parallel sides; some are _cylindrical_, as those of the wings of the
_Libellulina_[815]; and others, as those that move the legs in the
caterpillar of the _Cossus_, are triangular[816]. In the suctorious
mandibles of the grub of a common water-beetle[817] they are
_penniform_, or shaped like a feather; and some in the _Cossus_ are
forked[818]. Under this head I may also observe, that the muscles
are sometimes extremely slender threads, crossing each other, and
often curiously interwoven in various directions, so as to resemble
lace or fine gauze, as may be seen in the alimentary canal of some
caterpillars[819]; sometimes also they surround part of this organ,
like a series of minute rings[820].
iv. _Colour._ The most usual colour of the muscles of insects is
_white_: those for flight however, according to Chabrier, differ
from the rest, by being of a deeper and _reddish_ colour[821]; and
I have observed likewise that those in the head of the stag-beetle,
when dried at least, are _red_, and look something like the flesh of
warm-blooded animals.
v. _Kinds_ and _Denomination_. In general, muscles may be regarded as
divided into _primary_ and _secondary_--the _primary_ being the muscles
by which the _principal_ movements of any organs are effected, and the
_secondary_ their auxiliaries which are the cause of _subordinate_
movements[822]. Every muscle almost has its _antagonist_, the action of
which is in an opposite direction; so that when it is equal, the organ
to which they are attached remains without motion; but when that of one
preponderates, a movement in proportion takes place[823]. The principal
antagonist muscles that may be found in insects are the following.
1. _Levator muscles_ that _raise_ an organ, and _Depressors_ that
_depress_ it. 2. _Flexors_ that _bend_ an organ, and _Extensors_ that
_unbend_ or extend it. 3. _Abductors_ that draw an organ _back_, and
_Adductors_ that draw it _forwards_. 4. _Constrictors_ that _contract_
an opening, and _Laxators_ that _relax_ it. 5. _Supinators_ that _turn_
the underside of an organ upwards, and _Pronators_ that _return_ it
to its natural situation. Some of these muscles in insects, like some
of their articulations and their spinal chord[824], seem to exercise
a _double_ function,--thus the levators and depressors of the _wings_
are constrictors and laxators of the _trunk_[825]. At first it may
seem that insects, not having the power of turning up the hand, cannot
have the _Supinator_ and _Pronator_ muscles; but some muscle of this
kind must be in the _Gryllotalpa_, and in those that have a versatile
head[826].
v. _Attachment_ and _Insertion_. The attachment and insertion of the
muscles in insects in general is to the _interior_ of the crust, or
to some of its internal processes as a fulcrum, and to the organ to
be moved. In some cases, however, the muscles act upon the organ by
the intervention of other bodies. Thus, those that move the wings
are often attached to little _bones_, as Chabrier calls them[827],
which are connected with the base of the wings by ligaments. In the
_Dynastidæ_ and other Lamellicorns, and the _Libellulina_, &c., a
remarkable provision is made for giving a vast increment of force to
the muscles of the wings, by means of caps or cupules surmounted by
a tendon, which receive their extremity; the tendon terminating in a
fine point attached to the wing, and thus more muscles are brought to
bear upon it[828]. Chabrier seems to think that, in some cases, the
_back_ that intervenes between each pair of wings is the medium by
which the muscles act upon it[829].
vi. _Motions._ Irritability is the universal distinction of the
muscular fibre,--when put in action by the will or involuntarily, it
causes it to contract or become shorter; and the intermediate agents
of the will and other causes are the _nerves_, which, as galvanic
experiments seem in some degree to prove, are the conductors of an
invisible fluid or power which immediately causes that action. If a
nerve is divided, the muscles to which it renders obey it no longer,
evidently proving that the nerves cause muscular irritability[830].
How this contraction is immediately effected,--whether the fibre, as
some suppose, undergoes any _crispation_, or becomes zigzag[831], or
whether there is any sudden change in their _chemical_ composition
that rapidly and strongly augments their cohesion, as Cuvier
hints[832], cannot be clearly ascertained, unless a Bauer could
submit the _living_ fibre to his glasses. All that we know certainly
on the subject is, that muscles alternately contract and relax at
the bidding of the will or involuntarily, and so occasion all the
movements of animal bodies.
II. Having considered the muscles of insects in _general_, I must next
make a few observations, as far as my means of information will enable
me, upon those that move their different _parts_ and _organs_--at least
the principal ones; since to descend to minutiæ would be an endless
and unprofitable labour. As _larvæ_, except those whose metamorphosis
is _semicomplete_[833], differ widely in their system of muscles from
_perfect insects_, I shall begin my observations with them.
We owe by far the most accurate and detailed account of the muscles
of larvæ to the illustrious Lyonet, who, with incredible labour and
patience without example, dissected the caterpillar of the _Cossus_,
and has described every air-vessel, every nerve, and every muscle
that could be detected by the microscope. Cuvier also has given a
description of the muscles not only of caterpillars, but of the larvæ
of the Lamellicorn beetles, the _Hydrophili_, and the Capricorn
beetles[834]. From these sources are derived what I have now to lay
before you. If you look at one of Lyonet's plates[835], the layers of
_longitudinal_ muscles look like so many parallel ribands, others run
in an _oblique_, and others again in a _transverse_ direction[836].
He divides them into _dorsal_, _ventral_, and _lateral_ muscles[837],
terms which sufficiently explain themselves. Of the _longitudinal_
muscles there are _four_ principal rows[838], the others are more
numerous. The principal object of these muscles, which are flexors
and extensors, is to shorten or lengthen the body, or to act on any
particular segment as the circumstances of the animal may require. I
shall not here notice the muscles of the _head_ and _legs_, as they
are not _remarkably_ different from those of perfect insects. The
_prolegs_ are moved by _two_ muscles--the anterior one covering in
part the posterior--of a remarkable structure: one of their points
of attachment is by many branches or tails to the sole of the foot,
and by several heads to the skin of the animal; so that they can
draw the proleg within the body or push it out, and perform other
necessary movements[839].
I shall now call your attention to the muscles of the _perfect_
insect, as they move the _head_ and its organs; the _Trunk_; the
_Abdomen_; and the _Viscera_.
i. The _Head_. This part in insects moves upwards, downwards,
inwards, to right and left, is pushed forth or drawn in, is often
capable in part of a rotatory movement, and is sometimes versatile,
turning as it were upon a pivot. All these movements are of course
produced by an appropriate apparatus of _muscles_, which have their
attachment in the _anterior_ part of the trunk, mostly in the
_manitrunk_, while their insertion is in the _posterior_ part of
the head, in the margin of the occipital cavity. To enumerate and
describe them all would be tedious and uninteresting--I shall only
mention some of the principal ones. The _levators_ of the head are
usually a _pair_ of muscles situated in the manitrunk, to the upper
side of which they are attached, and perhaps in _Coleoptera_ and
some others to the _phragma_, which probably Cuvier means by the
_anterior_ part of the _scutellum_[840]; they are inserted in the
posterior margin of the upper part of the head, in _Coleoptera_ in
a _pair_ of notches (_Myoglyphides_[841]), or a single one[842].
In _Cordylia Palmarum_ these muscles as they approach the head, to
judge from the _dead_ animal, divide into _two_ branches or a fork:
thus, as the muscle-notches are wide in this insect, the muscle
acts upon each extremity of the sinus--these branches appear to be
_tendinous_[843]. The _depressors_ of the head are the antagonist
muscles to the above, and have their attachment to the _antepectus_
and its _antefurca_[844]. A circumstance distinguishes these muscles
in many _Coleoptera_, that seems hitherto to have been overlooked.
If you take the common dung-beetle (_Geotrupes stercorarius_), and
carefully extract the _head_ with its muscles from the trunk, you
will see on each side of the depressors a subovate corneous scale,
of a pitch colour[845], which is attached only to the muscle, and
designed to strengthen it: if you then examine the anterior cavity
of the _manitrunk_, you will perceive on each side, just within the
lower margin, a minute triangular scale, of a similar substance;
these ligaments, like the pax-wax, or _ligamenta nuchæ_, in
_mammalia_, though in a lower situation, are doubtless intended to
sustain the action of the muscles.
With regard to the moveable _organs_ of the head--the _antennæ_,
_maxillæ_, _palpi_, _tongue_, _mandibulæ_, &c., have each their
appropriate apparatus of muscles: but I shall only notice those
of the last, the _mandibulæ_. These are principally _abductors_
and _adductors_ to open and shut them: from the work that the jaws
of some insects have to do, you may conjecture that they must be
furnished with powerful muscles. In caterpillars and other larvæ,
in which state the action of the mandibles is most in requisition,
the muscles are what Cuvier calls _penniform_[846], and are
attached on each side to a tendinous lamina or cartilage. In
the grub of _Dytiscus_ the power and magnitude of the _adductor_
muscle is wonderful[847]. In the _Orthoptera_ this structure of the
mandibular muscles takes place also in the _imago_[848]; but in
the _Coleoptera_, at least in the stag-beetle and some others that
I have examined, these muscles in this state have no cartilage or
tendon. Their attachment is always to the _parietes_ of the head,
of the cavity of which the _adductors_, in some cases, occupy a
considerable portion[849]. As to their insertion--these last, in some
_Orthoptera_, enter more or less the interior of the mandible[850];
but commonly they are inserted at or near the _interior_ angle of the
mandibular basal cavity, and the _abductors_ at the _exterior_.
ii. The _Trunk_. We have little information with regard to the
muscles of the parts of the trunk itself, by which, in some insects,
the manitrunk is enabled to move independently of the alitrunk:
it is more probable that the levators have in part at least their
attachment to the anterior surface of the prophragm[851], than that
the levators of the head should be there fixed, as Cuvier seems to
think; since both the _phragma_ and the ligament that appears in many
cases to close the cavity of the manitrunk round the viscera[852],
would prevent all communication between those muscles and any part
connected with the scutellum: probably the depressors have their
attachment partly on the anterior face of the _medifurca_[853]. These
points, however, must be left to future investigators.
With regard to the _organs_ of the trunk, we have more certain
and satisfactory information;--the muscles of the _legs_ having
been described by Lyonet and Cuvier, and those of the wings most
particularly by Chabrier. In caterpillars, the muscles are situated
in the interior of the articulations that form the legs: they consist
of several bundles appropriated to each, which have their attachment
in the _parietes_ of the preceding joint, near the margin, and
are inserted in the margin of that they move[854]. Lyonet counted
_twenty-one_ muscles in the leg of the caterpillar of the _Cossus_;
but eight of these were appropriated to the claw, or rather formed a
pair of _semipenniform_ muscles, having their insertion at the inner
angle of its base[855]. In _perfect_ insects, according to Cuvier,
each joint of the legs is furnished with a _pair_ of antagonist
muscles--a flexor and extensor, the former being the _lower_, and
the latter the _upper_ muscle; and this pair has its insertion in
the joint it moves, and its attachment usually in the preceding one:
but those of the coxæ--which are _rotators_, causing it to turn
backwards or forwards--and the extensor of the thigh, have their
attachment in the _parietes_ of the trunk, and to the _endosternum_;
one of the _rotators_ of the _anterior_ coxa, and the extensor of
the _anterior_ thigh to the _antefurca_; of the _intermediate_ pairs
to the _medifurca_, and of the _posterior_ to the _postfurca_[856].
Every joint of the _tarsus_ has also its flexor and extensor. In
the ground- and water-beetles (_Eutrech_in_a_ and _Eunech_in_a_),
&c., whose posterior coxæ are immoveable, the thigh includes two
pair of antagonist muscles[857]. In extracting the posterior leg
of _Necrophorus Vespillo_ I observed more than a _single_ pair of
muscles that had their attachment in the coxa; and probably many
other variations in this respect exist.
Little was known with respect to the most interesting part of the
muscular apparatus of insects, that by which such wonderfully rapid
and varied motions are imparted to their organs of flight, till
Chabrier undertook to elucidate it; which he has done in a manner
that will confer a lasting honour upon his name, as one of the
most able successors to Swammerdam and Lyonet in their peculiar
department. He has given a most admirable account of the internal
anatomy of the trunk of insects in general, as far as it relates to
their flight; particularly of that of the cockchafer (_Melolontha
vulgaris_), of one of the _Libellulina_ (_Æshna grandis_), and of a
bumble-bee (_Bombus_); and I believe he has thus illustrated insects
of some of the other Orders, but his memoirs on these I have not had
an opportunity of consulting. What I have to say on this subject,
therefore, will be principally derived from what he has communicated
with respect to the above insects.
A considerable difference in the volume of the muscles of the wings
takes place in insects according to the force of their flight. Where
it is rapid and powerful, the alitrunk is nearly filled by them,
and the alimentary canal is much attenuated; but in those whose
flight is feeble, they occupy less space, and the alimentary canal
is proportionally enlarged[858]. In the _Lepidoptera_, _Hymenoptera_
and _Diptera_, the principal muscles of _both_ wings have their
attachment in the _anterior_ portion of the alitrunk[859]; in the
_Coleoptera_, in the _posterior_[860]; and in the _Libellulina_, those
of the _anterior_ wings are confined to the _anterior_ portion, and
those of the _posterior_ pair to the _posterior_[861]. The muscles
for flight in general differ from others by their mass, length,
and colour; the bundles of fibres are very distinct, strong, and
parallel; their direction is uniform, according to the motion they
are to produce; their fibres are either attached to the solid parts
to be moved, or to cupules, but they never terminate in a tendon;
the muscles are perfectly independent of each other, and the wings
can be moved by them separately[862]. As to their _denomination_ and
kind--the principal ones are the _levators_ and _depressors_, which
with respect to the _trunk_, as was before observed, are _constrictors_
and _laxators_. The _levator_ muscles form several distinct bundles
in _Coleoptera_, _Lepidoptera_, &c.; in the _Diptera_ there are
three[863]; in the _Libellulina_ they seem to be _single_, are all
environed with a blackish pellicle, with numerous aërial vesicles,
symmetrically arranged, filling the interstices[864]. The most common
number is a levator to each wing; there are often, however, as in
the cockchafer and the dragon-fly, _two_ depressors[865]: but in the
_Hemiptera_, _Lepidoptera_, and saw-flies (_Serrifera_) amongst the
_Hymenoptera_, the secondary wings have distinct levators, but not
depressors[866]; the other insects of that Order have only a pair of
each[867]. The other wing-muscles are of a _secondary_ description,
and auxiliary to the above. Their office is to extend and close the
wings: so that though the denomination of _extensor_ will suit the
former, that of _flexor_ is not so proper for their antagonists;
their office being not so much to _bend_, as to bring back the wing
to its station of repose. The folding of certain wings, as those of
_Coleoptera_, _Dermaptera_, the _Vespidæ_, &c., seems more the function
of the _abdomen_ than of the wing-muscles; this you may easily see,
as I have often done, if you attend to any _Staphylinus_, when after
alighting from flight it proceeds to fold up its wings under the
elytra. Perhaps the term _retractor_ might not be inapplicable to the
muscles in question. Both these and the extensors are usually small
slender muscles, but sometimes numerous[868]. They are larger in the
_Coleoptera_, _Lepidoptera_, and saw-flies[869]. The muscles that open
and shut the _elytra_ of _Coleoptera_, and probably of _Heteropterous
Hemiptera_, and which also aid their movements during flight, are
very slender[870]. With regard to the attachment and insertion of
the wing-muscles, it is according to two very distinct types, one of
which appertains to insects in general, and the other is peculiar to
the _Libellulina_. In insects in _general_, the principal muscles for
flight have not their insertion in the wings, but act upon their bases
by the intervention of small long pieces. The depressors occupy the
middle and upper region of the alitrunk, and are inserted anteriorly
and posteriorly upon the concave surfaces of two transverse horny
semi-partitions, adapted by their elasticity to dilate the trunk--and
thus acting the part of both diaphragm and ribs[871]: but in the
_Libellulina_, as in birds, these muscles are placed on each side
of the point of support of the humerus[872]; the _depressors_ being
attached immediately to the wings _without_ it, and the _levators
within_ it, with this sole difference, that they are connected to the
internal extremity of the base of the wing by the intervention of a
cupule terminating in a tendon; all are disposed _perpendicularly_
to the arms of the levers on which they act, and all incline more or
less _outwards_, the one to _dilate_, and the other to _contract_ the
trunk[873]. It may be observed in general, that in insects formed upon
the _first_ type, the _great_ action of these muscles is the dilatation
and contraction of the alitrunk, the main tendency of which is to
_depress_ and _raise_ the wings[874]. I shall add here a few words
upon the attachment of the wing-muscles in the different Orders: but
first I must request you to read what I have said on the partitions
and chambers of the alitrunk in a former letter[875]. In most insects
of the _first_ type, the depressors are longitudinal dorsal muscles
that have their _posterior_ point of attachment in the _metaphragm_
(_costale_ Chabr.); but the _anterior_ varies:--in those that have
_elytra_, _tegmina_, or _hemelytra_, the muscles for them seem to
be contained in the chamber, varying in size, that lies between the
_prophragm_ and _mesophragm_; and the anterior point of attachment of
their depressor muscles is the _mesophragm_: they are also attached
in some to the _metathorax_ or back of the posterior portion of the
alitrunk[876]. The levator muscles in _Coleoptera_, at least in the
cockchafer, by a long tendon have their posterior attachment in the
lower part of the posterior coxæ[877], their anterior attachment to
the solid parts to be moved. In the _Cockchafer_ and the _Dynastidæ_,
but _not_ in _Geotrupes_, on each side of the cavity of the metathorax
under the base of the wing is a large and small cupule, which from
their _lateral_ situation one would think must receive the _levator_
muscles--apparently unnoticed by M. Chabrier; but as there is a _pair_
of these cupules on each side, there must have been also a _pair_ of
muscles attached to them, which does not agree with his statement[878].
In the _Hymenoptera_ and _Diptera_ the anterior attachment of the
_depressors_ is to the back of the alitrunk and to the prophragm,
and the levators to the breast, and the sides of the back of the
trunk[879]. In the _Libellulina_ the depressors and levators that
terminate, by a tendon surmounting a cupule, in the base of the wings,
have their posterior attachment in the breast. These cylindrical
muscles with their cupule and tendon look like so many syringes[880].
Having thus described to you the powerful muscular apparatus by
which, either mediately or immediately, the _wings_ of insects
are moved, it will not be out of place if I add a few words upon
their _flight_ itself. The great object in this is to generate a
centrifugal force which may counteract the weight of the body.
Its wings are the _external_ organs by which the insect as it
were takes hold of the air when they fall, and is impelled by it
when they rise; its head makes way for it; its abdomen, as a
rudder, steers it; and by alternately increasing and diminishing
in volume, and rising and falling, enables it to win an easy way
through the fluctuations of the atmospheric sea. The trunk by its
elasticity admits the internal action of antagonist muscles, which
by turns compress and dilate it; an action promoting the elevation
and depression of the wings, and keeping up the elasticity of the
internal air, which is thus now rarified and now condensed: in the
_former_ state flowing like a tide, accompanied by the blood, into
the nervures of the wings[881], and thus increasing their tension and
centrifugal force;--in the _latter_ ebbing and receding to the trunk,
thus relaxing the one and diminishing the other. The spiracles by
which the air enters or is expelled, open and shut at the animal's
pleasure[882]; and besides, many insects are furnished, as we have
seen[883], with numerous vesicles or reservoirs, which can give out
a supply of internal air when wanted: and thus they can vary their
aërial motions, diminish or increase the counteracting centrifugal
force; rise and fall, and move onwards and in different directions,
as their occasions demand.
iii. The _Abdomen_ is perhaps capable of the greatest variety of
motions of the three primary sections of the body. Even when the
insect is reposing, a constant dilatation and contraction usually
takes place in it[884]; and from its annular structure, its parts
capable of separate motion are numerous:--it expands and contracts;
it rises and falls; it bends in various directions; and its segments
can often be lengthened or retracted. Besides all this, its spiracles
open and shut, and its reproductive and other anal organs have their
appropriate motions. In numerous _Coleoptera_, however, and some
_Hemiptera_, the _upper-side_ of the abdomen is almost the only part
that is moveable, especially near the trunk; the _under-side_, having
its first segments soldered together, is only capable of motion near
the tail[885]. The muscles that produce the various motions of this
part must be entitled to all the denominations stated above[886]. I
have on a former occasion explained to you how, in insects that have
a petiolate abdomen, that part is elevated and depressed[887]. In
those with a sessile one the base is attached to the metaphragm by
strong ligaments[888], and the muscles that move the first piece act
from one segment to another. The _partial_ movements of the segments
of this part, where they have place, are produced by muscular fibres
which extend from the whole _anterior_ margin of one to the whole
_posterior_ one of that which precedes it. If those, for example,
of the back contract, the abdomen becoming shorter above, bends
upwards; and if those of the sides or belly, it bends sideways or
downwards[889]: this is a beautiful as well as simple contrivance.