gives life to every part and portion of our globe, rendering even
the most arid desert interesting. From their visitations every leaf
and flower becomes animated; the very dust seems to quicken into
life, and the stones, like those thrown by Deucalion and Pyrrha, to
be metamorphosed into locomotive beings. In the variety of motions
which they exhibit, we see, as Cuvier remarks[402], those of every
other description of animals. They walk, run, and jump with the
quadrupeds; they fly with the birds; they glide with the serpents;
and they swim with the fish. And the provision made for these motions
in the structure of their bodies is most wonderful and various. "If
I was minded to expatiate," says the excellent Derham, "I might
take notice of the admirable mechanism in those that creep; the
curious oars in those amphibious insects that swim and walk; the
incomparable provision made in the feet of such as walk or hang upon
smooth surfaces; the great strength and spring in the legs of such
as leap; the strong-made feet and talons of such as dig; and, to
name no more, the admirable faculty of such as cannot fly, to convey
themselves with speed and safety, by the help of their webs, or some
other artifice, to make their bodies lighter than the air[403]."
Since the motions, and instruments of motion, of insects are usually
very different in their preparatory states, from what they are in the
imago or perfect state, I shall therefore consider them separately,
and divide my subject into--motions of larvæ,--motions of pupæ,--and
motions of perfect insects.
I. Amongst _larvæ_ there are two classes of movers--_Apodous_ larvæ,
or those that move without legs,--and _Pedate_ larvæ, or those that
move by means of legs. I must here observe, that by the term _legs_,
which I use strictly, I mean only jointed organs, that have free
motion, and can walk or step alternately; not those spurious legs
without joints, that have no free motion, and cannot walk or take
alternate steps; such as support the middle and anus of the larvæ of
most _Lepidoptera_ and saw-flies (_Serrifera_).
_Apodous_ larvæ seldom have occasion to take long journeys; and many
of them, except when about to assume the pupa, only want to change
their place or posture, and to follow their food in the substance,
whether animal or vegetable, to which, when included in the egg, the
parent insect committed them. Legs therefore would be of no great
use to them, and to these last a considerable impediment. They are
capable of three kinds of motion;--they either walk, or jump, or
swim. I use _walking_ in an improper sense, for want of a better term
equally comprehensive: for some may be said to move by gliding; and
others (I mean those that, fixing the head to any point, bring the
tail up to it, and so proceed) by stepping.
The motion of serpents was ascribed by some of the ancients (who were
unable to conceive that it could be effected naturally, unless by
the aid of legs, wings, or fins,) to a preternatural cause. It was
supposed to resemble the "_incessus deorum_," and procured to these
animals, amongst other causes, one of the highest and most honourable
ranks in the emblematical class of their false divinities[404]. Had
they known Sir Joseph Banks's late discovery,--that some serpents
push themselves along by the points of their ribs, which Sir E. Home
has found to be curiously constructed for this purpose,--their wonder
would have been diminished, and their serpent-gods undeified. But
though serpents can no longer make good their claim to motion _more
deorum_, some insects may take their places; for there are numbers of
larvæ, that having neither legs, nor ribs, nor any other points by
which they can push themselves forward on a plane, _glide_ along by the
alternate contraction and extension of the segments of their body. Had
the ancient Egyptians been aware of this, their catalogue of insect
divinities would have been wofully crowded. In this annular motion, the
animal alternately supports each segment of the body upon the plane of
position, which it is enabled to do by the little bundles of muscles
attached to the skin, that take their origin within the body[405].
I shall begin the list of _walkers_, the movements of which are
aided by various instruments, with one which is well known to most
people,--the grub of the nut-weevil (_Balaninus Nucum_). When placed
upon a table, after lying some time, perhaps, bent in a bow, with its
head touching its tail, at last it begins to move, which, though in
no certain direction, it does with more speed than might be expected.
Rösel fancied that this animal had feet furnished with claws; but
in this, as De Geer justly observes, he was altogether mistaken,
since it has not the least rudiment of them, its motion being
produced solely by the alternate contraction and extension of the
segments of the body, assisted, perhaps, by the fleshy prominences
of its sides.--Other larvæ have this annular motion aided by a slimy
secretion, which gives them further hold upon the plane on which they
are moving, and supplies in some degree the place of legs or claws.
That of the weevil of the common figwort (_Cionus Scrophulariæ_)
is always covered with slime, which enables it,--though it renders
its appearance disgusting,--to walk with steadiness, by the mere
lengthening and shortening of its segments, upon the leaves of that
plant[406].--Of this kind also are those larvæ, mentioned above[407],
received by De Geer from M. Ziervogel, which, adhering to each other
by a slimy secretion, glide along so slowly upon the ground as to be
a quarter of an hour in going the breadth of the hand, whence the
natives call their bands _Gårds-drag_[408].
As a further help, others again call in the assistance of their
unguiform mandibles. These, which are peculiar to grubs with a variable
membranaceous, or rather retractile, head[409], especially those of the
fly tribe (_Muscidæ_), when the animal does not use them, are retracted
not only within the head, but even within the segments behind it[410];
but when it is moving they are protruded, and lay hold of the surface
on which it is placed. They were long ago noticed by the accurate
Ray. "This blackness in the head," says he, speaking of the maggot of
the common flesh-fly, "is caused by two black spines or hooks, which
when in motion it puts forth, and fixing them in the ground, so drags
along its body[411]."--The larvæ of the aphidivorous flies (_Syrphus_,
&c.), the ravages of which amongst the Aphides I have before described
to you[412], transport themselves from place to place in the same
way, walking by means of their teeth. Fixing their hind part to the
substances on which they are moving, they give their body its greatest
possible tension; and, if I may so speak, thus take as long a step as
they can: next, laying hold of it with their mandibles, by setting free
the tail and relaxing the tension, the former is brought near the head.
Thus the animal proceeds, and thus will even walk upon glass[413]. Some
grubs, as the lesser house-fly (_Anthomyia canicularis_), have only
one of these claw-teeth; and in some they have the form as well as the
office of legs[414]. Bonnet mentions an apodous larva, that, before
it can use its mandibles, is obliged to spin, at certain intervals,
little hillocks or steps of silk; of which it then lays hold by them,
and so drags itself along.
Besides their mandibular hooks, some of these grubs supply the want
of legs by means of claws at their anus. Thus that of the flesh-fly,
Ray tells us in the place just quoted, pushes itself by the protruded
spines of its tail. The larva also of a long-legged gnat (_Limnobia
replicata_), which in that state lives in the water, is furnished
with these anal claws, which, in conjunction with its annular tension
and relaxation, and the hooks of its mouth, assist it in walking over
the aquatic plants[415].
A remarkable difference, according to their station, obtains in
the bots of gad-flies (_Œstridæ_); those that are subcutaneous
(_Cuticolæ_, Clark) having no unguiform mandibles; while those
that are gastric (_Gastricolæ_, Clark), and those that inhabit the
maxillary sinuses of animals (_Cavicolæ_, Clark), are furnished with
them. In this we evidently see Creative Wisdom adapting means to
their end. For the cuticular bots having no plane surface to move
upon, and imbibing a liquid food, in them the mandibular hooks would
be superfluous. But they are furnished with other means by which
they can accomplish such motions, and in contrary directions, as are
necessary to them; the anterior part of each segment being beset with
numbers of very minute spines, not visible except under a strong
magnifier, sometimes arranged in bundles, which all look towards the
anus; and the posterior part is as it were paved with similar hooks,
but smaller, which point to the head. Thus we may conceive, when the
animal wants to move forward, that it pushes itself by the first
set of hooks, keeping the rest, which would otherwise impede motion
in that direction, pressed close to its skin--or it may depress that
part of the segment; and when it would move backwards that it employs
the second[416]. The other descriptions of bots, not being embedded
in the flesh but fixed to a plane, are armed with the mandibles in
question, by which they can not only suspend themselves in their
several stations, but likewise, with the aid of the spines with which
their segments also are furnished, move at their pleasure[417]. Other
larvæ of flies, as well as the bots, are furnished with spines or
hooks--by which they take stronger hold--to assist them in their
motions. Those mentioned in my last letter as inhabiting the nests
of humble-bees[418], besides the six radii that arm their anus, and
which perhaps may assist them in locomotion, have the margin of their
body fringed with a double row of short spines[419], which are,
doubtless, useful in the same way.
The next order of walkers amongst apodous larvæ are those that move
by means of fleshy tuberculiform or pediform prominences,--which last
resemble the spurious legs of the caterpillars of most _Lepidoptera_.
Some, a kind of monopods, have only one of such prominences, which
being always fixed almost under the head, may serve, in some degree,
the purpose of an unguiform mandible. The grub of a kind of gnat
(_Chironomus stercorarius_), and also another, probably of the
Tipularian tribe (found by De Geer in a subputrescent stalk of
Angelica which he was unable to trace to the fly), have each a fleshy
leg on the underside of the first segment, which points towards
the head and assists them in their motions[420].--Others again go
a little further, and are supported at their anterior extremity by
a pair of spurious legs. An aquatic larva of a most singular form,
and of the same tribe, figured by Reaumur, is thus circumstanced. In
this case the processes in question proceed from the head, and are
armed with claws[421]. Would you think it--another Tipularian grub
is distinguished by _three_ legs of this kind? It was first noticed
by De Geer under the name of _Tipula maculata_ (_Tanypus monilis_,
Meig.), who gives the following account of its motions and their
organs:--It is found, he observes, in the water of swampy places and
in ditches, is not bigger than a horse-hair, and about a quarter of
an inch in length. Its mode of swimming is like that of a serpent,
with an undulating motion of the body, and it sometimes walks at the
bottom of the water and upon aquatic plants. The most remarkable
part of it are its legs, called by Latreille, but it should seem
improperly, tentacula. They resemble, by their length and rigidity,
wooden legs. The anterior leg is attached to the underside, but
towards the head, of the first segment of the body. It is long and
cylindrical, placed perpendicularly or obliquely, according to the
different movements the animal gives it, and terminates in two feet,
armed at their extremity by a coronet of long moveable hooks. These
feet, like the tentacula of snails, are retractile within the leg,
and even within the body, so that only a little stump, as it were,
remains without. The insect moves them both together, as a lame man
does his crutches, either backwards or forwards. The two posterior
legs are placed at the anal end of the body. They are similar to
the one just described, but larger, and entirely separate from each
other, being not, like them, retractile within the body, but always
stiff and extended. These also are armed with hooks. In walking, this
larva uses these two legs much as the caterpillars of the moths,
called _Geometræ_, do theirs. By the inflection of the anus it can
give them any kind of lateral movement, except that it can neither
bend nor shorten them, since like a wooden leg, as I have before
observed, they always remain stiff and extended[422]. Lyonet had
observed this larva, or a species nearly related to it; but he speaks
of it as having four legs, two before and two behind. Probably, when
he examined them, the common base, from which the feet are branches,
was retracted within the body[423].
Generally speaking, however, in these apodous walkers the place of
legs is supplied by fleshy and often retractile mamillæ or tubercles.
By means of these and a slimy secretion, unaided by mandibular hooks,
the caterpillar of a little moth (_Apoda Testudo_,) moves from place
to place[424].--A subcutaneous larva belonging to the same order, that
mines the leaves of the rose, moves also by tubercular legs assisted
by slime. It has eighteen homogeneous legs, with which, when removed
from its house of concealment, it will walk well upon any surface,
whether horizontal, inclined, or even vertical[425]. But the greatest
number of legs of this kind that distinguish any known larva, is to
be observed in that of a two-winged fly (_Syrphus Pyrastri_) that
devours the Aphides of the rose. This animal has six rows of tubercular
feet, with which it moves, each row consisting of seven, making in all
forty-two[426].--The grub of the weevil of the dock (_Hypera Rumicis_)
has twenty-four tubercular legs; but, what is remarkable, the six
anterior ones, being longer than the rest, seem to represent the real
legs, while the others represent the spurious ones, of lepidopterous
larvæ. These legs, however, are all fleshy tubercles, and have no
claws, the place of which is supplied by slime which covers all the
underside of the body, and hinders the animal from falling[427].
Another weevil (_Lixus paraplecticus_,) produces a grub inhabiting the
water-hemlock, which has only six tubercles that occupy the place and
are representatives of the legs of the perfect insect[428].
Some larvæ have these tubercles armed with claws. The maggot of a fly
described by De Geer (_Volucella plumata_,) has six pair of them, each
of which has three long claws. This animal has a radiated anus, and
seems related to those flies that live in the nests of humble-bees[429].
Insects in the peculiarities of their structure, as we have seen
in many instances, sometimes realize the wildest fictions of the
imagination. Should a traveller tell you that he had seen a quadruped
whose legs were on its back, you would immediately conclude that
he was playing upon your credulity, and had lost that regard to
truth which ought to distinguish the narratives of persons of his
description. What then will you say to me, when I affirm, upon
the evidence of two most unexceptionable witnesses, Reaumur and
De Geer, that there are insects which exhibit this extraordinary
structure? The grub of a little gall-fly, appearing to be _Cynips
Quercus inferus_ of Linné--which inhabits a ligneous gall resembling
a berry to be met with on the underside of oak-leaves--was found
by the former to have on its back, on the middle of each segment,
a retractile fleshy protuberance that resembled strikingly the
spurious legs of some caterpillars. A little attention will convince
any one, argues Reaumur, that the legs of insects circumstanced
like the one under consideration, if it has any, should be on its
back. For this grub--inhabiting a spherical cavity, in which it
lies rolled up as it were in a ring--when it wants to move, will be
enabled to do so, in this hollow sphere, with much more facility, by
means of legs on the middle of its back, than if they were in their
ordinary situation[430]. So wisely has Providence ordered every
thing.--Another similar instance is recorded by De Geer, which indeed
had previously been noticed, though cursorily, by the illustrious
Frenchman[431]. There is a little larva, he observes, to be found at
all seasons of the year, the depth of winter excepted, in stagnant
waters, which keeps its body always doubled as it were in two,
against the sides of ditches or the stalks of aquatic plants. If it
is placed in a glass half full of water, it so fixes itself against
the sides of it, that its head and tail are in the water while the
remainder of the body is out of it; thus assuming the form of a
siphon, the tail end being the longest. When this animal is disposed
to feed, it lifts its head and places it horizontally on the surface
of the water, so that it forms a right angle with the rest of the
body, which always remains in a situation perpendicular to the
surface. It then agitates, with vivacity, a couple of brushes, formed
of hairs and fixed in the anterior part of the head, which producing
a current towards the mouth, it makes its meal of the various species
of animalcula, abounding in stagnant waters, that come within the
vortex thus produced. As these animals require to be firmly fixed to
the substance on which they take their station, and their back is the
only part, when they are doubled as just described, that can apply to
it,--they are furnished with minute legs armed with black claws, by
which they are enabled to adhere to it. They have ten of these legs:
the four anterior ones, which point towards the head and are distant
from each other, are placed upon the fourth and fifth dorsal segments
of the body; and the six posterior ones, which point to the anus
and are so near to each other as at first to look like one leg, are
placed on the eighth, ninth, and tenth. When the animal moves, the
body continues bent, and the sixth segment, which is without feet and
forms the summit of the curve, goes first[432]. De Geer named the fly
it produces _Tipula amphibia_: it seems not clear, from his figure,
to which of the modern genera of the _Tipulariæ_ it belongs, nor is
it referred to by Meigen.
I come now to the jumping apodes, and one of this description will
immediately occur to your recollection,--that I mean which revels
in our richest cheeses, and produces a little black shining fly
(_Tyrophaga Casei_). These maggots have long been celebrated for
their saltatorious powers. They effect their tremendous leaps--laugh
not at the term, for they are truly so when compared with what
human force and agility can accomplish--in nearly the same manner
as salmon are stated to do when they wish to pass over a cataract,
by taking their tail in their mouth, and letting it go suddenly.
When it prepares to leap, our larva first erects itself upon its
anus, and then bending itself into a circle by bringing its head to
its tail, it pushes forth its unguiform mandibles, and fixes them
in two cavities in its anal tubercles. All being thus prepared,
it next contracts its body into an oblong, so that the two halves
are parallel to each other. This done, it lets go its hold with
so violent a jerk that the sound produced by its mandibles may be
readily heard, and the leap takes place. Swammerdam saw one, whose
length did not exceed the fourth part of an inch, jump in this
manner out of a box six inches deep; which is as if a man six feet
high should raise himself in the air by jumping 144 feet! He had
seen others leap a great deal higher[433]. The grub of a little gnat
lately noticed (_Chironomus stercorarius_) has a similar faculty,
though executed in a manner rather different. These larvæ, which
inhabit horse-dung, though deprived of feet, cannot move by annular
contraction and dilatation; but are able, by various serpentine
contortions, aided by their mandibles, to move in the substance which
constitutes their food. Should any accident remove them from it,
Providence has enabled them to recover their natural station by the
power I am speaking of. When about to leap, they do not, like the
cheese-fly, erect themselves so as to form an angle with the plane
of position; but lying horizontally, they bring the anus near the
head, regulating the distance by the length of the leap they mean to
take; when fixing it firmly, and then suddenly resuming a rectilinear
position, they are carried through the air sometimes to the distance
of two or three inches. They appear to have the power of flattening
their anal extremity, and even of rendering it concave: by means
of which it may probably act as a sucker, and so be more firmly
fixable[434].--The grub of a fly whose proceedings in that state I
have before noticed[435] (_Leptis Vermileo_), will, when removed from
its habitation, endeavour to recover it by leaping. Indeed this mode
of motion seems often to be given to this description of larvæ by
Providence, to enable them to return to their natural station, when
by any accident they have wandered away from it.
Many apodous larvæ inhabit the water, and therefore must be
furnished with means of locomotion proper to that element. To this
class belongs the common gnat (_Culex pipiens_), which being one
of our greatest torments, compels us to feel some curiosity about
its history. Its larva is a very singular creature, furnished with
a remarkable anal apparatus for respiration, by which it usually
remains suspended at the surface of the water. If disposed to
descend, it seems to sink by the weight of its body; but when it
would move upwards again, it effects its purpose by alternate
contortions of the upper and lower halves of it, and thus it moves
with much celerity. The laminæ or swimmers, which terminate its
anus[436], are doubtless of use to it in promoting this purpose.
It does not, that I ever observed, move in a lateral direction,
but only from the surface downwards, and _vice versa_.--Another
dipterous larva (_Corethra culiciformis_), which much resembles that
of the gnat in form, differs from it in its motions and station of
repose. For, instead of being suspended at the surface with its head
downwards, it usually, like fishes, remains in a horizontal position
in the middle of the water. When it ascends to the surface, it is
always by means of a few strokes of its tail, so that its motion is
not equable, _sed per saltus_. It descends again gradually by its
own weight, and regains its equilibrium by a single stroke of the
tail[437].--A well known fly (_Stratyomis Chamæleon_), in its first
state an aquatic animal, often remains suspended, by its radiated
anus, at the surface of the water, with its head downwards. But when
it is disposed to seek the bottom or to descend, by bending the radii
of its tail so as to form a concavity, it includes in them a bubble
of air, in brilliancy resembling silver or pearl; and then sinks with
it by its own weight. When it would return to the surface it is by
means of this bubble, which is, as it were, its air-balloon. If it
moves upon the surface or horizontally, it bends its body alternately
to the right and left, contracting itself into the form of the letter
S; and then extending itself again into a straight line, by these
alternate movements it makes its way slowly in the water[438].
I have dwelt longer upon the apodous larvæ, or those that are
without what may be called proper legs, analogous to those of
perfect insects, because the absence of these ordinary instruments
of motion is in numbers of them supplied in a way so remarkable
and so worthy to be known; and because in them the wisdom of the
Creator is so conspicuously, or, I should rather say, so strikingly
manifested--since it is doubtless equally conspicuous in the ordinary
routine of nature. But aberrations from her general laws, and modes,
and instruments of action, often of rare occurrence, impress us more
forcibly than any thing that falls under our daily observation.
* * * * *