or articulate legs. These legs (generally six in number, and attached
to the underside of the three first segments of the body) vary in
larvæ of the different orders: but they seem in most to have joints
answering to the hip (_coxa_); trochanter; thigh (_femur_); shank
(_tibia_); foot (_tarsus_), of perfect insects, the legs of which
they include. Cuvier, speaking of _Coleoptera_ and some _Neuroptera_,
mentions only three joints. But many in these orders (amongst which
he included the _Trichoptera_) have the joints I have enumerated. To
name no more, the _Lamellicornia_, _Dytisci_, _Silphæ_, _Staphylini_,
_Cicindelæ_, and _Gyrini_, &c. amongst coleopterous larvæ; and
the _Trichoptera_, as well as the _Libellulina_ and _Ephemerina_,
amongst Cuvier's _Neuroptera_,--have these joints, and in many the
last terminates in a double claw[439]. In some coleopterous genera
the tarsus seems absent or obsolete. The larva of the lady-bird
(_Coccinella_) affords an example of the former kind, and that of
_Chrysomela_ of the latter[440]. These joints are very visible in the
legs of caterpillars of _Lepidoptera_, and their tarsus is armed with
a single claw[441]. The larvæ that have these legs walk with them
sometimes very swiftly. In stepping they set forward at the same time
the anterior and posterior legs of one side, and the intermediate one
of the other; and so alternately on each side.
Pedate larvæ are of two descriptions: those that to perfect legs add
spurious ones with or without claws, and those that have only perfect
legs. I begin with the former--those that have both kinds of legs. But
first I must make a few remarks upon _spurious_ legs. Because their
muscles, instead of the horny substance that protects them in perfect
legs, are covered only by a soft membrane, they have been usually
denominated _membranaceous legs_: since, however, they are temporary,
vanishing altogether when the insect arrives at its perfect state,--are
merely used, for they do not otherwise assist in this motion, as props
to hinder its long body, when it walks, from trailing on the ground;
to push against the plane of position; and, by means of their hooks or
claws, to fix itself firmly to its station when it feeds or reposes,--I
shall therefore call them prolegs (_propedes_[442]). These organs
consist of three or four folds, and are commonly terminated, though
not always, by a coronet or semicoronet of very minute crooked claws
or hooks. These claws, which sometimes amount to nearly a hundred on
one proleg, are alternately longer and shorter. They are crooked at
both ends, and are attached to the proleg by the back by means of a
membrane, which covers about two-thirds of their length, leaving their
two extremities naked. Of these the upper one is sharp, and the lower
blunt. The sole, or part of the prolegs within the claws, is capable of
opening and shutting. When the animal walks, that they may not impede
its motion, it is shut, and the claws are laid flat with their points
inwards; but when it wishes to fix itself, the sole is opened, becoming
of greater diameter than before, and the claws stand erect with their
points outwards. Thus they can lay stronger hold of the plane of
position[443].
The number of these prolegs varies in different species and families.
In the numerous tribes of saw-flies (_Serrifera_), the larvæ of
which resemble those of _Lepidoptera_, and are called by Reaumur
spurious caterpillars (_fausses chenilles_), one family (_Lophyrus_)
has sixteen prolegs; a second (_Hylotoma_, &c.) fourteen; another
(_Tenthredo_, F.) twelve; and a fourth (_Lyda_) none at all, having
only the six perfect legs. The majority of larvæ of _Lepidoptera_
have ten prolegs, eight being attached, a pair on each, to the
sixth, seventh, eighth, and ninth segments of the body, and two to
the twelfth or anal segment[444]. The caterpillar of the puss-moth
(_Cerura Vinula_) and some others, instead of the anal prolegs, have
two tails or horns. A hemigeometer, described by De Geer, has only
six intermediate prolegs, the posterior pair of which are longer
than the rest, to assist the anal pair in supporting the body in
a posture more or less erect[445]. Other hemigeometers, of which
kind is the larva of _Plusia Gamma_[446], have only six prolegs,
four intermediate and two anal. The true geometers or surveyors
(_Geometræ_) have only two intermediate and two anal prolegs.
Many grubs of _Coleoptera_, especially those of _Staphylinidæ_,
_Silphidæ_, &c. which are long and narrow, are furnished with a stiff
joint at the anus, which they bend downwards and use as a prop to
prevent their body from trailing. This joint, though without claws,
may be regarded as a kind of proleg, which supports them when they
walk[447]; and probably may assist their motion by pushing against
the plane of position.
With respect to the larvæ that have only perfect legs, having just
given you an account of these organs, I have nothing more to state
relating to their structure. I shall therefore now consider the
motions of pedate larvæ, under the several heads of walking or
running, jumping, climbing, and swimming.
Amongst those that _walk_, some are remarkable for the slowness of
their motion, while others are extremely swift. The caterpillar of
the hawk-moth of the Filipendula (_Zygæna Filipendulæ_) is of the
former description, moving in the most leisurely manner; while that
of _Apatela leporina_, a moth unknown in Britain, is named after the
hare, from its great speed. The caterpillar of another moth, the
species of which seems not to be ascertained, is celebrated by De
Geer for the wonderful celerity of its motions. When touched it darts
away backwards as well as forwards, giving its body an undulating
motion with such force and rapidity, that it seems to fly from side
to side[448].--Cuvier observes, that the grubs of some coleopterous
and neuropterous insects, which have only the six perfect legs,
by means of them lay hold of any surrounding object, and, fixing
themselves to it, drag the rest of their body to that point; and that
those of many capricorn beetles and their affinities (but that of
_Callidium violaceum_ is an apode[449]) have these legs excessively
minute and almost nothing; that they move in the sinuosities which
they bore by the assistance of their mandibles, with which they fix
themselves, and also of several dorsal and ventral tubercles, by
which they are supported against the sides of their cavity, and push
themselves along, in the same manner as a chimney-sweeper--by the
pressure of his knees, elbows, shoulder-blades, and other prominent
parts--pushes himself up a chimney[450]. The larva of the ant-lion
(_Myrmeleon_)--with the exception of one species, which moves in the
common way--always walks backwards, even when its legs are cut off.
The _jumpers_ amongst pedate larvæ, as far as they are known, are not
very numerous, and will not detain you long. When the caterpillar of
_Lithosia Quadra_, a moth not uncommon, would descend from one branch
or leap to another, it approaches to the edge of the leaf on which
it is stationed, bends its body together, and retiring a little
backwards, as if to take a good situation, leaps through the air,
and, however high the jump, alights on its legs like a cat. That of
another moth (_Herminia rostralis_) will also leap to a considerable
height[451].
Another species of motion, which is peculiar to larvæ,--their mode I
mean of _climbing_,--as it merits particular attention, will occupy
more time. I have already related so many extraordinary facts in their
history, that I promise myself you will not disbelieve me if I assert
that insects either use ladders for this purpose, or a single rope. You
may often have seen the caterpillar of the common cabbage-butterfly
climbing up the walls of your house, and even over the glass of your
windows. When next you witness this last circumstance, if you observe
closely the square upon which the animal is travelling, you will find
that, like a snail, it leaves a visible track behind it. Examine this
with your microscope, and you will see that it consists of little
silken threads, which it has spun in a zigzag direction, forming a
rope-ladder, by which it ascends a surface it could not otherwise
adhere to. The silk as it comes from the spinners is a gummy fluid,
which hardens in the air; so that it has no difficulty in making
it stick to the glass.--Many caterpillars that feed upon trees,
particularly the geometers, have often occasion to descend from branch
to branch, and sometimes, especially previously to assuming the pupa,
to the ground. Had they to descend by the trunk, supposing them able
to traverse with ease its rugged bark, what a circuitous route must
they take before they could accomplish their purpose! Providence, ever
watchful over the welfare of the most insignificant of its creatures,
has gifted them with the means of attaining these ends, without all
this labour and loss of time. From their own internal stores they can
let down a rope, and prolong it indefinitely, which will enable them to
travel where they please. Shake the branches of an oak or other tree
in summer, and its inhabitants of this description, whether they were
reposing, moving, or feeding, will immediately cast themselves from the
leaves on which they were stationed; and however sudden your attack,
they are nevertheless still provided for it, and will all descend
by means of the silken cord just alluded to, and hang suspended in
the air. Their name of geometer was given them, because they seem to
measure the surface they pass over, as they walk, with a chain. If you
place one upon your hand, you will find that they draw a thread as they
go; when they move, their head is extended as far as they can reach
with it; then fastening their thread there, and bringing up the rest of
their body, they take another step; never moving without leaving this
clue behind them; the object of which, however, is neither to measure,
nor to mark its path that it may find it again; but thus, whenever the
caterpillar falls or would descend from a leaf, it has a cord always
ready to support it in the air, by lengthening which it can with ease
reach the ground. Thus it can drop itself without danger from the
summit of the most lofty trees, and ascend again by the same road. As
the silky matter is fluid when it issues from the spinners, it should
seem as if the weight of the insect would be too great, and its descent
too rapid, so as to cause it to fall with violence upon the earth. The
little animal knows how to prevent such an accident, by descending
gradually. It drops itself a foot or half a foot, or even less, at
a time; then making a longer or shorter pause, as best suits it, it
reaches the ground at last without a shock. From hence it appears that
these larvæ have power to contract the orifice of the spinners, so as
that no more of the silky gum shall issue from it; and to relax it
again when they intend to resume their motion downwards: consequently
there must be a muscular apparatus to enable them to effect this, or at
least a kind of sphincter, which, pressing the silk, can prevent its
exit. From hence also it appears that the gummy fluid which forms the
thread must have gained a degree of consistence even before it leaves
the spinner, since as soon as it emerges it can support the weight of
the caterpillar.--In ascending, the animal seizes the thread with its
jaws as high as it can reach it; and then elevating that part of the
back that corresponds with the six perfect legs, till these legs become
higher than the head, with one of the last pair it catches the thread;
from this the other receives it, and so a step is gained: and thus
it proceeds till it has ascended to the point it wishes to reach. At
this time if taken it will be found to have a packet of thread, from
which, however, it soon disengages itself; between the two last pairs
of perfect legs[452]. To see hundreds of these little animals pendent
at the same time from the boughs of a tree, suspended at different
heights, some working their way downwards and some upwards, affords
a very amusing spectacle. Sometimes, when the wind is high, they are
blown to the distance of several yards from the tree, and yet maintain
their threads unbroken. I witnessed an instance of this last summer,
when numbers were driven far from the most extended branches, and
looked as if they were floating in the air.
Having related to you what is peculiar in the motions of pedate
larvæ upon the earth and in the air, I must next say something
with respect to their locomotive powers in the _water_. Numbers of
this description inhabit that element.--Amongst the beetles, the
genera _Dytiscus_, _Hydrophilus_, _Gyrinus_, _Limnius_, _Parnus_,
_Heterocerus_, _Elophorus_, _Hydræna_, &c. amongst the bug tribes,
_Gerris_, _Velia_, _Hydrometra_, _Notonecta_, _Sigara_, _Nepa_,
_Ranatra_, _Naucoris_; a few _Lepidoptera_; the majority of
_Trichoptera_; _Libellula_, _Aeshna_, _Agrion_, _Sialis_, _Ephemera_,
&c. amongst the _Neuroptera_; _Culex_ and many of the _Tipulariæ_,
Latr. from the dipterous insects; and from the _Aptera_, _Atax_, some
_Poduræ_, and many of the _Oniscidæ_, &c.--All these, in their larva
state, are aquatic animals.
The motions of these creatures in this state are various. Some walk
on the ground under water; some move in midwater, either by the
same motion of the legs as they use in walking, or by strokes, as
in swimming; others for this purpose employ certain laminæ, which
terminate their tails, as oars; others again swim like fish, with an
equable motion; some move by the force of the water which they spirt
from their anus; others again swim about in cases, or crawl over the
submerged bottom; and others walk even on the surface of the water. I
shall not now enlarge on all these kinds of water-motion, since many
will come under consideration hereafter.
There are two descriptions of larvæ of _Hydrophili_, one furnished
with swimmers or anal appendages, by means of which they are enabled
to swim; the other have them not, and hence are not able to rise from
the bottom[453]. The larvæ of _Dytisci_, by means of these natatory
organs, will swim, though slowly, and every now and then rise to the
surface for the sake of respiration. Those of _Ephemeræ_, when they
swim, apply their legs to the body, and swim with the swiftness and
motions of fish[454]. Those of the true may-fly (_Sialis lutaria_),
on the contrary, use their legs in swimming, and at the same time,
by alternate inflexions, give to their bodies the undulations of
serpents[455]. But the larvæ of certain dragon-flies (_Aeshna_ and
_Libellula_,) will afford you the most amusement by their motions.
These larvæ commonly swim very little, being generally found walking
at the bottom on aquatic plants: when necessary, however, they can
swim well, though in a singular manner. If you see one swimming, you
will find that the body is pushed forward by strokes, between which
an interval takes place. The legs are not employed in producing this
progressive motion, for they are then applied close to the sides of
the trunk, in a state of perfect inaction. But it is effected by a
strong ejaculation of water from the anus. When I treat upon the
respiration of insects, I shall explain to you the apparatus by which
these animals separate the air from the water for that purpose; in
the present case it is subsidiary to their motions, since it is by
drawing in and then expelling the water that they are enabled to
swim. To see this, you have only to put one of these larvæ into a
plate with a little water. You will find that, while the animal
moves forward, a current of water is produced by this pumping, in
a contrary direction. As the larva, between every stroke of its
internal piston, has to draw in a fresh supply of water, an interval
must of course take place between the strokes. Sometimes it will lift
its anus out of the water, when a long thread of water, if I may so
speak, issues from it[456].
II. I am next to say something upon the motions of insects in their
_pupa_ state. This is usually to our little favourites a state of
perfect repose; but, as I long since observed[457], there are several
that, even when become pupæ, are as active and feed as rapaciously
as they do when they are either larvæ or perfect insects. The
_Dermaptera_, _Orthoptera_, _Hemiptera_, many of the _Neuroptera_,
and the majority of the _Aptera_, are of this description. With
respect to their motions, we may therefore consider pupæ as of two
kinds--_active_ pupæ, and _quiescent_ pupæ.
The motions of most insects whose pupæ are _active_, are so similar
in all their states, except where the wings are concerned, as not
to need any separate account. I shall therefore request you to wait
for what I have to say upon them, till I enter upon those of the
imago. One insect, however, of this kind, moving differently in its
preparatory states, is entitled to notice under the present head.--In
a late letter, I mentioned to you a bug (_Reduvius personatus_) which
usually covers itself with a mask of dust, and fragments of various
kinds, cutting a very grotesque figure[458]. Its awkward motions add
not a little to the effect of its appearance. When so disposed, it can
move as well and as fast as its congeners; yet this does not usually
answer its purpose, which is to assume the appearance of an inanimate
substance. It therefore hitches along in the most leisurely manner
possible, as if it was counting its steps. Having set one foot forward
(for it moves only one leg at a time), it stops a little before it
brings up its fellow, and so on with the second and third legs. It
moves its antennæ in a similar way, striking, as it were, first with
one, and then, after an interval of repose, with the other[459].--The
pupæ of gnats also, as well as those of many other aquatic _Diptera_,
retain their locomotive powers, not however the free motion of their
limbs. When not engaged in action, they ascend to the surface by the
natural levity of their bodies, and are there suspended by two auriform
respiratory organs in the anterior part of the trunk, their abdomen
being then folded under the breast; when disposed to descend the animal
unfolds it, and by sudden strokes which she gives with it and her anal
swimmers to the water, she swims, to the right and left as well as
downwards, with as much ease as the larva[460].
Bonnet mentions a pupa which climbs up and down in its cocoon,--and
that of the common glow-worm (_Lampyris noctiluca_) will sometimes
push itself along by the alternate extension and contraction of the
segments of its body[461].--Others turn round when disturbed. That
of a weevil (_Hypera Arator_) which spins itself a beautiful cocoon
like fine gauze, and which it fixes to the stalks of the common
spurrey (_Sagina arvensis_), upon my touching this stalk, whirled
round several times with astonishing rapidity.--The chrysalis of a
scarce moth (_Hypogymna dispar_) when touched turns round with great
quickness; but, as if fearful of breaking the thread by which it is
suspended by constantly twisting it in one direction, it performs
its gyrations alternately from left to right, and from right to
left[462]. Generally speaking, quiescent pupæ when disturbed show
that they have life, by giving their abdomen violent contortions.
But the most extraordinary motion of pupæ is jumping. In the year
1810 I received an account from a very intelligent young lady, who
collected and studied insects with more than common ardour and ability,
that a friend had brought her a chrysalis endued with this faculty.
It was scarcely a quarter of an inch in length; of an oval form; its
colour was a semitransparent brown, with a white opake band round the
middle. It was found attached, by one end, to the leaf of a bramble. It
repeatedly jumped out of an open pill-box that was an inch in height.
When put into a drawer in which some other insects were impaled, it
skipped from side to side, passing over their backs for nearly a
quarter of an hour with surprising agility. Its mode of springing
seemed to be by balancing itself upon one extremity of its case. About
the end of October one end of the case grew black, and from that time
the motion ceased; and about the middle of April, in the following
year, a very minute ichneumon made its appearance by a hole it had made
at the opposite end.--Some time after I had received this history, I
happened to have occasion to look at Reaumur's Memoir upon the enemies
of caterpillars, where I met with an account of a similar jumping
chrysalis, if not the same. Round the nests of the caterpillar of the
processionary moth, before noticed[463], he found numerous little
cocoons suspended by a thread three or four inches long to a twig or a
leaf, of a shortened oval form, and close texture, but so as the meshes
might be distinguished. These cocoons were rather transparent, of a
coffee-brown colour, and surrounded in the middle by a whitish band.
When put into boxes or glasses, or laid on the hand, they surprised
him by leaping. Sometimes their leaps were not more than ten lines, at
others they were extended to three or four inches, both in height and
length. When the animal leaps, it suddenly changes its ordinary posture
(in which the back is convex and touches the upper part of the cocoon,
and the head and anus rest upon the lower), and strikes the upper part
with the head and tail, before its belly, which then becomes the convex
part, touches the bottom. This occasions the cocoon to rise in the air
to a height proportioned to the force of the blow. At first sight this
faculty seems of no great use to an animal that is suspended in the
air; but the winds may probably sometimes place it in a different and
unsuitable position, and lodge it upon a leaf or twig: in this case
it has it in its power to recover its natural station. Reaumur could
not ascertain the fly that should legitimately come from this cocoon,
for different cocoons gave different flies: whence it was evident that
these ichneumons were infested by their own parasite[464]. This might
be the case with that of the lady just mentioned. Perhaps, properly
speaking, in this last instance the motions ought rather to be regarded
as belonging to a larva; but as it had ceased feeding, and had inclosed
itself in its cocoon, I consider it as belonging to the present head.
You may probably here feel some curiosity to be informed how the
numerous larvæ that are buried in their pupa state, either in the
heart of trees, under the earth, or in the waters, effect their
escape from their various prisons and become denizens of the air,
especially as you are aware that each is shrouded in a winding-sheet
and cased in a coffin. In most, however, if you examine this coffin
closely, you will see RESURGAM written upon it. What I mean is this.
The _puparium_ or case of the animal is furnished with certain
acute points (_adminicula_) generally single, but in some instances
forked, looking towards the anus, and usually placed upon transverse
ridges on the back of the abdomen, but sometimes arming the sides or
the margins of the segments. By this simple contrivance, aided by
new-born vigour, when the time for its great change is arrived, the
included prisoner of hope, if under ground, pushes itself gradually
upwards, till reaching the surface its head and trunk emerge, when
an opening in the latter being effected by its efforts, it escapes
from its confinement, and once more tastes the sweets of liberty
and the joys of life. Those that are inclosed in trees and spin a
cocoon, are furnished with points on the head, with which they make
an opening in the former. The pupa of the great goat-moth (_Cossus
ligniperda_) thus, by divers movements, keeps disengaging itself from
this envelope, till it arrives at a hole in the tree which it had
made when a caterpillar; when its anterior part having emerged, it
stops short, and so escapes a fall that might destroy it. After some
repose, in consequence of very violent efforts, the puparium opens,
and it escapes from its prison[465].