description is that of a moth, whose abode, except as to the
materials which compose it, is formed on the same general plan as
that just described, and the larva in like manner feeds only on the
parenchyma of the leaf. In the beginning of spring, if you examine
the leaves of your pear-trees, you will scarcely fail to meet with
some beset on the under surface with several perpendicular downy
russet-coloured projections, about a quarter of an inch high, and not
much thicker than a pin, of a cylindrical shape, with a protuberance
at the base, and altogether resembling at first sight so many spines
growing out of the leaf. You would never suspect that these could be
the habitations of insects; yet that they are is certain. Detach one
of them, and give it a gentle squeeze, and you will see emerge from
the lower end a minute caterpillar with a yellowish body and black
head. Examine the place from which you have removed it, and you will
perceive a round excavation in the cuticle and parenchyma of the
leaf, the size of the end of the tube by which it was concealed.
This excavation is the work of the above-mentioned caterpillar, which
obtains its food by moving its little tent from one part of the leaf
to the other, and eating away the space immediately under it. It
touches no other part; and when these insects abound, as they often
do to the great injury of pear-trees[796], you will perceive every
leaf bristled with them, and covered with little withered specks, the
vestiges of their former meals. The case in which the caterpillar
resides, and which is quite essential to its existence, is composed
of silk spun from its mouth almost as soon as it is excluded from
the egg. As it increases in size, it enlarges its habitation by
slitting it in two, and introducing a strip of new materials.
But the most curious circumstance in the history of this little
Arab is the mode by which it retains its tent in a perpendicular
posture. This it effects partly by attaching silken threads from the
protuberance at the base to the surrounding surface of the leaf. But
being not merely a mechanician, but a profound natural philosopher
well acquainted with the properties of air, it has another resource
when any extraordinary violence threatens to overturn its slender
turret. It forms a _vacuum_ in the protuberance at the base, and
thus as effectually fastens it to the leaf as if an air-pump had
been employed! This vacuum is caused by the insect's retreating on
the least alarm up its narrow case, which its body completely fills,
and thus leaving the space below free of air. In detaching one of
these cases you may easily convince yourself of the fact. If you
seize it suddenly while the insect is at the bottom, you will find
that it is readily pulled off, the silken cords giving way to a very
slight force; but if, proceeding gently, you give the insect time to
retreat, the case will be held so closely to the leaf as to require a
much stronger effort to loosen it. As if aware that, should the air
get admission from below, and thus render a vacuum impracticable,
the strongest bulwark of its fortress would be destroyed, our little
philosopher carefully avoids gnawing a hole in the leaf, contenting
itself with the pasturage afforded by the parenchyma above the lower
epidermis; and when the produce of this area is consumed, it gnaws
asunder the cords of its tent, and pitches it at a short distance as
before. Having attained its full growth, it assumes the pupa state,
and after a while issues out of its confinement a small brown moth,
with long hind legs, the _Phalæna Tinea serratella_ of Linné[797].
Some larvæ, which form their covering of pure silk, are not content
with a single coating, but actually envelop themselves in another,
open on one side and very much resembling a cloak; whence Reaumur
called them "_Teignes à fourreau à manteau_." What is very striking
in the construction of this cloak, is, that the silk, instead of
being woven into one uniform close texture, is formed into numerous
transparent scales over-wrapping each other, and altogether very much
resembling the scales of a fish[798]. These mantle-covered cases, one
of which I once had the pleasure of discovering, are inhabited by
the larva of a little moth apparently first described by Dr. Zincken
genannt Sommer, who calls it _Tinea palliatella_[799].
Various substances besides silk are fabricated into habitations by
other larvæ, though usually joined together either with silk or an
analogous gummy material. Thus _Diurnea? Lichenum_ forms of pieces
of lichen a dwelling resembling one of the turreted _Helices_, many
of which I observed in June 1812 on an oak in Barham. The larvæ of
another moth, which also feeds upon lichens, instead of employing these
vegetables in forming its habitation, composes it of grains of stone
eroded from the walls of buildings upon which its food is found, and
connected by a silken cement. These insects were the subject of a paper
in the Memoirs of the French Academy[800], by M. de la Voye, who, from
the circumstance of their being found in great abundance on mouldering
walls, attributed to them the power of eating stone, and regarded them
as the authors of injuries proceeding solely from the hand of time:
for the insects themselves are so minute, and the coating of grains
of stone composing their cases is so trifling, that Reaumur observes
they could scarcely make any perceptible impression on a wall from
which they had procured materials for ages[801].--Another lepidopterous
larva, but of a much larger size and different genus, the case of which
is preserved in the cabinet of the President of the Linnean Society,
who pointed it out to me, employs the spines apparently of some species
of _Mimosa_, which are ranged side by side so as to form a very elegant
fluted cylinder. A similar arrangement of pieces of small twigs is
observable in the habitation of the females[802] of the larvæ of a moth
referred by Von Scheven to _Bombyx vestita_, F.; which Ochsenheimer
regards as synonymous with _Psyche graminella_, while _P. Viciella_
of the _Wiener Verzeichniss_ covers itself with short portions of the
stems of grasses placed transversely, and united by means of silk into
a five- or six-sided case. The habitation of a third larva of the same
family, described and figured by Reaumur (_P. graminella_, Ochsenh.
just named), is composed of squarish pieces of the _leaves_ of grass
fastened only at one end, and overwrapping each other like the tiles of
a house; and that of another noticed by the same author, of portions
of the smallest twigs of broom arranged on the same plan[803]. Indeed
the larvæ of the whole of this tribe of moths, now separated into a
distinct genus (_Psyche_, Schrank, Ochsenh., _Fumea_, Haworth), but
which according to Germar needs further subdivision, reside in cases or
sacks (whence they are called by the Germans _Sackträger_) composed of
silk, and fragments of grass, bark, &c.
The larvæ of a small beetle (_Clytra longimana_) reside in oviform
cases apparently of a calcareous or earthy substance, joined by
a gummy cement and covered with red hairs, the origin of which,
Hübner, who first discovered them, could not account for: and from
the observations of Amstein and the French translator of Fuessly's
_Archives_, it seems probable that the larvæ of all the species
of _Clytra_, and according to Zschorn, at least of one species of
Cryptocephalus, (_C. duodecimpunctatus_) differing in this respect
from all other known _Coleoptera_, live in moveable cases[804]. I
have however found a species of _Limnius_ (_L. æneus_) inhabiting a
fixed case made of particles of stone or sand.
Wax is the principal substance employed in the habitations of the larvæ
before mentioned[805], occasionally so destructive to bee-hives. These
insidious depredators, which are mentioned by Aristotle[806], tying
together, with silk, grains of wax (which, and not honey, forms their
food), construct galleries of a considerable length, and thus concealed
from the sight, and protected from the stings of the armed people
whom they have attacked, push their mines into the very heart of the
fortress, and pursue their robberies in perfect safety[807].
As many of the habitations which I have been describing, fit the
body of the insects as close as a coat, they might perhaps with more
propriety be called _clothes_. This is certainly the most appropriate
designation of the abodes of some species of Tineæ (the clothes'
moths), which not only cover themselves with a coat, but employ the
very same material in its composition as we do in ours, forming it
of wool or hair curiously felted together. Like us, they are born
naked, but not like us helpless at that period, scarcely have they
breathed before they begin to clothe themselves; thus contradicting
Dr. Paley's assertion, that "the _human_ animal is the only one
which is naked, and the only one which can clothe itself[808]:" and
wisely inattentive to change of fashion, the same suit serves them
from their birth to mature age. The shape of their dress is adapted
to that of their body--a cylindrical case open at both ends. The
stuff of which it is composed is the manufacture of the larva of the
moth (_Tinea_), which incorporates wool or hair artfully cut from
our clothes or furniture, with silk drawn from its own mouth, into
a warm and thick tissue: and as this would not be soft enough for
its tender skin, it also lines the inside of its coat with a layer
of pure silk. Since this suit of clothes during the earliest age
of the insect accurately fits its body, you will readily conceive
that it will frequently require enlarging. This the little occupant
accomplishes as dexterously as any tailor. If the case merely
requires lengthening, the task is easy. All that is needful is to add
a new ring of hair or wool and silk to each end. But to enlarge it
in width is not so simple an affair. Yet it sets to work precisely
as we should, 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,
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 habit 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. If placed alternately upon cloths of different
hues, its dress will be parti-coloured like that of a Harlequin.--The
injury occasioned to us by these insects is not confined to the
quantity of materials consumed in clothing and feeding themselves.
In moving from place to place they seem to be as much incommoded by
the long hairs which surround them, as we are by walking amongst high
grass; and accordingly, marching scythe in hand, with their teeth
they cut out a smooth road, from time to time reposing themselves,
and anchoring their little case with small silken cables.
If, as I hope, you are induced to investigate the manners of
these insects, you have but to leave an old coat for a few months
undisturbed in a dark closet, and you may be pretty certain of
meeting with an abundant colony.
Not merely wool or hair, but another substance analogous to one
employed in our dress, is adopted for their clothing by other insects.
The larva of a fly which lives on the seeds of willows, makes itself a
very beautiful case of their cottony down, not only impervious to wet
and cold, but serving, if accidentally blown into the water, which from
the situation of these trees frequently happens, as a buoyant little
barge which is wafted safely to the shore[809].
The habitations which we have hitherto been considering, are formed
by larvæ that live on land, but others equally remarkable are
constructed by aquatic species, the larvæ of the various _Phryganeæ_
L., a tribe of four-winged insects which an ordinary observer would
call moths, but which are even of a distinct order (_Trichoptera_),
not having their wings covered by the scales which adorn the
lepidopterous race. If you are desirous of examining the insects to
which I am alluding, you have only to place yourself by the side of
a clear and shallow pool of water, and you cannot fail to observe
at the bottom little oblong moving masses resembling pieces of
straw, wood, or even stone. These are the larvæ in question, well
known to fishermen by the title of _Caddis-worms_, and which, if you
take them out of the water, you will observe to inhabit cases of
a very singular conformation. Of the larva itself, which somewhat
resembles the caterpillars of many _Lepidoptera_, nothing is to be
seen but the head and six legs by means of which it moves itself
in the water, and drags after it the case in which the rest of the
body is inclosed, and into which on any alarm it wholly retires. The
construction of these habitations is very various. Some select four
or five pieces of the leaves of grass, which they glue together into
a shapely polygonal case; others employ portions of the stems of
rushes, placed side by side so as to form an elegant fluted cylinder;
some arrange round them pieces of leaves like a spirally-rolled
ribband[810]; others inclose themselves in a mass of the leaves of
any aquatic plants united without regularity; and others again form
their abode of minute pieces of wood either fresh or decayed[811].
One, like the _Sabellæ_[812], forms a horn-shaped case composed of
grains of sand, so equal in size, and so nicely and regularly gummed
together, the sides throughout being of the thickness of one grain
only, that the first time I viewed it I could scarcely persuade
myself it could be the work of an insect. The case of _Leptocerus
bimaculatus_, which is less artificially constructed of a mixture
of mud and sand, is pyriform, and has its end curiously stopped by
a plate formed of grains of sand, with a central aperture[813].
Other species construct houses which may be called alive, forming
them of the shells of various aquatic snails of different kinds and
sizes even while inhabited, all of which are immoveably fixed to it,
and dragged about at its pleasure--a covering as singular as if a
savage, instead of clothing himself with squirrels' skins, should
sew together into a coat the animals themselves. However various may
be the form of the case externally, within it is usually cylindrical
and lined with silk; and though seldom apparently wider than just to
admit the body of the insect, some species have the power of turning
round in it, and of putting out their head at either end[814]. Some
larvæ constantly make their cases of the same materials; others
employ indifferently any that are at hand; and the new ones which
they construct as they increase in size (for they have not the
faculty, like the larva of the moth, of enlarging them) have often an
appearance quite dissimilar to that of the old. Even those that are
most careless about the nature of the materials of their house, are
solicitously attentive to one circumstance respecting them, namely,
their specific gravity. Not having the power of swimming, but only of
walking at the bottom of the water by aid of the six legs attached
to the fore part of the body which is usually protruded out of the
case, and the insect itself being heavier than water, it is of great
importance that its house should be of a specific gravity so nearly
that of the element in which it resides, as while walking neither to
incommode it by its weight, nor by too great buoyancy; and it is as
essential that it should be so equally _ballasted_ in every part as
to be readily moveable in any position. Under these circumstances our
Caddis-worms evince their proficiency in hydrostatics, selecting the
most suitable substances; and, if the cell be too heavy, glueing
to it a bit of leaf or straw; or, if too light, a shell or piece of
gravel. It is from this necessity of regulating the specific gravity,
that to the cases formed with the greatest regularity we often see
attached a seemingly superfluous piece of wood, leaf, or the like.
A larva of one of the aquatic _Tipulariæ_ lives in cases somewhat
similar to those of some _Phryganeæ_. Several of these of a fusiform
shape and brown colour, composed partly of silk and partly perhaps
of fragments of leaves, and inhabited by a red larva apparently of
a _Chironomus_, were found by Reaumur upon dead leaves in a pool of
water in the Bois de Boulogne[815].
In concluding this head I may observe, that here might have been
described the various abodes which solitary larvæ prepare for
themselves previously to assuming the pupa, and intended for their
protection in that defenceless stage of existence; but as I shall
have occasion again to refer to them in speaking of the larva state
of insects, I shall defer their description to that letter, to which
they more strictly belong.
* * * * *
From the next division of the habitations of insects--those formed
by solitary _perfect_ insects for their own accommodation--I shall
select for description only two, both the work of spiders, and
alluded to in a former letter, which indeed, with the exception of
the inartificial retreats made by the _Grylli_, _Cicindelæ_, and
perhaps a few others, are the only ones properly belonging to it.
The habitation of one of these (_Cteniza cæmentaria_) is
subterraneous, not a mere shallow cavity, but a tube or gallery
upwards of two feet in length and half an inch broad. This tunnel,
so vast compared with the size of the insect, it digs by means of
its strong jaws in a steep bank of bare clay, so that the rain
may readily run off without penetrating to its dwelling. Its next
operation is to line the whole from top to bottom with a web of fine
silk, which serves the double purpose of preventing the earth that
composes the walls from falling in, and, by its connexion with the
door of the orifice, of giving information to the spider of what
is passing above. You doubtless suppose that in saying _door_ I am
speaking metaphorically. It could never enter into your conception
that any animal, much less an insect, could construct any thing
really deserving of that name--any thing like our doors, turning
upon a hinge, and accurately fitted to the frame of the opening
which it is intended to close. Yet such a door, incredible as it may
seem, is actually framed by this spider. It does not indeed, like
us, compose it of wood, but of several coats of dried earth fastened
to each other with silk. When finished, its outline is as perfectly
circular as if traced with compasses; the inferior surface is convex
and smooth, the superior flat and rough, and so like the adjoining
earth as not to be distinguishable from it. This door the ingenious
artist fixes to the entrance of her gallery by a hinge of silk,
which plays with the greatest freedom, and allows it to be opened
and shut with ease; and as if acquainted with the laws of gravity,
she invariably fixes the hinge at the _highest_ side of the opening,
so that the door when pushed up shuts again by its own weight. She
has not less sagaciously left a little edge or groove just within
the entrance, upon which the door closes, and to which it fits with
such precision, that it seems to make but one surface with it. Such
is the astonishing structure of this little animal's abode; nor
is its defence of its subterraneous cavern less surprising. If an
observer adroitly insinuates the point of a pin under the edge of
the door, and elevates it a little, he immediately perceives a very
strong resistance.--What is its cause?--The spider, warned by the
vibrations of the threads which extend from the door to the bottom
of her gallery, runs with all speed to the door, fastens its legs to
it on one side, and on the other to the walls, and, turning upon its
back, pulls with all its might. Thus the door is alternately shut or
opened, as the exertions of the observer or of the spider prevail.
It is easy to guess which will in the end conquer; and the spider,
when it finds all resistance ineffectual, betakes itself to flight,
and retreats. If, to make a further experiment, the observer fastens
down the door so that it cannot be forced open, the next morning he
will find a new entrance, with a new door formed at a small distance;
or, if he take the door entirely away, another will be constructed in
less than twelve hours.
The habitation thus singularly formed and defended is not at all used
as a snare, but merely as a safe abode for the spider, which hunts
its prey at night only; and, when caught, devours it in security at
the bottom of its den, which is generally strewed with the remains
of coleopterous insects[816].--From some curious observations of
M. Dorthes on this species in the second volume of the _Linnean
Transactions_, it appears that both the male and female spider
and as many as thirty young ones occasionally inhabit one of these
galleries.--_Mygale Sauvagesii_ of Rossi, which is a distinct species
found in Corsica, forms a similar habitation[817].
The galleries just described are the work of an European species not
uncommon in the south of France; but similar ones are fabricated by
_Thomisus venatorius_, an inhabitant of the West India islands, as
well as by many other tropical species. I have seen one of these,
which had been dug out of the earth, in the cabinet of Thomas Hall,
Esq. F.L.S., that was nearly a foot in length, and above an inch in
diameter, forming a cylindrical bag of dark-coloured silk, closed at
the bottom, and accurately fitted at the top by a door or lid.