particular mention, although it is not remarkable in colour or figure.
It is without hair, of a greenish-white, and has all the vivacity of
the other leaf-rollers. Sorrel is the plant on which it feeds; and the
manner in which it rolls a portion of the leaf is very ingenious.
[Illustration: Leaf-rolling Caterpillars of the Sorrel.]
The structure which it contrives is a sort of conical pyramid, composed
of five or six folds lapped round each other. From the position of this
little cone the caterpillar has other labours to perform, beside that
of rolling the leaf. It first cuts across the leaf, its teeth acting
as a pair of scissors; but it does not entirely detach this segment.
It rolls it up very gradually, by attaching threads of silk to the
plane surface of the leaf, as we have before seen; and then, having
cut in a different direction, sets the cone upright, by weaving other
threads, attached to the centre of the roll and the plane of the leaf,
upon which it throws the weight of its body. This, it will be readily
seen, is a somewhat complicated effort of mechanical skill. It has been
minutely described by M. Réaumur; but the preceding representation will
perhaps make the process clearer than a more detailed account.
This caterpillar, like those of which we have already spoken, devours
all the interior of the roll. It weaves, also, in the interior, a small
and thin cocoon of white silk, the tissue of which is made compact and
close. It is then transformed into a chrysalis.
[Illustration: Nests of the Hesperia malvæ, with Caterpillar,
Chrysalis, and Butterflies.]
The caterpillars of two of our largest and handsomest butterflies,
the painted lady (_Cynthia cardui_, Stephens), and the admiral, or
_Alderman_ of the London fly-fanciers (_Vanessa atalanta_), are also
leaf-rollers. The first selects the leaves of the great spear-thistle,
and sometimes those of the stemless or star-thistle, which might be
supposed rather difficult to bend; but the caterpillar is four times
as large and strong as those which we have been hitherto describing. In
some seasons it is plentiful; in others it is rarely to be met with:
but the admiral is seldom scarce in any part of the country; and by
examining the leaves of nettles which appear folded edge to edge, in
July and August, the caterpillar may be readily found.
Another butterfly (_Hesperia malvæ_) is met with on dry banks where
mallows grow, in May, or even earlier, and also in August, but is not
indigenous. The caterpillar, which is grey, with a black head, and
four sulphur-coloured spots on the neck, folds around it the leaves of
the mallow, upon which it feeds. There is nothing, however, peculiarly
different in its proceedings from those above described; but the care
with which it selects and rolls up one of the smaller leaves, when it
is about to be transformed into a chrysalis, is worthy of remark; it
joins it, indeed, so completely round and round, that it has somewhat
the resemblance of an egg. Within this green cell it lies secure, till
the time arrives when it is ready to burst its cerements, and trust to
the quickness of its wings for protection against its enemies.
Among the nests of caterpillars which roll up parcels of leaves, we
know none so well contrived as those which are found upon willows and
a species of osier. The long and narrow leaves of these plants are
naturally adapted to be adjusted parallel to each other; for this is
the direction which they have at the end of each stalk, when they are
not entirely developed. One kind of small smooth caterpillar (_Tortrix
chlorana_), with sixteen feet, the under part of which is brown, and
streaked with white, fastens these leaves together, and makes them up
into parcels. There is nothing particularly striking in the mechanical
manner in which it constructs them. It does precisely what we should do
in a similar case: it winds a thread round those leaves which must be
kept together, from a little above their termination to a very short
distance from their extreme point; and as it finds the leaves almost
constantly lying near each other, it has little difficulty in bringing
them together, as is shown in the following cut, _a_.
The prettiest of these parcels are those which are made upon a kind
of osier, the borders of whose leaves sometimes form columnar bundles
before they become developed. A section of these leaves has the
appearance of filigree-work (see _b_).
[Illustration: Nest of Willow-leaf Roller.]
A caterpillar which feeds upon the willow, and whose singular attitudes
have obtained for it the trivial name of _Ziczac_, also constructs
for itself an arbour of the leaves, by drawing them together in an
ingenious manner. M. Roesel[BK] has given a tolerable representation of
this nest, and of the caterpillar. The caterpillar is found in June;
and the moth (_Notodonta ziczac_) from May to July in the following
year (see cut, p. 151).
Beside those caterpillars which live solitary in the folds of a leaf,
there are others which associate, employing their united powers to
draw the leaves of the plants they feed upon into a covering for
their common protection. Among these we may mention the caterpillar
of a small butterfly, the plantain or Glanville fritillary (_Melitea
cinxia_), which is very scarce in this country.
[Illustration: Ziczac Caterpillar and Nest.]
Although a colony of these caterpillars is not numerous, seldom
amounting to a hundred individuals, the place which they have selected
is not hard to discover. Their abode may be seen in the meadow
in form of a tuft of herbage covered with a white web, which may
readily be mistaken, at first view, for that of a spider, but closer
inspection soon corrects this notion. It is, in fact, a sort of common
tent, in which the whole brood lives, eats, and undergoes the usual
transformations. The shape of this tent, for the most part, approaches
the pyramidal, though that depends much upon the natural growth of the
herbage which composes it. The interior is divided into compartments
formed by the union of several small tents, as it were, to which others
have been from time to time added according to the necessities of the
community.
When they have devoured all the leaves, or at least those which are
most tender and succulent, they abandon their first camp, and construct
another contiguous to it under a tuft of fresh leaves. Several of these
encampments may sometimes be seen within the distance of a foot or
two, when they can find plantain (_Plantago lanceolata_) fit for their
purpose; but though they prefer this plant, they content themselves
with grass if it is not to be procured.
When they are about to cast their skins, but particularly when they
perceive the approach of winter, they construct a more durable
apartment in the interior of their principal tent. The ordinary web is
thin and semi-transparent, permitting the leaves to be seen through it;
but their winter canvas, if we may call it so, is thick, strong, and
quite opaque, forming a sort of circular hall without any partition,
where the whole community lie coiled up and huddled together.
Early in spring they issue forth in search of fresh food, and again
construct tents to protect them from cold and rain, and from the
mid-day sun.
M. Réaumur found upon trial, that it was not only the caterpillars
hatched from the eggs of the same mother which would unite in
constructing the common tent; for different broods, when put together,
worked in the same social and harmonious manner. We ourselves
ascertained, during the present summer (1829), that this principle
of sociality is not confined to the same species, nor even to the
same genus. The experiment which we tried was to confine two broods
of different species to the same branch, by placing it in a glass of
water to prevent their escape. The caterpillars which we experimented
on were several broods of the brown-tail moth (_Porthesia auriflua_)
and the lackey (_Clisiocampa neustria_). These we found to work with
as much industry and harmony in constructing the common tent as if
they had been at liberty on their native trees; and when the lackeys
encountered the brown-tails they manifested no alarm nor uneasiness,
but passed over the backs of one another, as if they had made only a
portion of the branch. In none of their operations did they seem to
be subject to any discipline, each individual appearing to work, in
perfecting the structure, from individual instinct, in the same manner
as was remarked by M. Huber in the case of the hive-bees. In making
such experiments, it is obvious that the species of caterpillars
experimented with must feed upon the same sort of plant.[BL] (J. R.)
The design of the caterpillars in rolling up the leaves is not only
to conceal themselves from birds and predatory insects, but also to
protect themselves from the cuckoo-flies, which lie in wait in every
quarter to deposit their eggs in their bodies, that their progeny may
devour them. Their mode of concealment, however, though it appear to be
cunningly contrived and skilfully executed, is not always successful,
their enemies often discovering their hiding-place. We happened to see
a remarkable instance of this last summer (1828), in the case of one of
the lilac caterpillars which had changed into a chrysalis within the
closely-folded leaf. A small ichneumon, aware it should seem of the
very spot where the chrysalis lay within this leaf, was seen boring
through it with her ovipositor, and introducing her eggs through the
punctures thus made into the body of the dormant insect. We allowed
her to lay all her eggs, about six in number, and then put the leaf
under an inverted glass. In a few days the eggs of the cuckoo-fly were
hatched, the grubs devoured the lilac chrysalis, and finally changed
into pupæ in a case of yellow silk, and into perfect insects like their
parents. (J. R.)
_CHAPTER IX._
INSECTS FORMING HABITATIONS OF DETACHED LEAVES.
The habitations of the insects which we have just described consist
of growing leaves, bent, rolled, or pressed together, and fixed in
their positions by silken threads. But there are other habitations of
a similar kind, which are constructed by cutting out and detaching
a whole leaf, or a portion of a leaf. We have already seen how
dexterously the upholsterer-bees cut out small parts of leaves and
petals with their mandibles, and fit them into their cells. Some of
the caterpillars do not exhibit quite so much neatness and elegance as
the leaf-cutting bees, though their structures answer all the purposes
intended; but there are others, as we shall presently see, that far
excel the bees, at least in the delicate minutiæ of their workmanship.
We shall first advert to those structures which are the most simple.
* * * * *
Not far from Longchamps, in a road through the Bois de Boulogne,
is a large marsh, which M. Réaumur never observed to be in a dry
state even during summer. This marsh is surrounded with very lofty
oaks, and abounds with pondweed, the water-plant named by botanists
_potamogeton_. The shining leaves of this plant, which are as large as
those of the laurel or orange-tree, but thicker and more fleshy, are
spread upon the surface of the water. Having pulled up several of these
about the middle of June, M. Réaumur observed, beneath one of the first
which he examined, an elevation of an oval shape, which was formed out
of a leaf of the same plant. He carefully examined it, and discovered
that threads of silk were attached to this elevation. Breaking the
threads, he raised up one of the ends, and saw a cavity, in which a
caterpillar (_Hydrocampa potamogeta_) was lodged. An indefatigable
observer, such as M. Réaumur, would naturally follow up this discovery;
and he has accordingly given us a memoir of the pondweed tent-maker,
distinguished by his usual minute accuracy.
In order to make a new habitation, the caterpillar fastens itself on
the under side of a leaf of the _Potamogeton_. With its mandibles it
pierces some part of this leaf, and afterwards gradually gnaws a curved
line, marking the form of the piece which it wishes to detach. When the
caterpillar has cut off, as from a piece of cloth, a patch of leaf of
the size and shape suited to its purpose, it is provided with half of
the materials requisite for making a tent. It takes hold of this piece
by its mandibles, and conveys it to the situation on the under side of
its own or another leaf, whichever is found most appropriate. It is
there disposed in such a manner that the under part of the patch--the
side which was the under part of the entire leaf--is turned towards the
under part of the new leaf, so that the inner walls of the cell or tent
are always made by the under part of two portions of leaf. The leaves
of the potamogeton are a little concave on the under side; and thus the
caterpillar produces a hollow cell, though the rims are united.
The caterpillar secures the leaf in its position by threads of white
silk. It then weaves in the cavity a cocoon, which is somewhat thin,
but of very close tissue. There it shuts itself up, to appear again
only in the form of the perfect insect, and is soon transformed into a
chrysalis. In this cocoon of silk no point touches the water; whilst
the tent of leaves, lined with silk, has been constructed underneath
the water. This fact proves that the caterpillar has a particular art
by which it repels the water from between the leaves.
When the caterpillar, which has thus conveyed and disposed a patch
of leaf against another leaf, is not ready to be transformed into a
chrysalis, it applies itself to make a tent or habitation which it
may carry everywhere about with it. It begins by slightly fixing the
piece against the perfect leaf, leaving intervals all round, between
the piece and leaf, at which it may project its head. The piece which
it has fixed serves as a model for cutting out a similar piece in the
other leaf. The caterpillar puts them accurately together, except at
one end of the oval, where an opening is left for the insect to project
its head through. When the caterpillar is inclined to change its
situation, it draws itself forward by means of its scaly limbs, riveted
upon the leaf. The membraneous limbs, which are riveted against the
inner sides of the tents, oblige it to follow the anterior part of the
body, as it advances. The caterpillar, also, puts its head out of the
tent every time it desires to eat.
There is found on the common chickweed (_Stellaria media_), towards
the end of July, a middle-sized smooth green caterpillar, having three
brown spots bordered with white on the back, and six legs and ten
pro-legs, whose architecture is worthy of observation. When it is about
to go into chrysalis, towards the beginning of August, it gnaws off,
one by one, a number of the leaves and smaller twigs of the chickweed,
and adjusts them into an oval cocoon, somewhat rough and unfinished
externally, but smooth, uniform, and finely tapestried with white silk
within. Here it undergoes its transformation securely, and, when the
period of its pupa trance has expired in the following July, it makes
its exit in the form of a yellowish moth, with several brown spots
above, and a brown band on each of its four wings below. It is also
furnished with a sort of tail.
On the cypress-spurge (_Euphorbia cyparissias_), a native woodland
plant, but not of very common occurrence, may be found, towards the
end of October, a caterpillar of a middle size, sparely tufted with
hair, and striped with black, white, red, and brown. The leaves of the
plant, which are in the form of short narrow blades of grass, are made
choice of by the caterpillar to construct its cocoon, which it does
with great neatness and regularity, the end of each leaf, after it has
been detached from the plant, being fixed to the stem, and the other
leaves placed parallel, as they are successively added. The other ends
of all these are bent inwards, so as to form a uniformly rounded oblong
figure, somewhat larger at one end than at the other.
A caterpillar which builds a very similar cocoon to the last-mentioned
may be found upon a more common plant--the yellow snap-dragon or
toad-flax (_Antirrhinum linaria_)--which is to be seen in almost every
hedge. It is somewhat shaped like a leech, is of a middle size, and the
prevailing colour pearl-grey, but striped with yellow and black. It
spins up about the beginning of September, forming the outer coating of
pieces of detached leaves of the plant, and sometimes of whole leaves
placed longitudinally, the whole disposed with great symmetry and
neatness. The moth appears in the following June.
[Illustration: Cypress-Spurge Caterpillar--(_Acronycta
Euphrasiæ_)--with a Cocoon, on a branch.]
It is worthy of remark, as one of the most striking instances of
instinctive foresight, that the caterpillars which build structures of
this substantial description are destined to lie much longer in their
chrysalis trance than those which spin merely a flimsy web of silk. For
the most part, indeed, the latter undergo their final transformation
in a few weeks; while the former continue entranced the larger portion
of a year, appearing in the perfect state the summer after their
architectural labours have been completed. (J. R.) This is a remarkable
example of the instinct which leads these little creatures to act as
if under the dictates of prudence, and with a perfect knowledge of the
time, be it long or short, which will elapse before the last change of
the pupa takes place. That the caterpillar, while weaving its cocoon
and preparing to assume the pupa state, exercises any reflective
faculties, or is aware of what is about to occur relative to its own
self, we cannot admit. It enters upon a work of which it has had no
previous experience, and which is performed, as far as contingencies
allow, in the same manner by every caterpillar of the same species.
Its labours, its mode of carrying them on, and the very time in which
they are to be commenced, are all pre-appointed; and an instinctive
impulse urges and guides; and with this instinct its organic endowments
are in precise harmony; nor does instinct ever impel to labours for
which an animal is not provided. "The same wisdom," says Bonnet, "which
has constructed and arranged with so much art the various organs of
animals, and has made them concur towards one determined end, has also
provided that the different operations which are the natural results
of the economy of the animal should concur towards the same end. The
creature is directed towards his object by an invisible hand; he
executes with precision, and by one effort, those works which we so
much admire; he appears to act as if he reasoned, to return to his
labour at the proper time, to change his scheme in case of need. But in
all this he only obeys the secret influence which drives him on. He is
but an instrument which cannot judge of each action, but is wound up
by that adorable Intelligence, which has traced out for every insect
its proper labours, as he has traced the orbit of each planet. When,
therefore, I see an insect working at the construction of a nest, or a
cocoon, I am impressed with respect, because it seems to me that I am
at a spectacle where the Supreme Artist is hid behind the curtain."[BM]
There is a small sort of caterpillar which may be found on old walls,
feeding upon minute mosses and lichens, the proceedings of which are
well worthy of attention. They are similar, in appearance and size, to
the caterpillar of the small cabbage-butterfly (_Pontia rapæ_), and
are smooth and bluish. The material which they use in building their
cocoons is composed of the leaves and branchlets of green moss, which
they cut into suitable pieces, detaching at the same time along with
them a portion of the earth in which they grow. They arrange these upon
the walls of their building, with the moss on the outside, and the
earth on the inside, making a sort of vault of the tiny bits of green
moss turf, dug from the surface of the wall. So neatly, also, are the
several pieces joined, that the whole might well be supposed to be a
patch of moss which had grown in form of an oval tuft, a little more
elevated than the rest growing on the wall. When these caterpillars are
shut up in a box with some moss, without earth, they construct with it
cells in form of a hollow ball, very prettily plaited and interwoven.
[Illustration: Moss-Cell of small Caterpillar (_Bryophila perla?_)]
In May last (1829), we found on the walls of Greenwich Park a great
number of caterpillars, whose manners bore some resemblance to those
of the grub described by M. Réaumur. (J. R.) They were of middle size,
with a dull-orange stripe along the back; the head and sides of the
body black, and the belly greenish. Their abodes were constructed with
ingenuity and care. A caterpillar of this sort appears to choose either
a part where the mortar contains a cavity, or it digs one suited to
its design. Over the opening of the hollow in the mortar it builds
an arched wall, so as to form a chamber considerably larger than is
usual with other architect caterpillars. It selects grains of mortar,
brick, or lichen, fixing them, by means of silk, firmly into the
structure. As some of these vaulted walls were from an inch to an inch
and a half long, and about a third of an inch wide and deep, it may
be well imagined that it would require no little industry and labour
to complete the work; yet it does not demand more than a few hours
for the insect to raise it from the foundation. Like all other insect
architects, this caterpillar uses its own body for a measuring-rule,
and partly for a mould, or rather a block or centre to shape the walls
by, curving itself round and round concentrically with the arch which
it is building.
We afterwards found one of these caterpillars, which had dug a cell in
one of the softest of the bricks, covering itself on the outside with
an arched wall of brick-dust, cemented with silk. As this brick was of
a bright-red colour, we were thereby able to ascertain that there was
not a particle of lichen employed in the structure.
The neatness mentioned by Réaumur, as remarkable in his moss-building
caterpillars, is equally observable in that which we have just
described; for, on looking at the surface of the wall, it would be
impossible for a person unacquainted with those structures to detect
where they were placed, as they are usually, on the outside, level with
the adjoining brick-work; and it is only when they are opened by the
entomologist, that the little architect is perceived lying snug in his
chamber. If a portion of the wall be thus broken down, the caterpillar
immediately commences repairing the breach, by piecing in bits of
mortar and fragments of lichen, till we can scarcely distinguish the
new portion from the old.
_CHAPTER X._
CADDIS-WORMS AND CARPENTER-CATERPILLARS.
[Illustration: Leaf Nest of Caddis-Worm.]
[Illustration: Reed Nest of Caddis-Worm.]
There is a very interesting class of grubs which live under water,
where they construct for themselves moveable tents of various materials
as their habits direct them, or as the substances they require can
be conveniently procured. Among the materials used by these singular
grubs, well-known to fishermen by the name of _caddis-worms_, and
to naturalists as the _larvæ_ of the four-winged flies in the order
_Trichoptera_ of Kirby and Spence, we may mention sand, stones,
shells, wood, and leaves, which are skilfully joined and strongly
cemented. One of these grubs forms a pretty case of leaves glued
together longitudinally, but leaving an aperture sufficiently large
for the inhabitant to put out its head and shoulders when it wishes
to look about for food. Another employs pieces of reed cut into
convenient lengths, or of grass, straw, wood, &c., carefully joining
and cementing each piece to its fellow as the work proceeds; and he
frequently finishes the whole by adding a broad piece longer than the
rest to shade his door-way overhead, so that he may not be seen from
above. A more laborious structure is reared by the grub of a beautiful
caddis-fly (_Phryganea_), which weaves together a group of the leaves
of aquatic plants into a roundish ball, and in the interior of this
forms a cell for its abode. The fallowing figure from Roesel will give
a more precise notion of this structure than a lengthened description.
[Illustration]
Another of these aquatic architects makes choice of the tiny shells of
young fresh-water mussels and snails (_Planorbis_), to form a moveable
grotto; and as these little shells are for the most part inhabited, he
keeps the poor animals close prisoners, and drags them without mercy
along with him. These grotto-building grubs are by no means uncommon in
ponds; and in chalk districts, such as the country about Woolwich and
Gravesend, they are very abundant.
[Illustration: Shell Nests of Caddis-Worms.]
One of the most surprising instances of their skill occurs in the
structures of which small stones are the principal material. The
problem is to make a tube about the width of the hollow of a
wheat-straw or a crow-quill, and equally smooth and uniform. Now the
materials being small stones full of angles and irregularities, the
difficulty of performing this problem will appear to be considerable,
if not insurmountable: yet the little architects, by patiently
examining their stones and turning them round on every side, never fail
to accomplish their plans. This, however, is only part of the problem,
which is complicated with another condition, and which we have not
found recorded by former observers, namely, that the under-surface
shall be flat and smooth, without any projecting angles which might
impede its progress when dragged along the bottom of the rivulet where
it resides. The selection of the stones, indeed, may be accounted for,
from this species living in streams where, but for the weight of its
house, it would to a certainty be swept away. For this purpose, it is
probable that the grub makes choice of larger stones than it might
otherwise want; and therefore also it is that we frequently find a case
composed of very small stones and sand, to which, when nearly finished,
a large stone is added by way of ballast. In other instances, when the
materials are found to possess too great specific gravity, a bit of
light wood, or a hollow straw, is added to buoy up the case.
[Illustration: Stone Nest of Caddis-Worm.]
[Illustration: Sand Nest balanced with a Stone.]
[Illustration: Nest of Caddis-Worm balanced with Straws.]
It is worthy of remark, that the cement, used in all these cases,
is superior to pozzolana[BN] in standing water, in which it is
indissoluble. The grubs themselves are also admirably adapted for their
mode of life, the portion of their bodies which is always enclosed in
the case being soft like a meal-worm, or garden-caterpillar, while
the head and shoulders, which are for the most part projected beyond
the door-way in search of food, are firm, hard, and consequently less
liable to injury than the protected portion, should it chance to be
exposed.
We have repeatedly tried experiments with the inhabitants of those
aquatic tents, to ascertain their mode of building. We have deprived
them of their little houses, and furnished them with materials for
constructing new ones, watching their proceedings from their laying the
first stone or shell of the structure. They work at the commencement
in a very clumsy manner, attaching a great number of chips to whatever
materials may be within their reach with loose threads of silk, and
many of these they never use at all in their perfect building. They
act, indeed, much like an unskilful workman trying his hand before
committing himself upon an intended work of difficult execution. Their
main intention is, however, to have abundance of materials within
reach: for after their dwelling is fairly begun, they shut themselves
up in it, and do not again protrude more than half of their body to
procure materials; and even when they have dragged a stone, a shell, or
a chip of reed within building reach, they have often to reject it as
unfit. (J. R.)
[We have here some examples of the latter kind of nest, _i. e._, those
habitations which are made of stones and shells. Beginning at the upper
left-hand figure, we find one that is made of moderately-sized stones
cemented together in a way that reminds the observer of the manner
in which a builder forms irregular stones into a wall. Next to it is
another, in which the stones are larger and narrower, and are arranged
much as some of the caddis-worms arrange pieces of stick and straw.
In the second, and on the left-hand side, is a very long and simple
tube, made of a grass stem, and balanced by three little sticks
attached to its centre. The next figure represents a number of
sand-tubes attached to each other. These are built up laboriously
of single particles of sand, and are remarkable for their peculiar
horn-like shape, the tube having the same regular curve as the horn of
an ox or antelope, and tapering gradually from the base to the top. A
somewhat similar tube, but of larger size, is shown in the right-hand
figure.
[Illustration]
Any one who wishes to see one of these creatures rebuild its house can
do so by carefully removing it from its tube, and supplying it with
fresh material. Very great care must be taken in the removal, as the
grub is easily damaged, and it holds so tightly to the tube with a pair
of pincers at the end of its body, that it must rather be coaxed than
driven out.
If desirable, they can be made to build their new houses of most
singular materials. A lady, Miss Smee, was very successful in a series
of experiments which she made with these insects, forcing them to
make tubes of different colours and patterns, by supplying them with
coloured sand, pieces of stained glass of various hues, gold dust, and
similar materials. Although there was scarcely any material which they
would not use, they seemed to consider a certain amount of angularity
as essential, and rejected any object, such as a bead, of which the
surface was perfectly rounded, while they would accept the same, if it
were broken or indented.
When the caddis-grub has ceased from feeding, and is about to pass into
the perfect stage, it spins over the mouth of the tube a strong silken
web. This web is made in quite a pretty pattern, and being woven with
rather wide meshes, it allows the water to flow through the tube while
it prevents any aquatic foes from penetrating and destroying the pupa.
The remaining figures of the illustration represent tubes, around which
are built a quantity of small shells. Generally, stones are mixed
with the shells; but in some cases, shells seem to be almost the only
material.]
Carpenter-Caterpillars.
Insects, though sometimes actuated by an instinct apparently blind,
unintelligent, or unknown to themselves, manifest in other instances
a remarkable adaptation of means to ends. We have it in our power
to exemplify this in a striking manner by the proceedings of the
caterpillar of a goat-moth (_Cossus ligniperda_) which we kept till it
underwent its final change.
This caterpillar, which abounds in Kent and many other parts of the
island, feeds on the wood of willows, oaks, poplars, and other trees,
in which it eats extensive galleries; but it is not contented with the
protection afforded by these galleries during the colder months of
winter, before the arrival of which it scoops out a hollow in the tree,
if it does not find one ready prepared, sufficiently large to contain
its body in a bent or somewhat coiled-up position. On sawing off a
portion of an old poplar in the winter of 1827, we found such a cell
with a caterpillar coiled up in it.
[Illustration: Caterpillar of Goat-Moth in a Willow Tree.]
[Illustration: Winter Nest of the Goat-Caterpillar.]
It had not, however, been contented with the bare walls of the retreat
which it had hewn out of the tree, for it had lined it with a fabric
as thick as coarse broadcloth, and equally warm, composed of the
raspings of the wood scooped out of the cell, united with the strong
silk which every species of caterpillar can spin. In this snug retreat
our caterpillar, if it had not been disturbed, would have spent the
winter without eating; but upon being removed into a warm room and
placed under a glass along with some pieces of wood, which it might eat
if so inclined, it was roused for a time from its dormant state, and
began to move about. It was not long, however, in constructing a new
cell for itself, no less ingenious than the former. It either could not
gnaw into the fir plank, where it was now placed with a glass above it,
or it did not choose to do so; for it left it untouched, and made it
the basis of the edifice it began to construct. It formed, in fact, a
covering for itself precisely like the one from which we had dislodged
it,--composed of raspings of wood detached for the purpose from what
had been given it as food, the largest piece of which was employed as
a substantial covering and protection for the whole. It remained in
this retreat, motionless, and without food, till revived by the warmth
of the ensuing spring, when it gnawed its way out, and began to eat
voraciously, to make up for its long fast.
[Illustration: Nest of Goat-Moth.--Figured from specimen, and raised to
show the Pupa.]
These caterpillars are three years in arriving at their final change
into the winged state; but as the one just mentioned was nearly full
grown, it began, in the month of May, to prepare a cell, in which it
might undergo its metamorphosis. Whether it had actually improved its
skill in architecture by its previous experience we will not undertake
to say, but its second cell was greatly superior to the first. In the
first there was only one large piece of wood employed; in the second,
two pieces were placed in such a manner as to support each other, and
beneath the angle thus formed an oblong structure was made, composed,
as before, of wood-raspings and silk, but much stronger in texture than
the winter cell. In a few weeks (four, if we recollect aright) the moth
came forth. (J. R.)
[I have now before me a series of three cocoons, made by one
caterpillar of the goat-moth, showing its increase in size during the
three years that it remained in the larval state. They were found in
an old willow tree, and occupied different parts of the same burrow.
The ravages which a goat-moth caterpillar can make in a tree are almost
incredible to those who have not seen the long and tortuous burrows
which the insect will construct, burrows which at first are small and
insignificant, but which afterwards become large enough to admit a
man's finger.
Sometimes the tunnel runs just under the bark, and sometimes it goes
straight towards the centre of the tree; and no small labour is
required before it can be fully traced. Still, the result is worth
the labour, for it is most interesting to trace the creature through
its whole existence, from the tiny hole which it made soon after its
exit from the egg, to the large aperture through which it emerged as a
moth. The whole of the tunnel is strongly imbued with the peculiar and
unpleasant odour which has given to the goat-moth its popular name; and
the scent is so persistent, that it adheres to the fingers which have
touched the sides of the tunnel, and can scarcely be removed even by
repeated washings.
The moth itself is a well-known insect, though rarely seen except by
night. It is large, brown, round bodied; the wings are covered with a
soft and downy clothing, which strongly reminds the observer of the
plumage of an owl.]
A wood-boring caterpillar, of a species of moth much rarer than the
preceding (_Ægeria asiliformis_, Stephens), exhibits great ingenuity in
constructing a cell for its metamorphosis. We observed above a dozen
of them during this summer (1829) in the trunk of a poplar, one side
of which had been stripped of its bark. It was this portion of the
trunk which all the caterpillars selected for their final retreat,
not one having been observed where the tree was covered with bark. The
ingenuity of the little architect consisted in scooping its cell almost
to the very surface of the wood, leaving only an exterior covering of
unbroken wood, as thin as writing-paper. Previous, therefore, to the
chrysalis making its way through this feeble barrier, it could not
have been suspected that an insect was lodged under the smooth wood.
We observed more than one of these in the act of breaking through this
covering, within which there is, besides, a round moveable lid of a
sort of brown wax. (J. R.)
[Illustration: Larva of Ægeria.]
Another architect caterpillar, frequently to be met with in July on
the leaves of the willow and the poplar, is, in the fly-state, called
the puss-moth (_Cerura vinula_). The caterpillar is produced from
brown-coloured shining eggs, about the size of a pin's head, which
are deposited--one, two, or more together--on the upper surface of a
leaf. In the course of six or eight weeks (during which time it casts
its skin thrice) it arrives at its full growth, when it is about as
thick, and nearly as long, as a man's thumb, and begins to prepare a
structure in which the pupa may sleep securely during the winter. As we
have, oftener than once, seen this little architect at work, from the
foundation till the completion of its edifice, we are thereby enabled
to give the details of the process.
[Illustration: Eggs of the Puss-Moth.]
[Illustration: Rudiments of the Cell of the Puss-Moth.]
The puss, it may be remarked, does not depend for protection on the
hole of a tree, or the shelter of an overhanging branch, but upon
the solidity and strength of the fabric which it rears. The material
it commonly uses is the bark of the tree upon which the cell is
constructed; but when this cannot be procured, it is contented to
employ whatever analogous materials may be within reach. One which we
had shut up in a box substituted the marble paper it was lined with for
bark, which it could not procure.[BO] With silk it first wove a thin
web round the edges of the place which it marked out for its edifice,
then it ran several threads in a spare manner from side to side, and
from end to end, but very irregularly in point of arrangement; these
were intended for the skeleton or framework of the building. When this
outline was finished, the next step was to strengthen each thread of
silk by adding several (sometimes six or eight) parallel ones, all of
which were then glued together into a single thread, by the insect
running its mandibles, charged with gluten, along the line. The meshes,
or spaces, which were thus widened by the compression of the parallel
threads, were immediately filled up with fresh threads, till at length
only very small spaces were left. It was in this stage of the operation
that the paper came into requisition, small portions of it being gnawed
off the box and glued into the meshes. It was not, however, into the
meshes only that the bits of paper were inserted; for the whole fabric
was in the end thickly studded over with them. In about half a day
from the first thread of the framework being spun the building was
completed. It was at first, however, rather soft, and yielded to slight
pressure with the finger; but as soon as it became thoroughly dry, it
was so hard that it could with difficulty be penetrated with the point
of a penknife. (J. R.)
[One puss-moth larva, which I reared, made its nest in a rather curious
manner. After it had ceased feeding it had been placed on a marble
mantelpiece under a glass tumbler, as a temporary residence until a
more appropriate dwelling could be found for it. But its instincts
urged it to make its nest without delay, and it accordingly set to
work, and spun itself up in a cocoon composed entirely of its own silk,
neither the glass tumbler or the mantelpiece affording it any material
with which to harden the walls of its dwelling.
[Illustration: Cell built by the Larva of the Puss-Moth.]
Consequently, the texture of the cocoon was of a rather singular
nature. The silken threads had been fused together so as to form a
translucent cocoon, looking as if it had been made of gelatine, and
being nearly equally transparent, the chrysalis being plainly visible
through its walls. The cocoon was thin and elastic, as if it had been
made of very thin horn; and it was so tightly fixed to the mantelpiece
as well as to the tumbler, that it could not be removed without damage.
The moth suffered no injury from the privation which the larva had to
undergo.
The cocoons of the puss-moth are to be found upon the trunks of trees,
but they are so rough, and so greatly resembling the bark, with which,
indeed, their walls are strengthened, that an inexperienced eye would
fail to detect them. Even when they have been pointed out to a novice
in practical entomology, he has failed to find them again whenever his
eye has been taken off their rugged outlines.]
A question will here suggest itself to the curious inquirer, how the
moth, which is not, like the caterpillar, furnished with mandibles
for gnawing, can find its way through so hard a wall. To resolve
this question, it is asserted by recent naturalists (see Kirby and
Spence, vol. iii. p. 15) that the moth is furnished with a peculiar
acid for dissolving itself a passage. We have a specimen of the case
of a puss-moth, in which, notwithstanding its strength, one of the
ichneumons had contrived to deposit its eggs. In the beginning of
summer, when we expected the moth to appear, and felt anxious to
observe the recorded effects of the acid, we were astonished to find a
large orange cuckoo-fly make its escape; while another, which attempted
to follow, stuck by the way and died. On detaching the cell from the
box, we found several others, which had not been able to get out, and
had died in their cocoons. (J. R.)
[Illustration: Ichneumon (_Cphion luteum_), figured from the one
mentioned.]
Among the carpenter-grubs may be mentioned that of the purple
capricorn-beetle (_Callidium violaceum_), of which the Rev. Mr. Kirby
has given an interesting account in the fifth volume of the 'Linnæan
Transactions.' This insect feeds principally on fir timber which has
been felled some time without having had the bark stripped off; but
it is often found on other wood. Though occasionally taken in this
kingdom, it is supposed not to have been originally a native. The
circumstance of this destructive little animal attacking only such
timber as had not been stripped of its bark ought to be attended to
by all persons who have any concern in this article; for the bark is
a temptation not only to this, but to various other insects; and much
of the injury done in timber might be prevented, if the trees were all
barked as soon as they were felled. The female is furnished, at the
posterior extremity of her body, with a flat retractile tube, which
she inserts between the bark and the wood, to the depth of about a
quarter of an inch, and there deposits a single egg. By stripping off
the bark, it is easy to trace the whole progress of the grub, from
the spot where it is hatched, to that where it attains its full size.
It first proceeds in a serpentine direction, filling the space which
it leaves with its excrement, resembling sawdust, and so stopping all
ingress to enemies from without. When it has arrived at its utmost
dimensions, it does not confine itself to one direction, but works in
a kind of labyrinth, eating backwards and forwards, which gives the
wood under the bark a very irregular surface: by this means its paths
are rendered of considerable width. The bed of its paths exhibits,
when closely examined, a curious appearance, occasioned by the
gnawings of its jaws, which excavate an infinity of little ramified
canals. When the insect is about to assume its chrysalis state, it
bores down obliquely into the solid wood, to the depth sometimes of
three inches, and seldom if ever less than two, forming holes nearly
semi-cylindrical, and of exactly the form of the grub which inhabits
them. At first sight one would wonder how so small and seemingly so
weak an animal could have strength to excavate so deep a mine; but when
we examine its jaws, our wonder ceases. These are large, thick, and
solid sections of a cone divided longitudinally, which, in the act of
chewing, apply to each other the whole of their interior plane surface,
so that they grind the insect's food like a pair of millstones. Some
of the grubs are hatched in October; and it is supposed that about the
beginning of March they assume their chrysalis state. At the place
in the bark opposite to the hole from whence they descended into the
wood, the perfect insects gnaw their way out, which generally takes
place betwixt the middle of May and the middle of June. These insects
are supposed only to fly in the night, but during the day they may
generally be found resting on the wood from which they were disclosed.
The grubs are destitute of feet, pale, folded, somewhat hairy, convex
above, and divided into thirteen segments. Their head is large and
convex.[BP]
[Illustration: Magnified Cells of Pyralis strigulalis?
_a._ The walls before they are joined. _b._ Walls joined, but not
closed at top. _c._ Side view of structure complete.]
It would not be easy to find a more striking example of ingenuity
than occurs in a small caterpillar which may be found in May, on the
oak, and is supposed by Kirby and Spence to be that of the _Pyralis
strigulalis_. It is of a whitish-yellow colour, tinged with a shade of
carnation, and studded with tufts of red hairs on each segment, and
two brown spots behind the head. It has fourteen feet, and the upper
part of its body is much flatter than is common in caterpillars. When
this ingenious little insect begins to form its cell, it selects a
smooth young branch of the oak, near an offgoing of the branchlets
whose angle may afford it some protection. It then measures out, with
its body for a rule, the space destined for its structure, the basement
of which is of a triangular form, with the apex at the lower end. The
building itself is composed of small, rectangular, strap-shaped pieces
of the outer bark of the branch cut out from the immediate vicinity;
the insect indeed never travels further for materials than the length
of its own body. Upon the two longest sides of the triangular base it
builds uniform walls, also of a triangular shape, and both gradually
diverging from each other as they increase in height. These are formed
with so much mathematical precision, that they fit exactly when they
are afterwards brought into contact. As soon as the little architect
has completed these walls, which resemble very much the feathers of
an arrow, it proceeds to draw them together in a manner similar to
that which the leaf-rolling caterpillars employ in constructing their
abodes, by pulling them with silken cords till they bend and converge.
Even when the two longest sides are thus joined, there is an opening
left at the upper end, which is united in a similar manner. When the
whole is finished, it requires close inspection to distinguish it from
the branch, being formed of the same materials, and having consequently
the same colour and gloss. Concealment, indeed, may be supposed,
with some justice, to be the final object of the insect in producing
this appearance, the same principle being extensively exemplified in
numerous other instances.
_CHAPTER XI._
EARTH-MASON CATERPILLARS.
Many species of caterpillars are not only skilful in concealing
themselves in their cocoons, but also in the concealment of the cocoon
itself; so that even when that is large, as in the instance of the
death's-head hawk-moth (_Acherontia atropos_), it is almost impossible
to find it. We allude to the numerous class of caterpillars which,
previous to their changing into the pupa state, bury themselves in the
earth. This circumstance would not be surprising, were it confined
to those which are but too well known in gardens, from their feeding
upon and destroying the roots of lettuce, chicory, and other plants,
as they pass a considerable portion of their lives under ground; nor
is it surprising that those which retire under ground during the
day, and come abroad to feed in the night, should form their cocoons
where they have been in the habit of concealing themselves. But it is
very singular and unexpected, that caterpillars which pass the whole
of their life on plants and even on trees, should afterwards bury
themselves in the earth. Yet, the fact is, that perhaps a greater
number make their cocoons under than above ground, particularly those
which are not clothed with hair.
Some of those caterpillars which go into the ground previous to their
change make no cocoon at all, but are contented with a rude masonry of
earth as a nest for their pupæ: into the details of their operations
it will not be so necessary for us to go, as into those which exhibit
more ingenuity and care. When one of the latter is dug up it has the
appearance of nothing more than a small clod of earth, of a roundish
or oblong shape, but, generally, by no means uniform. The interior,
however, when it is laid open, always exhibits a cavity, smooth,
polished, and regular, in which the cocoon or the chrysalis lies secure
(Fig. B, p. 221). The polish of the interior is precisely such as might
be given to soft earth by moistening and kneading it with great care.
But beside this, it is usually lined with a tapestry of silk, more or
less thick, though this cannot always be discovered without the aid of
a magnifying glass. This species of caterpillars, as soon as they have
completed their growth, go into the earth, scoop out, as the cossus
does in wood, a hollow cell of an oblong form, and line it with pellets
of earth, from the size of a grain of sand to that of a pea--united, by
silk or gluten, into a fabric more or less compact, according to the
species, but all of them fitted for protecting the inhabitant, during
its winter sleep, against cold and moisture.
[Illustration: Outside view of Nests of Earth-mason Caterpillars.]
One of the examples of this occurs in the ghost-moth (_Hepialus
humuli_), which, before it retires into the earth, feeds upon the
roots of the hop or the burdock. Like other insects which construct
cells under ground, it lines the cemented earthen walls of its cell
with a smooth tapestry of silk, as closely woven as the web of the
house-spider.
Inaccurate observers have inferred that these earthen structures were
formed by a very rude and unskilful process--the caterpillar, according
to them, doing nothing more than roll itself round, while the mould
adhered to the gluey perspiration with which they describe its body
to be covered. This is a process as far from the truth as Aristotle's
account of the spider spinning its web from wool taken from its body.
Did the caterpillar do nothing more than roll itself in the earth, the
cavity would be a long tube fitted exactly to its body (Fig. c): it is
essentially different.
[Illustration: Nests, &c., of an Earth-mason Caterpillar.]
It does not indeed require very minute observation to perceive that
every grain of earth in the structure is united to the contiguous
grains by threads of silk; and that consequently, instead of the whole
having been done at once, it must have required very considerable time
and labour. This construction is rendered more obvious by throwing one
of these earthen cases into water, which dissolves the earth, but does
not act on the silk which binds it together. To understand how this is
performed, it may not be uninteresting to follow the little mason from
the beginning of his task.
When one of those burrowing caterpillars has done feeding, it enters
the earth to the depth of several inches, till it finds mould fit
for its purpose. Having nowhere to throw the earth which it may dig
out, the only means in its power of forming a cavity is to press it
with its body; and, by turning round and round for this purpose, an
oblong hollow is soon made. But were it left in this state, as Réaumur
well remarks, though the vault might endure the requisite time by the
viscosity of the earth alone, were no change to take place in its
humidity, yet, as a great number are wanted to hold out for six, eight,
and ten months, they require to be substantially built; a mere lining
of silk, therefore, would not be sufficient, and it becomes necessary
to have the walls bound with silk to some thickness.
When a caterpillar cannot find earth sufficiently moist to bear
kneading into the requisite consistence, it has the means of moistening
it with a fluid which it ejects for the purpose; and as soon as it has
thus prepared a small pellet of earth, it fits it into the wall of the
vault, and secures it with silk. As the little mason, however, always
works on the _inside_ of the building, it does not, at first view,
appear in what manner it can procure materials for making one or two
additional walls on the inside of the one first built. As the process
takes place under ground, it is not easy to discover the particulars,
for the caterpillars will not work in glazed boxes. The difficulty was
completely overcome by M. Réaumur, in the instance of the caterpillar
of the water-betony moth (_Cucullia scrophulariæ_, Schrank), which he
permitted to construct the greater part of its underground building,
and then dug it up and broke a portion off from the end, leaving about
a third part of the whole to be rebuilt. Those who are unacquainted
with the instinct of insects might have supposed that, being disturbed
by the demolition of its walls, it would have left off work; but
the stimulus of providing for the great change is so powerful, that
scarcely any disturbance will interrupt a caterpillar in this species
of labour.
[Illustration: Earth-mason Caterpillars' Nests, with the perfect Moth,
&c.]
The little builder accordingly was not long in recommencing its task
for the purpose of repairing the disorder, which it accomplished in
about four hours. At first it protruded its body almost entirely
beyond the breach which had been made, to reconnoitre the exterior for
building materials. Earth was put within its reach, of the same kind
as it had previously used, and it was not long in selecting a grain
adapted to its purpose, which it fitted into the wall and secured with
silk. It first enlarged the outside of the wall by the larger and
coarser grains, and then selected finer for the interior. But before it
closed the aperture, it collected a quantity of earth on the inside,
wove a pretty thick net-work tapestry of silk over the part which
remained open, and into the meshes of this, by pushing and pressing,
it thrust grains of earth, securing them with silk till the whole
was rendered opaque; and the further operations of the insect could
no longer be watched, except that it was observed to keep in motion,
finishing, no doubt, the silken tapestry of the interior of its little
chamber. When it was completed, M. Réaumur ascertained that the portion
of the structure which had been built under his eye was equally thick
and compact with the other, which had been done under ground.
The grubs of several of the numerous species of may-fly (_Ephemera_)
excavate burrows for themselves in soft earth, on the banks of rivers
and canals, under the level of the water, an operation well described
by Scopoli, Swammerdam, and Réaumur. The excavations are always
proportioned to the size of the inhabitant; and consequently, when it
is young and small, the hole is proportionally small, though, with
respect to extent, it is always at least double the length of its body.
The hole, being under the level of the river, is always filled with
water, so that the grub swims in its native element, and while it is
secure from being preyed upon by fishes, it has its own food within
easy reach. It feeds, in fact, if we may judge from its _egesta_, upon
the slime or moistened clay with which its hole is lined.
In the bank of the stream at Lee, in Kent, we had occasion to take
up an old willow stump, which, previous to its being driven into the
bank, had been perforated in numerous places by the caterpillar of the
goat-moth (_Cossus ligniperda_). From having been driven amongst the
moist clay, these perforations became filled with it, and the grubs of
the ephemeræ found them very suitable for their habitation: for the
wood supplied a more secure protection than if their galleries had been
excavated in the clay. In these holes of the wood we found several
empty, and some in which were full-grown grubs. (J. R.)
The architecture of the grub of a pretty genus of beetles, known to
entomologists by the name of _Cincindela_, is peculiarly interesting.
It was first made known by the eminent French naturalists, Geoffroy,
Desmarest, and Latreille. This grub, which may be met with during
spring, and also in summer and autumn, in sandy places, is long,
cylindric, soft whitish, and furnished with six scaly brown feet.
The head is of a square form, with six or eight eyes, and very large
in proportion to the body. They have strong jaws, and on the eighth
joint of the body there are two fleshy tubercles, thickly clothed with
reddish hairs, and armed with a recurved horny spine, the whole giving
to the grub the form of the letter Z.
[Illustration: Nest of the Grubs of Ephemeræ.
A, The Grub. B, Perforations in a river bank. C, One laid open to show
the parallel structure.]
[Illustration: Nests of Ephemeræ in holes of Cossus.]
With their jaws and feet they dig into the earth to the depth of
eighteen inches, forming a cylindrical cavity of greater diameter
than their body, and furnished with a perpendicular entrance. In
constructing this, the grub first clears away the particles of earth
and sand by placing them on its broad trapezoidal head, and carrying
the load in this manner beyond the area of the excavation. When it gets
deeper down, it climbs gradually up to the surface with similar loads
by means of the tubercles on its back, above described. This process
is a work of considerable time and difficulty, and in carrying its
loads the insect has often to rest by the way to recover strength for
a renewed exertion. Not unfrequently, it finds the soil so ill adapted
to its operations, that it abandons the task altogether, and begins
anew in another situation. When it has succeeded in forming a complete
den, it fixes itself at the entrance by the hooks of its tubercles,
which are admirably adapted for the purpose, forming a fulcrum or
support, while the broad plate on the top of the head exactly fits the
aperture of the excavation, and is on a level with the soil. In this
position the grub remains immovable, with jaws expanded, and ready
to seize and devour every insect which may wander within its reach,
particularly the smaller beetles; and its voracity is so great, that
it does not spare even its own species. It precipitates its prey into
the excavation, and in case of danger it retires to the bottom of its
den, a circumstance which renders it not a little difficult to discover
the grub. The method adopted by the French naturalists was to introduce
a straw or pliant twig into the hole, while they dug away, by degrees
and with great care, the earth around it, and usually found the grub at
the bottom of the cell, resting in a zig-zag position like one of the
caterpillars of the geometric moths.
When it is about to undergo its transformation into a pupa, it
carefully closes the mouth of the den, and retires to the bottom in
security.
It does not appear that the grub of the genus _Cincindela_ uses the
excavation just described for the purpose of a trap or pitfall, any
further than that it can more effectually secure its prey by tumbling
them down into it; but there are other species of grubs which
construct pitfalls for the express purpose of traps. Among these is
the larva of a fly (_Rhagio vermileo_), not unlike the common flesh
maggot. The den which it constructs is in the form of a funnel, the
sides of which are composed of sand or loose earth. It forms this
pitfall of considerable depth, by throwing out the earth obliquely on
all sides; and when its trap is finished, it stretches itself along the
bottom, remaining stiff and motionless, like a piece of wood. The last
segment of the body is bent at an angle with the rest, so as to form a
strong point of support in the struggles which it must often have to
encounter with vigorous prey. The instant that an insect tumbles into
the pitfall, the grub pounces upon it, writhes itself round it like a
serpent, transfixes it with its jaws, and sucks its juices at its ease.
Should the prey by any chance escape, the grub hurls up jets of sand
and earth, with astonishing rapidity and force, and not unfrequently
succeeds in again precipitating it to the bottom of its trap.
The Ant-Lion.
The observations of the continental naturalists have made known to us a
pitfall constructed by an insect, the details of whose operations are
exceedingly curious; we refer to the grub of the ant-lion (_Myrmeleon
formicarius_), which, though marked by Dr. Turton and Mr. Stewart as
British, has not (at least of late years) been found in this country.
As it is not, however, uncommon in France and Switzerland, it is
probable it may yet be discovered in some spot hitherto unexplored, and
if so, it will well reward the search of the curious.
The ant-lion grub being of a grey colour, and having its body composed
of rings, is not unlike a wood-louse (_Oniscus_), though it is larger,
more triangular, has only six legs, and most formidable jaws, in
form of a reaping-hook, or a pair of calliper compasses. These jaws,
however, are not for masticating, but are perforated and tubular,
for the purpose of sucking the juices of ants, upon which it feeds.
Vallisnieri was therefore mistaken, as Réaumur well remarks, when he
supposed that he had discovered its mouth. Its habits require that
it should walk backwards, and this is the only species of locomotion
which it can perform. Even this sort of motion it executes very slowly;
and were it not for the ingenuity of its stratagems, it would fare
but sparingly, since its chief food consists of ants, whose activity
and swiftness of foot would otherwise render it impossible for it to
make a single capture. Nature, however, in this, as in nearly every
other case, has given a compensating power to the individual animal,
to balance its privations. The ant-lion is slow, but it is extremely
sagacious; it cannot follow its prey, but it can entrap it.
[Illustration: Grub of the Ant-Lion, magnified, with one perfect Trap,
and another begun.]
The snare which the grub of the ant-lion employs consists of a
funnel-shaped excavation formed in loose sand, at the bottom of which
it lies in wait for the ants that chance to stumble over the margin,
and cannot, from the looseness of the walls, gain a sufficient footing
to effect their escape.
By shutting up one of these grubs in a box with loose sand, it has been
repeatedly observed constructing its trap of various dimensions, from
one to nearly three inches in diameter, according to circumstances.
In the 'Magazine of Natural History,' 1838, p. 601, Mr. Westwood gives
a very interesting account of the mode in which the ant-lion proceeds
in the excavation of its pitfall, as witnessed by himself in specimens
procured in the Parc de Belle Vue, near Paris, where, at the foot of a
very high sand-bank, these pits were numerous, and of various sizes,
but none exceeded an inch and a half or two inches in diameter, and
two-thirds of an inch deep. "The ant-lions were of various sizes,
corresponding to the size of their retreats. I brought many of them
to Paris, placing several together in a box filled with sand. They,
however, destroyed one another whilst shut up in these boxes; and I
only succeeded in bringing three of them alive to England, one of
which almost immediately afterwards (on the 23rd of July) enclosed
itself in a globular cocoon of fine sand. The other two afforded me
many opportunities of observing their proceedings. They were unable to
walk forwards,--an anomalous circumstance, and not often met with in
animals furnished with well-developed legs. It is generally backwards,
working in a spiral direction, that the creature moves, pushing itself
backwards and downwards at the same time, the head being carried
horizontally, and the back much arched, so that the extremity of the
body is forced into the sand. In this manner it proceeds backwards (to
use an Hibernianism), forming little mole-hills in the sand. But it
does not appear to me that this retrograde motion has anything to do
with the actual formation of the cell, since, as soon as it has fixed
upon a spot for its retreat, it commences throwing up the sand with the
back of its head, jerking the sand either behind its back or on one or
the other side. It shuts its long jaws, forming them into a kind of
shovel, the sharp edges of which it thrusts laterally into the sand
on each side of its head, and thereby contrives to lodge a quantity
of the sand upon the head as well as the jaws. The motion is in fact
something like that of the head of a goat, especially when butting
sideways in play. In this manner it contrives to throw away the sand,
and by degrees to make a hole entirely with its head, the fore legs
not affording the slightest assistance in the operation. During this
performance the head only is exposed, the insect having previously
pushed itself beneath the surface of the sand; but when it has made the
hole sufficiently deep, it withdraws the head also, leaving only the
jaws exposed, which are spread open in a line, and laid on the sand so
as to be scarcely visible. If alarmed, the insect immediately takes a
step backwards, withdrawing the jaws; but when an insect falls into
the hole, the jaws are instinctively and instantaneously closed, and
the insect seized by the leg, wing, or body, just as it may chance to
fall within the reach of the ant-lion's jaws. If, however, the insect
be not seized, but attempts to escape, no matter in what direction, the
ant-lion immediately begins twisting its head about, and shovelling up
the sand with the greatest agility, jerking it about on each side and
backwards, but never forwards, as misrepresented in some figures, until
the hole is made so much deeper, and such a disturbance caused in the
sides of the hole, that the insect is almost sure to be brought down
to the bottom, when it is seized by the ant-lion, which immediately
endeavours to draw it beneath the sand; and if it be very boisterous,
the ant-lion beats it about, holding it firmly by the jaws until it is
too weak for further resistance. Hence, as the head of the ant-lion is
immersed in the sand, it is evident that the accounts given in popular
works of the instinct by which it throws the sand in the direction of
the escaping prey are not quite correct. The act of throwing up the
sand, when an insect has fallen into the pit and attempts to escape,
has evidently for its chief object that of making the pit deeper and
more conical, and therefore more difficult of ascent."
[Illustration: Ant-Lion's Pitfalls, in an experimenting-box.]
It is by the action of the hinder pair of its legs that the ant-lion
drags itself backwards, the other four pair being extended trailing
after it, and leaving an impression on the surface of the fine sand
over which it has passed; and when burrowing its way beneath the
surface of the sand, it proceeds by short steps backwards. A portion
of sand at each step is thrown on the head, owing to the hump-like
form of the back: this is immediately jerked away, the body at the
same time advancing another step in its backward and spiral motion.
Where it rests, a little hillock of sand is raised by the body of the
ant-lion underneath; while its jaws emerge and spread flat on the
surface. It now probably commences its pitfall, the mode of excavating
which we have given in detail. From the spiral course described by the
ant-lion in its backward progress appears to have arisen the idea of
its tracing out a circle as the outline of its pitfall--as would an
architect or engineer; but whence sprang the often-repeated statement,
that the ant-lion loads its head with sand by means of one of its legs,
that nearest the centre of the circle, we cannot conjecture. Nor do we
know how, as it works entirely buried with the exception of the head,
the ant-lion can act when it meets with a stone or other obstacle, as
M. Bonnet states he has repeatedly witnessed. He observes that if the
stone be small, it can manage to jerk it out in the same manner as the
sand; but when it is two or three times larger and heavier than its
own body, it must have recourse to other means of removal. The larger
stones it usually leaves till the last; and when it has removed all
the sand which it intends, it then proceeds to try what it can do with
the less manageable obstacles. For this purpose it crawls backwards
to the place where a stone may be, and thrusting its tail under it,
is at great pains to get it properly balanced on its back, by an
alternate motion of the rings composing its body. When it has succeeded
in adjusting the stone, it crawls up the side of the pit with great
care, and deposits its burthen on the outside of the circle. Should
the stone happen to be round, the balance can be kept only with the
greatest difficulty, as it has to travel with its load upon a slope
of loose sand, which is ready to give way at every step; and often
when the insect has carried it to the very brink, it rolls off its
back and tumbles down to the bottom of the pit. This accident, so far
from discouraging the ant-lion, only stimulates it to more persevering
efforts. Bonnet observed it renew these attempts to dislodge a stone
five or six times. It is only when it finds it utterly impossible to
succeed, that it abandons the design and commences another pit in
a fresh situation. When it succeeds in getting a stone beyond the
line of its circle, it is not contented with letting it rest there;
but, to prevent it from again rolling in, it goes on to push it to a
considerable distance. We maybe pardoned for pausing before we give
full credence to these details.
The ant-lion feeds only on the blood or juice of insects; and as soon
as it has extracted these, it tosses the dry carcase out of its den.
When it is about to change into a pupa, it proceeds in nearly the
same manner as the caterpillar of the water-betony moth (_Cucullia
scrophulariæ_). It first builds a case of sand, the particles of which
are secured by threads of silk, and then tapestries the whole with a
silken web. Within this it undergoes its transformation into a pupa,
and in due time it emerges in form of a four-winged fly, closely
resembling the dragon-flies (_Libellulæ_), vulgarly and erroneously
called _horse-stingers_.
The instance of the ant-lion naturally leads us to consider the design
of the Author of Nature in so nicely adjusting, in all animals, the
means of destruction and of escape. As the larger quadrupeds of prey
are provided with a most ingenious machinery for preying on the weaker,
so are those furnished with the most admirable powers of evading their
destroyers.
In the economy of insects, we constantly observe that the means of
defence, not only of the individual creatures, but of their larvæ
and pupæ, against the attacks of other insects, and of birds, is
proportioned, in the ingenuity of their arrangements, to the weakness
of the insect employing them. Those species which multiply the quickest
have the greatest number of enemies. Bradley, an English naturalist,
has calculated that two sparrows carry, in the course of a week, above
three thousand caterpillars to the young in their nests. But though
this is, probably, much beyond the truth, it is certain that there is
a great and constant destruction of individuals going forward; and yet
the species is never destroyed. In this way a balance is kept up, by
which one portion of animated nature cannot usurp the means of life and
enjoyment which the world offers to another portion. In all matters
relating to reproduction, Nature is prodigal in her arrangements.
Insects have more stages to pass through before they attain their
perfect growth than other creatures. The continuation of the species
is, therefore, in many cases, provided for by a much larger number
of eggs being deposited than ever become fertile. How many larvæ are
produced, in comparison with the number which pass into the pupa
state; and how many pupæ perish before they become perfect insects!
Every garden is covered with caterpillars; and yet how few moths and
butterflies, comparatively, are seen, even in the most sunny season?
Insects which lay few eggs are, commonly, most remarkable in their
contrivances for their preservation. The dangers to which insect life
is exposed are manifold; and therefore are the contrivances for its
preservation of the most perfect kind, and invariably adapted to the
peculiar habits of each tribe. The same wisdom determines the food of
every species of insect; and thus some are found to delight in the
rose-tree, and some in the oak. Had it been otherwise, the balance of
vegetable life would not have been preserved. It is for this reason
that the contrivances which an insect employs for obtaining its
food are curious, in proportion to the natural difficulties of its
structure. The ant-lion is carnivorous, but he has not the quickness
of the spider, nor can he spread a net over a large surface, and
issue from his citadel to seize a victim which he has caught in his
out-works. He is therefore taught to dig a trap, where he sits like
the unwieldy giants of fable, waiting for some feeble one to cross
his path. How laborious and patient are his operations--how uncertain
the chances of success! Yet he never shrinks from them, because his
instinct tells him that by these contrivances alone can he preserve his
own existence, and continue that of his species.
_CHAPTER XII._
CLOTHES-MOTH AND OTHER TENT-MAKING CATERPILLARS.--LEAF AND BARK MINERS.
There are at least five different species of moths similar in manners
and economy, the caterpillars of which feed upon animal substances,
such as furs, woollen cloths, silk, leather, and, what to the
naturalist is no less vexing, upon the specimens of insects and other
animals preserved in his cabinet. The moths in question are of the
family named _Tinea_ by entomologists, such as the tapestry-moth
(_Tinea tapetzella_), the fur-moth (_Tinea pellionella_), the wool-moth
(_Tinea vestianella_), the cabinet-moth (_Tinea destructor_, Stephens),
&c.
The moths themselves are, in the winged state, small and well fitted
for making their way through the most minute hole or chink, so that it
is scarcely possible to exclude them by the closeness of a wardrobe or
a cabinet.[BQ] If they cannot effect an entrance when a drawer is out,
or a door open, they will contrive to glide through the key-hole; and
if they once get in, it is no easy matter to dislodge or destroy them,
for they are exceedingly agile, and escape out of sight in a moment.
Moufet is of opinion that the ancients possessed an effectual method of
preserving stuffs from the moth, because the robes of Servius Tullius
were preserved up to the death of Sejanus, a period of more than five
hundred years. On turning to Pliny to learn this secret, we find him
relating that stuff laid upon a coffin will be ever after safe from
moths; in the same way as a person once stung by a scorpion will never
afterwards be stung by a bee, or a wasp, or a hornet! Rhasis, again,
says that cantharides suspended in a house drive away moths; and he
adds that they will not touch anything wrapped in a lion's skin!--the
poor little insects, says Réaumur, sarcastically, being probably in
bodily fear of so terrible an animal.[BR] Such are the stories which
fill the imagination even of philosophers, till real science entirely
expels them.
The effluvium of camphor or turpentine, or fumigation by sulphur or
chlorine, may sometimes kill them, when in the winged state, but this
will have no effect upon their eggs, and seldom upon the caterpillars;
for they wrap themselves up too closely to be easily reached by any
agent except heat. This, when it can be conveniently applied, will
be certain either to dislodge or to kill them. When the effluvium of
turpentine, however, reaches the caterpillar, Bonnet says it falls into
convulsions, becomes covered with livid blotches, and dies.[BS]
The mother insect takes care to deposit her eggs on or near such
substances as she instinctively foreknows will be best adapted for the
food of the young, taking care to distribute them so that there may
be a plentiful supply and enough of room for each. We have found, for
example, some of those caterpillars feeding upon the shreds of cloth
used in training wall-fruit trees; but we never saw more than two
caterpillars on one shred. This scattering of the eggs in many places
renders the effects of the caterpillars more injurious, from their
attacking many parts of a garment or a piece of stuff at the same time.
(J. R.)
When one of the caterpillars of this family issues from the egg, its
first care is to provide itself with a domicile, which indeed seems no
less indispensable to it than food; for, like all caterpillars that
feed under cover, it will not eat while it remains unprotected. Its
mode of building is very similar to that which is employed by other
caterpillars that make use of extraneous materials. The foundation
or framework is made of silk secreted by itself, and into this it
interweaves portions of the material upon which it feeds. It is said by
Bingley, that "after having spun a fine coating of silk _immediately
around its body_, it cuts the filaments of the wool or fur close by
the thread of the cloth, or by the skin, with its teeth, which act
in the manner of scissors, into convenient lengths, and applies the
bits, one by one, with great dexterity, to the _outside_ of its silken
case."[BT] This statement, however, is erroneous, and inconsistent with
the proceedings not only of the clothes-moth, but of every caterpillar
that constructs a covering. None of these build from within outwards,
but uniformly commence with the exterior wall, and finish by lining the
interior with the finest materials. Réaumur, however, found that the
newly-hatched caterpillars lived at first in a case of silk.
We have repeatedly witnessed the proceedings of these insects from the
very foundation of their structures; and, at the moment of writing
this, we turned out one from the carcase of an "old lady moth" (_Mormo
maura_, Ochsenheim) in our cabinet, and placed it on a desk covered
with green cloth, where it might find materials for constructing
another dwelling. It wandered about for half a day before it began its
operations; but it did not, as is asserted by Bonnet, and Kirby and
Spence, "in moving from place to place, seem to be as much incommoded
by the long hairs which surround it, as we are by walking amongst high
grass," nor, "accordingly, marching scythe in hand," did it, "with its
teeth, cut out a smooth road."[BU] On contrary, it did not cut a single
hair till it selected one for the foundation of its intended structure.
This it cut very near the cloth, in order, we suppose, to have it
as long as possible; and placed it on a line with its body. It then
immediately cut another, and placing it parallel to the first, bound
both together with a few threads of its own silk. The same process
was repeated with other hairs, till the little creature had made a
fabric of some thickness, and this it went on to extend till it was
large enough to cover its body; which (as is usual with caterpillars)
it employed as a model and measure for regulating its operations. We
remarked that it made choice of longer hairs for the outside than for
the parts of the interior, which it thought necessary to strengthen by
fresh additions; but the chamber was ultimately finished by a fine and
closely-woven tapestry of silk. We could see the progress of its work
by looking into the opening at either of the ends; for at this stage
of the structure the walls are quite opaque, and the insect concealed.
It may be thus observed to turn round, by doubling itself and bringing
its head where the tail had just been; of course, the interior is left
wide enough for this purpose, and the centre, indeed, where it turns,
is always wider than the extremities. (J. R.)
[Illustration: Cases, &c., of the Clothes-Moth (_Tinea
pellionella_).--_a_. Caterpillar feeding in a case, which has been
lengthened by ovals of different colours; _b_. Case cut at the ends for
experiment; _c_. Case cut open by the insect for enlarging it; _d_,
_e_. The clothes-moths in their perfect state, when, as they cease to
eat, they do no further injury.]
When the caterpillar increases in length, it takes care to add to
the length of its house, by working in fresh hairs at either end;
and if it be shifted to stuffs of different colours, it may be made
to construct a party-coloured tissue, like a Scotch plaid. Réaumur
cut off with scissors a portion at each end, to compel the insect to
make up the deficiency. But the caterpillar increases in thickness
as well as in length, so that, its first house becoming too narrow,
it must either enlarge it, or build a new one. It prefers the former
as less troublesome, and accomplishes its purpose "as dexterously,"
says Bonnet, "as any tailor, and sets to work precisely as we should
do, slitting the case on the two opposite sides, and then adroitly
inserting between them two pieces of the requisite size. It does not,
however, cut open the case from one end to the other at once; the
sides would separate too far asunder, and the insect be left naked. It
therefore first cuts each side about half-way down, beginning sometimes
at the centre and sometimes at the end (Fig. _c_), and then, after
having filled up the fissure, proceeds to cut the remaining half; so
that, in fact, four enlargements are made, and four separate pieces
inserted. The colour of the case is always the same as that of the
stuff from which it is taken. Thus, if its original colour be blue,
and the insect, previously to enlarging it, be put upon red cloth,
the circles at the end, and two stripes down the middle, will be
red."[BV] Réaumur found that they cut these enlargements in no precise
order, but sometimes continuously, and sometimes opposite each other,
indifferently.
The same naturalist says he never knew one leave its old dwelling
in order to build a new; though, when once ejected by force from
its house, it would never enter it again, as some other species of
caterpillars will do, but always preferred building another. We, on
the contrary, have more than once seen them leave an old habitation.
The very caterpillar, indeed, whose history we have above given, first
took up its abode in a specimen of the ghost-moth (_Hepialus humuli_),
where, finding few suitable materials for building, it had recourse to
the cork of the drawer, with the chips of which it made a structure
almost as warm as it would have done from wool. Whether it took offence
at our disturbing it one day, or whether it did not find sufficient
food in the body of the ghost-moth, we know not; but it left its cork
house, and travelled about eighteen inches, selected "the old lady,"
one of the largest insects in the drawer, and built a new apartment,
composed partly of cork as before, and partly of bits clipt out of the
moth's wings. (J. R.)
We have seen these caterpillars form their habitations of every sort
of insect, from a butterfly to a beetle; and the soft, feathery wings
of moths answer their purpose very well: but when they fall in with
such hard materials as the musk beetle (_Cerambyx moschatus_), or the
large scolopendra of the West Indies, they find some difficulty in the
building.
[Illustration: Transformations of the honeycomb-moths. _a_ _a_ _a_,
Galleries of the cell-boring caterpillar; _b_, the female; _c_, the
male moth (_Galleria alvearia_); _d_ _d_ _d_ _d_, galleries of the
wax-eating caterpillar, _e_, seen at the entrance; _f_, the same
exposed; _g_, its cocoon; _h_, the moth (_Galleria cereana_).]
When the structure is finished, the insect deems itself secure to feed
on the materials of the cloth or other animal matter within its reach,
provided it is dry and free from fat or grease, which Réaumur found it
would not touch. This may probably be the origin of the practice of
putting a bit of candle with furs, &c., to preserve them from the moth.
For building, it always selects the straightest and loosest pieces of
wool, but for food it prefers the shortest and most compact; and to
procure these it eats into the body of the stuff, rejecting the pile
or nap, which it necessarily cuts across at the origin, and permits
to fall, leaving it threadbare, as if it had been much worn. It must
have been this circumstance which induced Bonnet to fancy (as we have
already mentioned) that it cuts the hairs to make itself a smooth,
comfortable path to walk upon. It would be equally correct to say that
an ox or a sheep dislikes walking amongst long grass, and therefore
eats it down in order to clear the way.
[There is a little insect closely allied to these moths, which does a
vast amount of harm to the bee-combs. This is the honeycomb-moth, of
which there are in England two species, both belonging to the genus
Galleria. This little creature is continually trying to make its
way into the hives, and is as continually opposed by the bees, who
instinctively know their enemy. If it once slips past the guards, the
unfortunate bees are doomed to lose a considerable amount of their
stored treasures, and have sometimes been so worried that they have
been obliged to leave the hive altogether.
As soon as it can hide itself in an empty cell--an easy matter enough
for so tiny a moth, which harmonizes exactly in colour with the
bee-combs--it proceeds to lay its eggs, and, having discharged its
office, dies. The eggs soon hatch into little grubs and caterpillars
with very hard horny heads and soft bodies. As soon as they come into
the dark world of the hive, they begin to eat their way through the
combs, spinning the while a tunnel of silk, which entirely protects
them from the stings of the bees. They can traverse these tunnels with
tolerable speed, so that the bees do not know where to find their
enemies; and if perchance they should discover one of them at the mouth
of its burrow, the hard, horny head is all that is visible, and against
its polished surface the sting of the bee is useless. The rapidity
with which they drive the silken tubes through the comb is really
marvellous; and even if they get among a collection of empty bee-combs,
they make as much havoc as if they were bred in the hive from which the
combs were taken.
In the accompanying illustration are seen figures of the two species
of honey-moths, together with their tunnels. The species may be easily
distinguished by the shape of the wings, _Galleria alvearia_ having, as
seen at Figs. _b_, _c_, the ends of its wings rounded, and _Galleria
cereana_ having them squared.
Some moths, also belonging to the vast Family Tineidæ, do much damage
to grain, and have also the habit of spinning silken tissues as they
eat their way through the grain. One of them is more plentiful on the
Continent than in England, but is known in this country by the name of
the mottled woollen moth (_Tinea granella_)].
The caterpillar, which is smooth and white, ties together with silk
several grains of wheat, barley, rye, or oats, weaving a gallery
between them, from which it projects its head while feeding; the
grains, as Réaumur remarks, being prevented from rolling or slipping
by the silk which unites them. He justly ridicules the absurd notion
of its filing off the outer skin of the wheat by rubbing upon it with
its body, the latter being the softer of the two, and he disproved, by
experiment, Leeuwenhoeck's assertion that it will also feed on woollen
cloth. It is from the end of May till the beginning of July that the
moths, which are of a silvery grey, spotted with brown, appear and lay
their eggs in granaries.
[Illustration: Transformations of the Grain-moths. _a_, Grain of
barley, including a caterpillar; _b_, _c_, the grain cut across,
seen to be hollowed out, and divided by a partition of silk; _d_,
the moth (_Tinea Hordei_); _e_, grains of wheat tied together by
the caterpillar; _f_, _g_, the caterpillar and moth (_Euplocamus
granella_).]
The caterpillar of another still more singular grain-moth (_Tinea
Hordei_, Kirby and Spence) proves sometimes very destructive of
granaries. The mother-moth, in May or June, lays about twenty or
more eggs on a grain of barley or wheat; and when the caterpillars
are hatched they disperse, each selecting a single grain. M. Réaumur
imagines that sanguinary wars must sometimes arise, in cases of
preoccupancy, a single grain of barley being a rich heritage for one
of these tiny insects; but he confesses he never saw such contests.
When the caterpillar has eaten its way into the interior of the grain,
it feeds on the farina, taking care not to gnaw the skin nor even to
throw out its excrements, so that except the little hole, scarcely
discernible, the grain appears quite sound. When it has eaten all the
farina, it spins itself a case of silk within the now hollow grain, and
changes to a pupa in November.
Tent-making Caterpillars.
The caterpillars of a family of small moths (_Tineidæ_), which feed on
the leaves of various trees, such as the hawthorn, the elm, the oak,
and most fruit-trees, particularly the pear, form habitations which are
exceedingly ingenious and elegant. They are so very minute that they
require close inspection to discover them; and to the cursory observer,
unacquainted with their habits, they will appear more like the withered
leaf-scales of the tree, thrown off when the buds expand, than
artificial structures made by insects. It is only, indeed, by seeing
them move about upon the leaves, that we discover they are inhabited by
a living tenant, who carries them as the snail does its shell.
These tents are from a quarter of an inch to an inch in length, and
usually about the breadth of an oat-straw. That they are of the colour
of a withered leaf is not surprising; for they are actually composed of
a piece of leaf; not, however, cut out from the whole thickness, but
artfully separated from the upper layer, as a person might separate one
of the leaves of paper from a sheet of pasteboard.
The tents of this class of caterpillars, which are found on the
elm, the alder, and other trees with serrated leaves, are much in
the shape of a minute goldfish. They are convex on the back, where
the indentations of the leaf out of which they have been cut add to
the resemblance, by appearing like the dorsal fins of the fish. By
depriving one of those caterpillars common on the hawthorn of its
tents, for the sake of experiment, we put it under the necessity of
making another; for, as Pliny remarks of the clothes-moth, they will
rather die of hunger than feed unprotected. When we placed it on a
fresh hawthorn leaf, it repeatedly examined every part of it, as if
seeking for its lost tent, though, when this was put in its way, it
would not again enter it; but, after some delay, commenced a new one.
(J. R.)
[Illustration: A Caterpillar's tent upon a leaf of the elm.--_a_, _a_,
The part of the leaf from which the tent has been cut out; _b_, the
tent itself.]
For this purpose, it began to eat through one of the two outer
membranes which compose the leaf and enclose the pulp (_parenchyma_),
some of which, also, it devoured, and then thrust the hinder part
of its body into the perforation. The cavity, however, which it had
formed, being yet too small for its reception, it immediately resumed
the task of making it larger. By continuing to gnaw into the pulp
_between_ the membranes of the leaf (for it took the greatest care not
to puncture or injure the membranes themselves), it soon succeeded in
mining out a gallery rather larger than was sufficient to contain its
body. We perceived that it did not throw out as rubbish the pulp it dug
into, but devoured it as food--a circumstance not the least remarkable
in its proceedings.
As the two membranes of leaf thus deprived of the enclosed pulp
appeared white and transparent, every movement of the insect within
could be distinctly seen; and it was not a little interesting to
watch its ingenious operations while it was making its tent from the
membranes prepared as we have just described. These, as Réaumur has
remarked, are in fact to the insect like a piece of cloth in the hands
of a tailor; and no tailor could cut out a shape with more neatness
and dexterity than this little workman does. As the caterpillar is
furnished in its mandibles with an excellent pair of scissors, this
may not appear to be a difficult task; yet, when we examine the matter
more minutely, we find that the peculiar shape of the two extremities
requires different curvatures, and this, of course, renders the
operation no less complex, as Réaumur subjoins, than the shaping of
the pieces of cloth for a coat.[BW] The insect, in fact, shapes the
membranes slightly convex on one side and concave on the other, and at
one end twice as large as the other. In the instance which we observed,
beginning at the larger end, it bent them gently on each side by
pressing them with its body thrown into a curve. We have not said it
_cuts_, but _shapes_ its materials; for it must be obvious that if the
insect had cut both the membranes at this stage of its operations, the
pieces would have fallen and carried it along with them.
To obviate such an accident it proceeded to join the two edges, and
secure them firmly with silk, before it made a single incision to
detach them. When it had in this manner joined the two edges along one
of the sides, it inserted its head on the outside of the joining, first
at one end and then at the other, gnawing the fibres till that whole
side was separated. It proceeded in the same manner with the other
side, joining the edges before it cut them: and when it arrived at the
last fibre, the only remaining support of its now finished tent, it
took the precaution, before snipping it, to moor the whole to the uncut
part of the leaf by a cable of its own silk. Consequently, when it does
cut the last nervure, it is secure from falling, and can then travel
along the leaf, carrying its tent on its back, as a snail does its
shell. (J. R.)
We have just discovered (Nov. 4th, 1829) upon the nettle a tent of a
very singular appearance, in consequence of the materials of which it
is made. The caterpillar seems, indeed, to have proceeded exactly in
the same manner as those which we have described, mining first between
the two membranes of the leaf, and then uniting these and cutting
out his tent. But the tent itself looks singular from being all over
studded with the stinging bristles of the nettle, and forming a no less
formidable coat of mail to the little inhabitant than the spiny hide
of the hedgehog. In feeding it does not seem to have mined into the
leaf, but to have eaten the whole of the lower membrane, along with the
entire pulp, leaving nothing but the upper membrane untouched. (J. R.)
During the summer of 1830 we discovered a very large tent which had
been formed out of a blade of grass; and another stuck all over with
chips of leaves upon the common maple.
[Illustration: _a_, The Caterpillar occupying the space it has eaten
between the cuticles of the leaf; _b_, a portion of the upper cuticle,
cut out for the formation of the tent; _c_, the tent nearly completed;
_d_, the perfect tent, with the caterpillar protruding its head.]
Tents of Stone-Mason Caterpillars.
The caterpillar of a small moth (_Tinea_) which feeds upon the lichens
growing on walls, builds for itself a moveable tent of a very singular
kind. M. de la Voye was the first who described these insects; but
though they are frequently overlooked, from being very small, they are
by no means uncommon on old walls. Réaumur observed them regularly
for twenty years together on the terrace-wall of the Tuileries at
Paris; and they may be found in abundance in similar situations in this
country. This accurate observer refuted by experiment the notion of M.
de la Voye that the caterpillars fed upon the stones of the wall; but
he satisfied himself that they detached particles of the stone for the
purpose of building their tents or sheaths (_fourreaux_), as he calls
their dwellings. In order to watch their mode of building, Réaumur
gently ejected half-a-dozen of them from their homes, and observed
them detach grain after grain from a piece of stone, binding each
into the wall of their building with silk till the cell acquired the
requisite magnitude, the whole operation taking about twenty-four hours
of continued labour. M. de la Voye mentions small granular bodies of a
greenish colour, placed irregularly on the exterior of the structure,
which he calls eggs; but we agree with Réaumur in thinking it more
probable that they are small fragments of moss or lichen intermixed
with the stone: in fact, we have ascertained that they are so. (J. R.)
[Illustration: Lichen-Tents and Caterpillars, both of their natural
size and magnified.]
When these little architects prepare for their change into chrysalides
before becoming moths, they attach their tents securely to the stone
over which they have hitherto rambled, by spinning a strong mooring
of silk, so as not only to fill up every interstice between the main
entrance of the tent and the stone, but also weaving a close, thick
curtain of the same material, to shut up the entire aperture.
It is usual for insects which form similar structures to issue, when
they assume the winged state, from the broader end of their habitation;
but our little stone-mason proceeds in a different manner. It leaves
open the apex of the cone from the first, for the purpose of ejecting
its excrements, and latterly it enlarges this opening a little, to
allow of a free exit when it acquires wings; taking care, however, to
spin over it a canopy of silk, as a temporary protection, which it can
afterwards burst through without difficulty. The moth itself is very
much like the common clothes-moth in form, but is of a gilded-bronze
colour, and considerably smaller.
In the same locality, M. de Maupertuis found a numerous brood of small
caterpillars, which employed grains of stone, not, like the preceding,
for building feeding-tents, but for their cocoons. This caterpillar
was of a brownish-grey colour, with a white line along the back, on
each side of which were tufts of hair. The cocoons which it built were
oval, and less in size than a hazel-nut, the grains of the stone being
skilfully woven into irregular meshes of silk.
In June, 1829, we found a numerous encampment of the tent-building
caterpillars described by MM. de la Voye and Réaumur, on the brick
wall of a garden at Blackheath, Kent. (J. R.) They were so very
small, however, and so like the lichen on the wall, that had not our
attention been previously directed to their habits, we should have
considered them as portions of the wall; for not one of them was in
motion, and it was only by the neat, turbinated, conical form in which
they had constructed their habitations that we detected them. We tried
the experiment above mentioned, of ejecting one of the caterpillars
from its tent, in order to watch its proceedings when constructing
another; but probably its haste to procure shelter, or the artificial
circumstances into which it was thrown, influenced its operations, for
it did not form so good a tent as the first, the texture of the walls
being much slighter, while it was more rounded at the apex, and of
course not so elegant. Réaumur found, in all his similar experiments,
that the new structure equalled the old; but most of the trials of this
kind which we have made correspond with the inferiority which we have
here recorded. The process indeed is the same, but it seems to be done
with more hurry and less care. It may be, indeed, in some cases, that
the supply of silk necessary to unite the bits of stone, earth, or
lichen employed, is too scanty for perfecting a second structure.
We remarked a very singular circumstance in the operations of our
little architect, which seems to have escaped the minute and accurate
attention of Réaumur. When it commenced its structure, it was
indispensable to lay a foundation for the walls about to be reared;
but as the tent was to be moveable like the shell of a snail, and not
stationary, it would not have answered its end to cement the foundation
to the wall. We had foreseen this difficulty, and felt not a little
interested in discovering how it would be got over. Accordingly, upon
watching its movements with some attention, we were soon gratified
to perceive that it used its own body as the primary support of the
building. It fixed a thread of silk upon one of its right feet, warped
it over to the corresponding left foot, and upon the thread thus
stretched between the two feet it glued grains of stone and chips of
lichen, till the wall was of the required thickness. Upon this, as a
foundation, it continued to work till it had formed a small portion in
form of a parallelogram; and proceeding in a similar way, it was not
long in making a ring a very little wider than sufficient to admit its
body. It extended this ring in breadth, by working on the inside only,
narrowing the diameter by degrees, till it began to take the form of a
cone. The apex of this cone was not closed up, but left as an aperture
through which to eject its excrements.
It is worthy of remark, that one of the caterpillars which we deprived
of its tent attempted to save itself the trouble of building a new
one, by endeavouring to unhouse one of its neighbours. For this
purpose, it got upon the outside of the inhabited tent, and, sliding
its head down to the entrance, tried to make its way into the interior.
But the rightful owner did not choose to give up his premises so
easily, and fixed his tent down so firmly upon the table where we had
placed it, that the intruder was forced to abandon his attempt. The
instant, however, that the other unmoored his tent and began to move
about, the invader renewed his efforts to eject him, persevering in
the struggle for several hours, but without a chance of success. At
one time we imagined that he would have accomplished his felonious
intentions; for he bound down the apex of the tent to the table with
cables of silk. But he attempted his entrance at the wrong end. He
ought to have tried the aperture in the apex, by enlarging which a
little he would undoubtedly have made good his entrance; and as the
inhabitant could not have turned upon him for want of room, the castle
must have been surrendered. This experiment, however, was not tried,
and there was no hope for him at the main entrance.
Muff-shaped Tents.
The ingenuity of man has pressed into his service not only the wool,
the hair, and even the skins of animals, but has most extensively
searched the vegetable kingdom for the materials of his clothing.
In all this, however, he is rivalled by the tiny inhabitants of the
insect world, as we have already seen; and we are about now to give
an additional instance of the art of a species of caterpillars which
select a warmer material for their tents than even the caterpillar
of the clothes-moth. It may have been remarked by many who are not
botanists, that the seed-catkins of the willow become, as they ripen,
covered with a species of down or cotton, which, however, is too short
in the fibre to be advantageously employed in our manufactures. But the
caterpillars, to which we have alluded, find it well adapted for their
habitations.
The muff-looking tent in which we find these insects does not require
much trouble to construct; for the caterpillar does not, like the
clothes-moth caterpillar, join the willow-cotton together, fibre by
fibre--it is contented with the state in which it finds it on the seed.
Into this it burrows, lines the interior with a tapestry of silk, and
then detaches the whole from the branch where it was growing, and
carries it about with it as a protection while it is feeding.[BX]
[Illustration: _a_, Branch of the Willow, with seed-spikes covered with
cotton; _b_, Muff-tents, made of this cotton by _c_, the Caterpillar.]
An inquiring friend of Réaumur having found one of these insects
floating about in its muff-tent upon water, concluded that they feed
upon aquatic plants; but he was soon convinced that it had only been
blown down by an accident, which must frequently happen, as willows so
often hang over water. May it not be, that the buoyant materials of the
tent were intended to furnish the little inhabitant with a life-boat,
in which, when it chanced to be blown into the water, it might sail
safely ashore and regain its native tree?
Leaf-mining Caterpillars.
The process of mining between the two membranes of a leaf is carried on
to a farther extent by minute caterpillars allied to the tent-makers
above described. The tent-maker never deserts his house, except when
compelled, and therefore can only mine to about half the length of his
own body; but the miners now to be considered make the mine itself
their dwelling-place, and as they eat their way, they lengthen and
enlarge their galleries. A few of these mining caterpillars are the
progeny of small weevils (_Curculionidæ_), some of two-winged flies
(_Diptera_), but the greater number are produced from a genus of minute
moth (_Oecophora_, Latr.), which, when magnified, appear to be amongst
the most splendid and brilliant of Nature's productions, vying even
with the humming-birds and diamond-beetles of the tropics in the rich
metallic colours which bespangle their wings. Well may Bonnet call them
"tiny miracles of Nature," and regret that they are not _en grand_.[BY]
There are few plants or trees whose leaves may not, at some season of
the year, be found mined by these caterpillars, the track of whose
progress appears on the upper surface in winding lines. Let us take one
of the most common of these for an example,--that of the rose-leaf,
produced by the caterpillar of Ray's golden-silver spot (_Argyromiges
Rayella_? Curtis), of which we have just gathered above a dozen
specimens from one rose-tree. (J. R.)
It may be remarked that the winding line is black, closely resembling
the tortuous course of a river on a map,--beginning like a small brook,
and gradually increasing in breadth as it proceeds. This representation
of a river exhibits, besides, a narrow white valley on each side of
it, increasing as it goes, till it terminates in a broad delta. The
valley is the portion of the inner leaf from which the caterpillar has
eaten the pulp (_parenchyma_), while the river itself has been formed
by the liquid _ejectamenta_ of the insect, the watery part becoming
evaporated. In other species of miners, however, the dung is hard and
dry, and consequently these only exhibit the valley without the river
(see p. 255).
[Illustration: Leaf of the Monthly Rose (_Rose Indica_), mined by
Caterpillars of Argyromiges?]
On looking at the back of the leaf, where the winding line begins,
we uniformly find the shell of the very minute egg from which the
caterpillar has been hatched, and hence perceive that it digs into
the leaf the moment it escapes from the egg, without wandering a
hair's-breadth from the spot; as if afraid lest the air should visit it
too roughly. The egg is, for the most part, placed upon the mid-rib of
the rose-leaf, but sometimes on one of the larger nervures. When once
it has got within the leaf, it seems to pursue no certain direction,
sometimes working to the centre, sometimes to the circumference,
sometimes to the point, and sometimes to the base, and even,
occasionally, crossing or keeping parallel to its own previous track.
The most marvellous circumstance, however, is the minuteness of its
workmanship; for though a rose-leaf is thinner than this paper, the
insect finds room to mine a tunnel to live in, and plenty of food,
without touching the two external membranes. Let any one try with
the nicest dissecting instruments to separate the two plates of a
rose-leaf, and he will find it impossible to proceed far without
tearing one or other. The caterpillar goes still further in minute
nicety; for it may be remarked, that its track can only be seen on the
upper, and not on the under surface of the leaf, proving that it eats
as it proceeds only half the thickness of the pulp, or that portion of
it which belongs to the upper membrane of the leaf.
We have found this little miner on almost every sort of rose-tree, both
wild and cultivated, including the sweet-briar, in which, the leaf
being very small, it requires nearly the whole parenchyma to feed one
caterpillar. They seem, however, to prefer the foreign monthly rose to
any of our native species, and there are few trees of this where they
may not be discovered.
[Illustration: Leaf of the Dew-berry Bramble (_Rubus cæsius_), mined by
Caterpillars.]
Tunnels very analogous to the preceding may be found upon the common
bramble (_Rubus fruticosus_); and on the holly, early in spring, one
which is in form of an irregular whitish blotch. But in the former
case, the little miner seems to proceed more regularly, always, when
newly hatched, making directly for the circumference, upon or near
which also the mother-moth deposits her egg, and winding along for half
the extent of the leaf close upon the edge, following, in some cases,
the very indentations formed by the terminating nervures.
The bramble-leaf miner seems also to differ from that of the rose-leaf,
by eating the pulp both from the upper and under surface, at least the
track is equally distinct above and below; yet this may arise from the
different consistence of the leaf pulp, that in the rose being firm,
while that of the bramble is soft and puffy.
On the leaves of the common primrose (_Primula veris_), as well as
on the garden variety of it, the polyanthus, one of those mining
caterpillars may very frequently be found. It is, however, considerably
different from the preceding, for there is no black trace--no river to
the valley which it excavates: its ejectamenta, being small and solid,
are seen, when the leaf is dried, in little black points like grains
of sand. This miner also seems more partial than the preceding to the
mid-rib and its vicinity, in consequence of which its path is seldom
so tortuous, and often appears at its extremity to terminate in an
area comparatively extensive, arising from its recrossing its previous
tracks. (J. R.)
[Illustration: Leaf of the Primrose (_Primula veris_), mined by a
Caterpillar.]
Swammerdam describes a mining caterpillar which he found on the leaves
of the alder, though it did not, like those we have just described,
excavate a winding gallery; it kept upon the same spot, and formed
only an irregular area. A moth was produced from this, whose upper
wings, he says, "shone and glittered most gloriously with crescents
of gold, silver, and brown, surrounded by borders of delicate black."
Another area miner which he found on the leaves of willows, as many as
seventeen on one leaf, producing what appeared to be rusty spots, was
metamorphosed into a very minute weevil (_Curculio Rhionoc._). He says
he has been informed that, in warm climates, worms an inch long are
found in leaves, and adds, with great simplicity, "on these many fine
experiments might have been made, if the inhabitants had not laboured
under the cursed thirst of gold."[BZ]
The vine-leaf miner, when about to construct its cocoon, cuts, from
the termination of its gallery, two pieces of the membrane of the
leaf, deprived of their pulp, in a similar manner to the tent-makers
described above, uniting them and lining them with silk. This she
carries to some distance before she lays herself up to undergo her
change. Her mode of walking under her burthen is peculiar, for,
not contented with the security of a single thread of silk, she
forms, as Bonnet says, "little mountains (_monticules_) of silk,
from distance to distance, and seizing one of these with her teeth,
drags herself forward, and makes it a scaffolding from which she
can build another."[CA] Some of the miners, however, do not leave
their galleries, but undergo their transformations there, taking the
precaution to mine a cell, not in the upper, but in the under surface;
others only shift to another portion of the leaf.
Social Leaf-Miners.
The preceding descriptions apply to caterpillars who construct their
mines in solitude, there being seldom more than one on a leaf or
leaflet, unless when two mother-flies happen to lay their eggs on the
same leaf; but there are others, such as the miners of the leaves of
the henbane (_Hyoscyamus niger_), which excavate a common area in
concert--from four to eight forming a colony. These are very like
flesh-maggots, being larger than the common miners; the leaves of this
plant, from being thick and juicy, giving them space to work and plenty
to eat.