with which their bodies supply them. These are
principally of two kinds. The larvæ of _Lasiocampa Neustria_, _Arctia
Salicis_, &c. after spinning their cocoons, cast from their anus three
or four masses of a soft and paste-like matter, which they apply with
their head all round the inside of the cavity; and which, drying in a
short time, becomes a powder that effectually renders it opake. This
is not, as might be conjectured, an excrement, but a true secretion,
evidently intended for this very purpose: and, according to Reaumur,
a similar powder, but white, derived from the varicose intestines, is
used by the caterpillars of _Gastropacha quercifolia_, &c.[529] The
other material, which is still more frequently employed, and which is
occasionally mixed with the former, is the _hair_ which everyone has
observed to cover so thickly the bodies of some caterpillars. This,
after spinning a sufficient envelope, they tear, or in some instances
cut off with their mandibles, and distribute all round them, pushing
it with their head amongst the interstices of the silk, so as to make
the whole of a regular thick texture. After this process, which leaves
the body completely denuded, and often seems to give them great pain,
they conclude by spinning another tissue of slight silk, in order to
protect the forthcoming pupa from the surrounding prickly points. It
should be observed, however, that though many hairy larvæ, as those
of _Noctua Aceris_, _Arctia Caja_, and others, employ their hairs in
the composition of their cocoons, the rule is not general, several
never making any such use of them. Nor do all that do so employ them
distribute them in the same manner as those above described, which
rarely attempt to arrange them in any regular position. Reaumur has
noticed a small hairy caterpillar that feeds on lichens, which is more
methodical: this actually places its hairs upright, side by side, as
regularly as the pales in a palisade, in an oval ring around its body,
connecting them by a slight tissue of silk, which forces them to bend
into a sort of roof at the top; and under this curiously-formed cocoon
assumes its state of pupa[530]. Some larvæ make so much hair and so
little silk enter into the composition of their cocoons, that on the
first inspection they would be pronounced wholly composed of it[531];
others, thickening the interior of their cocoon with hair, line the
whole with a viscid matter like varnish[532].
The larvæ of some saw-flies (_Tenthredo_ L.) are remarkable for
inclosing themselves in a double cocoon, in which the inner is not,
as in the silk-worm &c., connected with the outer, but perfectly
distinct from it. Some species, as _T. Rosæ_ (_Cryptus_ Jur.), which
have but a small stock of silk, compose the outer cocoon of thick
silken cords crossing at right angles, and forming an oval net;
which at the same time that it protects them effectually from the
ants, which are always ready to attack them, demands much less silk
than a covering of a closer texture. But the tender nymph itself
requires to be inclosed in a case of a softer and more delicate
substance; and accordingly the inner cocoon is composed of fine silk,
woven so closely that the threads are scarcely perceptible under a
microscope[533]. Reaumur mentions a hymenopterous larva belonging
to Latreille's _Fossores_ (_Sphex_ L.) which thickened its cocoon
with the legs, wings, and other relics of the flies which it had
devoured[534]: trophies--like the drinking-cups of some savages, made
of the skulls of their enemies, or the skull pyramid near Ispahan--of
its powers of devastation.
It is a general rule, that those larvæ which spin cocoons, never
in ordinary circumstances become pupæ without having thus inclosed
themselves. An exception, however, is met with in the larva of
a species of ant noticed by De Geer (_Formica fusca_ L.), some
of the individuals of which inclose themselves in cocoons; while
others neglect this precaution, and undergo their metamorphosis
uncovered[535]. Rösel also made nearly the same observation on the
larva of the flea[536].
I must say something with regard to the _situation_, often very
remote from their place of feeding, in which larvæ fabricate their
cocoons. A very considerable number, probably the majority, form
them either partially (_Arctia lubricipeda_) or wholly under ground;
others beneath dead leaves, moss, or in the chinks of the trees;
others within the wood in substances on which they have fed; the
larva of _Cossus_ leaves in these a communication with the open air
by which the imago emerges; and a large number attach them to the
leaves and branches of trees and plants; the cocoon of _Donacia
fasciata_(?) is fastened by one side to the roots or surculi of
_Typha latifolia_. There is usually nothing very remarkable in
the mode of fixing them, the exterior threads being merely gummed
irregularly to different portions of the objects which support them.
But some effect this with greater art. I have one from New Holland,
very long, which is suspended from a twig by a long riband, as it
were, which entirely girths the twig. The larva of the magnificent
silk-moth, _Attacus Paphia_, actually forms a solid silken stalk
to its cocoon, an inch and half in length and a line in diameter,
fastened by the other extremity to a twig, which it closely surrounds
as if with a ring, at first sight resembling a fruit of a very
singular appearance[537]. I have specimens of this cocoon with both
stalk and ring. A bell-shaped cocoon fastened by a foot-stalk, but
of softer consistence, to a blade of grass, found by Mr. Sheppard, I
can also show you; and my friend Mr. Wilkin had a similar one out of
the late Mr. Hudson's collection. Most larvæ spin their cocoons in
solitude: some of those, however, which live in society do it close
together under their common tent.
There are other cocoons that should be noticed here, such as those
formed by the larva of _Zygæna Filipendulæ_, and some _Bombyces_,
saw-flies (_Tenthredo_ L.), and beetles (_Curculio_, _Donacia_ F.),
&c. These are formed of a substance which seems more analogous to gum
than silk, yet furnished from the silk reservoirs, and usually present
the appearance externally of parchment or membrane. That of the insect
first mentioned is coated, however, with a slight interior silken
lining; as indeed are almost all cocoons, of whatever substance.
* * * * *
The _second_ class, into which I have divided larvæ that inclose
themselves in cocoons, includes those which form their coverings
not solely or principally of silk, but in which other materials are
mixed more or less. The cocoons of some of these larvæ are merely
composed of a few leaves slightly tied together, either irregularly,
or arranged, particularly when they are of a linear figure, with
considerable symmetry. The grubs of many beetles, as of the
rose-beetle, _Cetonia aurata_, &c., prepare themselves a cocoon,
composed of earth, pieces of rotten wood, and any substances within
their reach: which they fasten together with a glutinous secretion.
The same material is employed by others in forming a cocoon wholly
of earth; which is sometimes, as that of the stag-beetle, _Lucanus
Cervus_, exceedingly hard; at others, as that of some moths, _Noctua
ambigua_, &c, so slight as to fall to pieces as soon as touched[538].
Other cocoons are formed of grains of earth. Reaumur has given a
very interesting account of the procedures of a larva in repairing
one of these cocoons, from which he had broken off the top when just
completed. Without quitting the interior of the walls that remained,
it put out its head from the breach, and for more than an hour
employed itself in selecting one by one grains of earth, which it
conveyed with its mandibles and deposited within its case: it next
spun all round the opening threads of silk, to which it attached
grains of earth taken from the previously-stored heap, uniting them
compactly by means of other silken threads. After employing three
hours in this laborious process, the industrious little mason had
reduced the diameter of the breach to a few lines. Reaumur was very
curious to know how it would fill up this orifice, which would no
longer admit the protrusion of its head outside the walls, as in
its previous operations. He concluded, that while the rest of the
cocoon was exteriorly formed of earth, this opening would be merely
closed with silk. He was mistaken, however: the artist knew how to
vary its manœuvres, and make its vault of one uniform texture. It
spun across the opening a little net of silk, between the meshes of
which it thrust grains of earth so dexterously that they projected
as far as the outer surface, retained there probably by silken
lines previously attached and fastened within. It then finished its
habitation by fortifying the inside of the orifice with another layer
of earth[539]. The ant-lion (_Myrmeleon_) spins a globular cocoon
with its anus, which it covers with grains of sand[540]. One that I
took in the forest of Fontainebleau, in the quarry that produces the
crystallized sandstone called the _Fontainebleau fossil_, was covered
with large and shining grains. Instead of the grains of earth or sand
employed by these larvæ, those of another tribe substitute grains
of stone detached from the softer walls, upon whose lichens they
previously feed, which they unite into solid oval cocoons[541]. Those
of a fourth form their cocoons of patches of short moss arranged with
the roots downwards, and forming a vault, as it were, of verdant
turf, admirably adapted for concealment[542]. The larvæ of some moths
form their cocoons of irregular pieces of bark tied together with
silk, and resembling when completed a knotty protuberance of the twig
on which they are fixed. That of _Pyralis tuberculana_ constructs a
pannier-shaped one of the parenchyma of the leaves of plants[543].
All these cocoons, however, must yield in point of singularity of
construction, materials, and ingenuity, to one formed by a small
caterpillar, described by the illustrious naturalist lately quoted,
which feeds upon the oak. This cocoon is wholly composed of small
rectangular strap-shaped pieces of the fine upper skin, or epidermis
of the twig upon which it rests, regularly fastened to each other
in a longitudinal direction with very slender silken cords. But the
mode of its construction is even more remarkable than the substance
of which it is fabricated. The caterpillar's first process is to
form its slips of bark into two flat triangular wing-like pieces,
projecting opposite to each other from each side of the twig,
somewhat like the feathers of an arrow. It does not, perhaps, require
any great degree of intelligence in a larva to give its cocoon the
usual oval form, when it begins to arrange its materials in that
shape from the very first, and round so good a mould as its own bent
body; but we surely must admit that it is a task to which no stupid
artist would be competent, to form first a multitude of strap-shaped
laminæ into two triangular plates, and then to bend these plates
into a case resembling the longitudinal section of a cone, with an
elliptical and protuberant base,--the figure which the cocoon of this
insect assumes. All the minutiæ of the manœuvres which it employs in
this nice operation could not be comprehended without a more diffuse
explanation than I have here room to give: suffice it to say, that
the caterpillar fastens silken lines to each exterior opposite and
longer side of the laminæ, and by applying all the weight of its
body forces them to bend and approach each other, in which position
it secures them by other shorter lines. It next repeats the same
process with the upper and shorter sides of the plates; which when
joined form the base of the cocoon. Both these tasks are accomplished
in less than an hour, and the seams are so nicely joined as to be
imperceptible. A fine inner tapestry of silk, covering all the
asperities of the exterior walls, concludes its labours[544]. It
is to be lamented that Reaumur was unacquainted with the moth that
proceeds from the pupæ inclosed in these ingenious cocoons; which
being small, and precisely of the same colour as the bark of the
twig that supports them, are not to be discovered but by a very
narrow inspection. It would seem, however, to be _Noctua Strigula_
of Berkhausen, _Pyralis strigulalis_ of Hubner[545]. The larva, he
informs us, is found in May: its body is flatter than common, of
a yellowish flesh-colour, clothed with tufts of red hair on each
segment, and furnished with fourteen feet. Should this description
enable you to detect it upon your oaks, a view of its ingenious
procedures would amply repay you for the trouble of seeking for it.
The larvæ of _Cerura vinula_, _Stauropus Fagi_, and several other
moths, form their cocoons of grains of wood gnawed from the trees on
which they feed. These grains they masticate, mixed with a glutinous
fluid secreted from the mouth, into a paste, which forms a covering
of an uniform smooth texture, and so hard as not readily to yield
to a knife. Of a substance apparently nearly similar is composed
the cocoon of a weevil related to _Liparus Pini_; which with its
inhabitant was given me by the ingenious Mr. Bullock. A little moth,
whose ravages have been before noticed[546], lines the interior of
the grain of barley, of which it has devoured the contents, with
silk; divides it into two apartments, into one of which it pushes the
excrement it had voided, and in the other assumes the pupa[547].
These, and the other larvæ mentioned above, commonly form their
cocoons of the substances I have indicated; but when by any cause
they are prevented from access to them, they often substitute such
other materials as are at hand. Reaumur fed a larva that formed its
cocoon of minute fragments of paper, which with its mandibles it
had cut from the piece that covered the glass vessel that contained
it[548]: and the same circumstance happened to Bonnet.
Upon a former occasion I described to you the cases of various kinds
formed and inhabited by the insects of the _Trichoptera_ Order
(_Phryganea_ L.) commonly called case-worms[549]. As these serve
for the pupa as well as the larva, they may be regarded as a kind
of cocoon. I shall not repeat here what I then said; but having
purchased from the collection of the late Mr. Francillon some that
seem to belong to this or some cognate tribe, that are of a curious
construction, I shall give you some account of two or three of
them in this place. The first is not quite three inches long, of a
sublanceolate shape, but rather widest towards one end. It consists
of an internal tough and thick bag or cocoon, of a silk resembling
fine wool of a dirty white colour, which is closely covered
transversely by pieces of the stalk of a plant, about three-fourths
of an inch in length, and crossing each other at an obtuse angle.
The next is thicker and shorter: the internal bag is just covered
with small fragments of wood like sawdust; over these are fastened
irregularly, short stout pieces of a pithy stick or stalk, and the
whole is clothed with a very close-woven ash-coloured web. It seems
difficult to conceive how the inclosed animal could contrive to
cover her habitation with this web without going wholly out of it.
The third is the most curious and remarkable of all. It is nearly
six inches long, and about four-fifths of an inch in diameter. It
consists of a bag of thick cinereous silk web, to which are fastened,
in a sextuple series, pieces of stick about an inch long, the end of
one mostly resting upon the base of another: between each series a
space of about three-tenths of an inch intervenes, but at the apex
they all converge. This probably imitates the branch or stem of some
tree or plant, in which the leaves are linear, and diverge but little
from the stem. A label upon it states its country to be New Holland.
I suspect the inhabitants of the two last cocoons to be terrestrial
animals: the first is probably a true aquatic case-worm.
* * * * *
The same purpose for which the cocoons above described serve, is
answered in the case of numerous _Dipterous_ insects, by a humble and
less artificial contrivance--the skin, namely, of the larva; which,
as was before observed[550], is never cast, but, when the insect is
about to enter into the pupa state, assumes a different form and
colour; becomes of a thicker and more rigid texture; and defends the
included pupa, which is separate from it, till its exclusion. In
this case the mouth of the larva is constantly different from that
of the perfect insect, or at least has not with it those relations
as to number and kind of organs, which have been observed in the
mouth of other larvæ compared with the insects that they produce.
The animal, immediately after it is clothed with this skin, if it
is opened, exhibits only a soft gelatinous pulp, in the surface of
which the exterior organs of the adult insect cannot yet be detected.
Nature requires more time for their elaboration, or at least for
the appearance of their outline, and to consolidate them. This pulp
first takes an oblong form (_Boule allongée_ Reaum.), and afterwards
that of the insect it is destined to give birth to[551]. The skin of
the larva also serves for a cocoon to the pupæ of male _Cocci_[552].
The grub of the genus _Anthrenus_, so destructive to our cabinets
of natural objects[553], when it assumes the pupa does not quit
its skin, but only splits it open longitudinally on the back, and
when it becomes an imago makes its exit through the orifice[554].
Some Lepidopterous larvæ even (_Alucita pentadactyla_, _Callimorpha
rosea_, &c.) assume the pupa state within their last skin[555].
* * * * *