long, in a horizontal direction, either in the ground or in the trunk
of a rotten willow-tree, or occasionally in other decaying wood. This
cavity she fills with six or seven cells wholly composed of portions
of leaf, of the shape of a thimble, the convex end of one closely
fitting into the open end of another. Her first process is to form
the exterior coating, which is composed of three or four pieces of
larger dimensions than the rest, and of an oval form. The second
coating is formed of portions of equal size, narrow at one end but
gradually widening towards the other, where the width equals half
the length. One side of these pieces is the serrate margin of the
leaf from which it was taken, which, as the pieces are made to lap
one over the other, is kept on the outside, and that which has been
cut within. The little animal now forms a third coating of similar
materials, the middle of which, as the most skilful workman would do
in similar circumstances, she places over the margins of those that
form the first tube, thus covering and strengthening the junctures.
Repeating the same process, she gives a fourth and sometimes a
fifth coating to her nest, taking care, at the closed end or narrow
extremity of the cell, to bend the leaves so as to form a convex
termination. Having thus finished a cell, her next business is to
fill it to within half a line of the orifice, with a rose-coloured
conserve composed of honey and pollen, usually collected from the
flowers of thistles; and then having deposited her egg, she closes
the orifice with three pieces of leaf so exactly circular, that a
pair of compasses could not define their margin with more truth; and
coinciding so precisely with the walls of the cell, as to be retained
in their situation merely by the nicety of their adaptation. After
this covering is fitted in, there remains still a concavity which
receives the convex end of the succeeding cell; and in this manner
the indefatigable little animal proceeds until she has completed the
six or seven cells which compose her cylinder.
The process which one of these bees employs in cutting the pieces
of leaf that compose her nest is worthy of attention. Nothing can
be more expeditious: she is not longer about it than we should be
with a pair of scissors. After hovering for some moments over a
rose-bush, as if to reconnoitre the ground, the bee alights upon the
leaf which she has selected, usually taking her station upon its
edge so that the margin passes between her legs. With her strong
mandibles she cuts without intermission in a curve line so as to
detach a triangular portion. When this hangs by the last fibre, lest
its weight should carry her to the ground, she balances her little
wings for flight, and the very moment it parts from the leaf flies
off with it in triumph; the detached portion remaining bent between
her legs in a direction perpendicular to her body. Thus without rule
or compasses do these diminutive creatures mete out the materials
of their work into portions of an ellipse, into ovals or circles,
accurately accommodating the dimensions of the several pieces of each
figure to each other. What other architect could carry impressed
upon the tablet of his memory the entire idea of the edifice which
he has to erect, and, destitute of square or plumb-line, cut out his
materials in their exact dimensions without making a single mistake?
Yet this is what our little bee invariably does. So far are human art
and reason excelled by the teaching of the Almighty[771].
Other insects besides bees construct habitations of different kinds
for their young, as various species of burrowing wasps (_Fossores_),
_Geotrupes_, &c., which deposit their eggs in cylindrical excavations
that become the abode of the future larvæ. In the procedures of most of
these, nothing worth particularizing occurs; but one species called by
Reaumur the mason-wasp, (_Odynerus muraria_,) referred to in a former
letter, works upon so singular a plan, that it would be improper to
pass it over in silence, especially as these nests may be found in this
country in most sandy banks exposed to the sun. This insect bores a
cylindrical cavity from two to three inches deep, in hard sand which
its mandibles alone would be scarcely capable of penetrating, were it
not provided with a slightly glutinous liquor which it pours out of
its mouth, that, like the vinegar with which Hannibal softened the
Alps, acts upon the cement of the sand, and renders the separation of
the grains easy to the double pickaxe with which our little pioneer is
furnished. But the most remarkable circumstance is the mode in which it
disposes of the excavated materials. Instead of throwing them at random
on a heap, it carefully forms them into little oblong pellets, and
arranges them round the entrance of the hole so as to form a tunnel,
which, when the excavation is completed, is often not less than two or
three inches in length. For the greater part of its height this tunnel
is upright, but towards the top it bends into a curve, always however
retaining its cylindrical form. The little masses are so attached to
each other in this cylinder, as to leave numerous vacuities between
them, which give it the appearance of filagree-work. You will readily
divine that the excavated hole is intended for the reception of an egg,
but for what purpose the external tunnel is meant is not so apparent.
One use, and perhaps the most important, would seem to be to prevent
the incursions of the artful Ichneumons, _Chrysidæ_, &c. which are
ever on the watch to insinuate their parasitic young into the nests of
other insects: it may render their access to the nest more difficult;
they may dread to enter into so long and dark a defile. I have seen,
however, more than once a _Chrysis_ come out of these tunnels. That
its use is only temporary, is plain from the circumstance that the
insect employs the whole fabric, when its egg is laid and store of food
procured, in filling up the remaining vacuity of the hole; taking down
the pellets, which are very conveniently at hand, and placing them in
it until the entrance is filled[772].--Latreille informs us, that a
nearly similar tunnel, but composed of grains of earth, is built at the
entrance of its cell by a bee of his family of _pioneers_[773].
Under this head, too, may be most conveniently arranged the very
singular habitations of the larvæ of the Linnæan genus _Cynips_,
the gall-fly, though they can with no propriety be said to be
_constructed_ by the mother, who, provided with an instrument as
potent as an enchanter's wand, has but to pierce the site of the
foundation, and commodious apartments, as if by magic, spring up
and surround the germe of her future descendants. I allude to those
vegetable excrescencies termed _galls_, some of which resembling
beautiful berries and others apples, you must have frequently
observed on the leaves of the oak, and of which one species, the
Aleppo gall, as I have before noticed, is of such importance in the
ingenious art "_de peindre la parole et de parler aux yeux_[774]."
All these tumours owe their origin to the deposition of an egg in
the substance out of which they grow. This egg, too small almost for
perception, the parent insect, a little four-winged fly, introduces
into a puncture made by her curious spiral sting, and in a few hours
it becomes surrounded with a fleshy chamber, which not only serves
its young for shelter and defence, but also for food; the future
little hermit feeding upon its interior and there undergoing its
metamorphosis. Nothing can be more varied than these habitations.
Some are of a globular form, a bright red colour, and smooth fleshy
consistence, resembling beautiful fruits, for which indeed, as you
have before been told, they are eaten in the Levant: others, beset
with spines or clothed with hair, are so much like seed-vessels, that
an eminent modern chemist has contended respecting the Aleppo gall,
that it is actually a capsule[775]. Some are exactly round; others
like little mushrooms; others resemble artichokes; while others
again might be taken for flowers: in short, they are of a hundred
different forms, and of all sizes from that of a pin's head to that
of a walnut. Nor is their situation on the plant less diversified.
Some are found upon the leaf itself; others upon the footstalks
only; others upon the roots; and others upon the buds[776]. Some of
them cause the branches upon which they grow to shoot out into such
singular forms, that the plants producing them were esteemed by the
old botanists distinct species. Of this kind is the _Rose-willow_,
which old Gerard figures and describes as "not only making a gallant
shew, but also yeelding a most cooling aire in the heat of summer,
being set up in houses for the decking of the same." This willow
is nothing more than one of the common species, whose twigs, in
consequence of the deposition of the egg of a Cynips in their
summits, there shoot out into numerous leaves totally different
in shape from the other leaves of the tree, and arranged not much
unlike those composing the flower of a rose, adhering to the stem
even after the others fall off. Sir James Smith mentions a similar
_lusus_ on the Provence willows, which at first he took for a tufted
lichen[777]. From the same cause the twigs of the common wild rose
often shoot out into a beautiful tuft of numerous reddish moss-like
fibres wholly dissimilar from the leaves of the plant, deemed by the
old naturalists a very valuable medical substance, to which they
erroneously gave the name of Bedeguar. None of these variations is
accidental or common to several of the tribe, but each peculiar to
the galls formed by a single and distinct species of _Cynips_.
How the mere insertion of an egg into the substance of a leaf or
twig, even if accompanied, as some imagine, by a peculiar fluid,
should cause the growth of such singular protuberances around it,
philosophers are as little able to explain, as why the insertion of
a particle of variolous matter into a child's arm should cover it
with pustules of small pox. In both cases the effects seem to proceed
from some action of the foreign substance upon the secreting vessels
of the animal or vegetable: but of the nature of this action we know
nothing. Thus much is ascertained by the observations of Reaumur
and Malpighi--that the production of the gall, which however large
attains its full size in a day or two[778], is caused by the egg or
some accompanying fluid: not by the larva, which does not appear
until the gall is fully formed[779]; that the galls which spring from
leaves almost constantly take their origin from nerves[780]; and that
the egg, at the same time that it causes the growth of the gall,
itself derives nourishment from the substance that surrounds it,
becoming considerably larger before it is hatched than it was when
first deposited[781].--When chemically analysed, galls are found to
contain only the same principles as the plant from which they spring,
but in a more concentrated state.
No productions of nature seem to have puzzled the ancient
philosophers more than galls. The commentator on Dioscorides,
Mathiolus, who agreeably to the doctrine of those days ascribed their
origin to spontaneous generation, gravely informs us that weighty
prognostications as to the events of the ensuing year may be deduced
from ascertaining whether they contain spiders, worms, or flies.
Other philosophers, who knew that except by rare accident no other
animals are to be found in galls, besides grubs of different kinds
which they rationally conceived to spring from eggs, were chiefly at
a loss to account for the conveyance of these eggs into the middle
of a substance in which they could find no external orifice. They
therefore inferred that they were the eggs of insects deposited in
the earth, which had been drawn up by the roots of trees along with
the sap, and after passing through different vessels had stopped,
some in the leaves, others in the twigs, and had there hatched and
produced galls! Redi's solution of the difficulty was even more
extraordinary. This philosopher, who had so triumphantly combated the
absurdities of spontaneous generation, fell himself into greater.
Not having been able to witness the deposition of eggs by the parent
flies in the plants that produce galls, he took it for granted that
the grubs which he found within them could not spring from eggs:
and he was equally unwilling to admit their origin from spontaneous
generation,--an admission which would have been fatal to his own
most brilliant discoveries. He therefore cut the knot, by supposing
that to the same _vegetative soul_ by which fruits and plants are
produced, is committed the charge of creating the larvæ found in
galls[782]! An instance truly humiliating, how little we can infer
from a man's just ideas on one point, that he will not be guilty of
the most pitiable absurdity on another!
Though by far the greater part of the vegetable excrescencies
termed galls, are caused by insects of the genus _Cynips_, they do
not always originate from this tribe. Some are produced by weevils
belonging to Schüppel's genus _Ceutorhynchus_; as those on the roots
of kedlock (_Sinapis arvensis_), which I have ascertained to be
inhabited by the larvæ of _Curculio contractus_ Marsh., _Rhynchænus
assimilis_, F. From the knob-like galls on turnips called in some
places the _anbury_, I have bred another of these weevils, (_Curculio
pleurostigma_, Marsh., _Rhynchænus sulcicollis_, Gyll.) and I have
little doubt that the same insects, or species allied to them, cause
the clubbing of the roots of cabbages. It seems to be a beetle of the
same family that is figured by Reaumur[783], as causing the galls on
the leaves of the lime-tree. Others owe their origin to moths, as
those resembling a nutmeg which Reaumur received from Cyprus[784];
and others again to two-winged flies, as the woody galls of the
thistle caused by _Trypeta Cardui_[785], and the cottony galls found
on ground ivy, wild thyme, &c. as well as a very singular one on
the juniper resembling a flower, described by De Geer[786], all
which are the work of minute gall-gnats (_Cecidomyiæ_, Latr.). Some
of these last convert even the flowers of plants into a kind of
galls, as _T. Loti_ of De Geer[787], which inhabits the blossoms of
_Lotus corniculatus_; and one which I have myself observed to render
the flowers of _Erysimum Barbarea_ like a hop blossom. A similar
monstrous appearance is communicated to the flowers of _Teucrium
supinum_ by a little field-bug, _Tingis Teucrii_ of Host[788],
and to another plant of the same genus by one of the same tribe
described by Reaumur[789]. In these two last instances, however,
the habitations do not seem strictly entitled to the appellation of
galls, as they originate not from the egg, but from the larva, which,
in the operation of extracting the sap, in some way imparts a morbid
action to the juices, causing the flower to expand unnaturally: and
the same remark is applicable to the gall-like swellings formed by
many Aphides, as _A. Pistaciæ_, which causes the leaves of different
species of _Pistacia_ to expand into red finger-like cavities; _A.
Abietis_, which converts the buds or young shoots of the fir into a
very beautiful gall, somewhat resembling a fir-cone, or a pine-apple
in miniature; and _A. Bursariæ_, which with its brood inhabits
angular utriculi on the leafstalk of the black poplar, numbers of
which I have observed on those trees by the road-side from Hull
to Cottingham.--The majority of galls are what entomologists have
denominated monothalamous, or consisting of only one chamber or cell;
but some are polythalamous, or consisting of several.
* * * * *
Having thus described the most remarkable of the habitations
constructed by the parent insects for the accommodation of their
future young, I proceed to the second kind mentioned, namely, those
which are formed by the insect itself for its own use. These may be
again subdivided into such as are the work of the insects in their
larva state; and such as are formed by perfect insects.
Many larvæ of all orders need no other habitations than the holes which
they form in seeking for, or eating, the substances upon which they
feed. Of this description are the majority of subterranean larvæ, and
those which feed on wood, as the _Bostrichi_ or labyrinth beetles; the
_Anobia_ which excavate the little circular holes frequently met with
in ancient furniture and the wood work of old houses; and many larvæ
of other orders, particularly _Lepidoptera_. One of these last, the
larva of _Cossus ligniperda_ differs from its congeners in fabricating
for its residence during winter a habitation of pieces of wood lined
with fine silk[790]. Under this division, too, come the singular
habitations of the subcutaneous larvæ, so called from the circumstance
of their feeding upon the parenchyma included between the upper and
under cuticles of the leaves of plants, between which, though the
whole leaf is often not thicker than a sheet of writing-paper, they
find at once food and lodging. You must have been at some time struck
by certain white zigzag or labyrinth-like lines on the leaves of the
dandelion, bramble, and numerous other plants: the next-time you meet
with one of them, if you hold it up to the light you will perceive that
the colour of these lines is owing to the pulpy substance of the leaf
having there been removed; and at the further end you will probably
remark a dark-coloured speck, which, when carefully extricated from
its covering, you will find to be the little miner of the tortuous
galleries which you are admiring. Some of these minute larvæ, to which
the parenchyma of a leaf is a vast country, requiring several weeks to
be traversed by the slow process of mining which they adopt--that of
eating the excavated materials as they proceed--are transformed into
beetles (_Cionus Thapsi_, &c.); others into flies; and a still greater
number into very minute moths, as _Gracillaria? Wilkella_, _Clerkella_,
&c. Many of these last are little miracles of nature, which has
lavished on them the most splendid tints tastefully combined with gold,
silver and pearl: so that, were they but formed upon a larger scale,
they would far eclipse all other animals in richness of decoration.
Another tribe of larvæ, not very numerous, content themselves
for their habitations with simple holes, into which they retire
occasionally. Many of these are merely cylindrical burrows in the
ground, as those formed by the larvæ of field-crickets, Cicindelæ
and Ephemeræ. But the larvæ of the very remarkable lepidopterous
genus (_Nycterobius_ of Mr. MacLeay) before alluded to[791], excavate
for themselves dwellings of a more artificial construction; forming
cylindrical holes in the trees of New Holland, particularly the
different species of _Banksia_, to which they are very destructive,
and defending the entrance against the attacks of the Mantes and
other carnivorous insects by a sort of trap-door composed of silk
interwoven with leaves and pieces of excrement, securely fastened
at the upper end, but left loose at the lower for the free passage
of the occupant. This abode they regularly quit at sun-set, for the
purpose of laying in a store of the leaves on which they feed.
These they drag by one at a time into their cell until the approach
of light, when they retreat precipitately into it, and there remain
closely secluded the whole day, enjoying the booty which their
nocturnal range has provided. One species lifts up the loose end of
its door by its tail, and enters backward, dragging after it a leaf
of _Banksia serrata_, which it holds by the footstalk[792].
A third description of larvæ, chiefly of the two lepidopterous
tribes _Tortricidæ_ and _Tineidæ_, form into convenient habitations
the leaves of the plants on which they feed. Some of these merely
connect together with a few silken threads several leaves so as to
form an irregular packet, in the centre of which the little hermit
lives. Others confine themselves to a single leaf, of which they
simply fold one part over the other. A third description form and
inhabit a sort of roll, by some species made cylindrical, by others
conical, resembling the papers into which grocers put their sugar,
and as accurately constructed, only there is an opening left at the
smaller extremity for the egress of the insect in case of need. If
you were to see one of these rolls, you would immediately ask by
what mechanism it could possibly be made--how an insect without
fingers could contrive to bend a leaf into a roll, and to keep it
in that form until fastened with the silk which holds it together?
The following is the operation. The little caterpillar first fixes a
series of silken cables from one side of the leaf to the other. She
next pulls at these cables with her feet; and when she has forced
the sides to approach, she _fastens_ them together with shorter
threads of silk. If the insect finds that one of the larger nerves
of the leaf is so strong as to resist her efforts, she weakens it
by gnawing it here and there half through. What engineer could act
more sagaciously?--To form one of the conical or horn-shaped rolls,
which are not composed of a whole leaf, but of a long triangular
portion cut out of the edge, some other manœuvres are requisite.
Placing herself upon the leaf, the caterpillar cuts out with her
jaws the piece which is to compose her roll. She does not however
entirely detach it: it would then want a base. She detaches that
part only which is to form the contour of the horn. This portion is
a triangular strap, which she rolls as she cuts. When the body of
the horn is finished, as it is intended to be fixed upon the leaf
in nearly an upright position, it is necessary to elevate it. To
effect this, she proceeds as we should with an inclined obelisk. She
attaches threads or little cables towards the point of the pyramid,
and raises it by the weight of her body[793].
A still greater degree of dexterity is manifested in fabricating the
habitations of the larvæ of some other moths which feed on the leaves
of the rose-tree, apple, elm, and oak, on the under-side of which
they may in summer be often found. These form an oblong cavity in the
interior of a leaf by eating the parenchyma between the two membranes
composing its upper and under side, which, after having detached them
from the surrounding portion, it joins with silk so artfully that the
seams are scarcely discoverable even with a lens, so as to compose
a case or horn, cylindrical in the middle, its anterior orifice
circular, its posterior triangular. Were this dwelling cylindrical in
every part, the form of the two pieces that compose it would be very
simple; but the different shape of the two ends renders it necessary
that each side should have peculiar and dissimilar curvatures; and
Reaumur assures us, that these are as complex and difficult to
imitate as the contours of the pieces of cloth that compose the back
of a coat. Some of this tribe, whose proceedings I had the pleasure
of witnessing a short time since upon the alders in the Hull Botanic
Garden, more ingenious than their brethren, and willing to save the
labour of sewing up two seams in their dwelling, insinuate themselves
near the edge of a leaf instead of in its middle. Here they form
their excavation, mining into the very crenatures between the two
surfaces of the leaf, which, being joined together at the edge, there
form one seam of the case, and from their dentated figure give it
a very singular appearance, not unlike that of some fishes which
have fins upon their backs. The opposite side they are necessarily
forced to cut and sew up, but even in this operation they show an
ingenuity and contrivance worthy of admiration. The moths, which cut
out their suit from the middle of the leaf, wholly detach the two
surfaces that compose it before they proceed to join them together,
the serrated incisions made by their teeth, which, if they do not
cut as fast, in this respect are more effective than any scissors,
interlacing each other so as to support the separated portions until
they are properly joined. But it is obvious that this process cannot
be followed by those moths which cut out their house from the edge
of a leaf. If these were to detach the inner side before they had
joined the two pieces together, the builder as well as his dwelling
would inevitably fall. They therefore, before making any incision,
prudently _run_ (as a sempstress would call it) loosely together
in distant points the two membranes on that side. Then putting out
their heads they cut the intermediate portions, carefully avoiding
the larger nerves of the leaf; afterwards they sew up the detached
sides more closely, and only intersect the nerves when their labour
is completed[794].--The habitation made by a moth, which lives upon a
species of _Astragalus_, is in like manner formed of the epidermis of
the leaves, but in this several corrugated pieces project over each
other, so as to resemble the furbelows once in fashion[795].