LETTER XLIII.

_INTERNAL ANATOMY AND PHYSIOLOGY OF INSECTS, CONCLUDED._

MOTION.

We have seen upon a former occasion the great variety of movements that insects can perform, and of the _external_ organs with which they perform them[801]: but we are now to consider the _internal_ apparatus, by the immediate action of which they take place--their system of _muscles_. When we reflect upon the wonderful velocity, their size considered, with which many insects move, and the unparalleled degree of muscular force that many exert[802], we feel no small degree of curiosity to know something of that part of their internal structure that produces these almost incredible effects. I shall in the present letter endeavour in some degree to gratify that curiosity, and give you an account of the _muscles_ of these little animals,--first considering them in _general_; and then, as far as my information goes, adverting to those in _particular_ that move the different parts and organs of an insect's body.

I. The muscles of insects may be considered in general as to their _Origin_; _Substance_ and _Parts_; _Shape_; _Colour_; _Kinds_; _Attachment_; and _Motions_.

i. _Origin._ The origin of the muscular fibre in the higher animals is from the _blood_, the globules of which, by their coagulation in a _series_, appear to form it[803]; and in insects it is derived from the same universal source of nutrition and accretion, but not till it has been concreted into the adipose tissue or _epiploon_ before noticed[804]. In the pupa of the cabbage-butterfly, Herold observed that this substance first assumed a fine flocky appearance and a blue-green colour, and that from it so changed were produced tender bundles of muscular fibres, extending in various directions, the epiploon itself decreasing in proportion as they were formed[805].

ii. _Substance_ and _Parts_. The muscular fibre in vertebrate animals appears to consist of globules arranged in a series, and of no larger diameter than those of the blood,--the mean diameter of which in the human subject, when measured under the microscope by a micrometer, is found to be about 1/5000th part of an inch[806]. When Cuvier published his immortal work in 1805, the powers of any magnifier then constructed were not sufficient to enable this great physiologist to arrive at the simple fibre[807]; but Mr. Bauer, by the use of improved glasses, amongst other discoveries that will immortalize his name, was the first to detect, under the directions of Sir E. Home, the ultimate thread of which the muscular bundles are composed[808]. Chemists distinguish the substance of which we are speaking, by the name of _fibrine_. By the abundance of azote or nitrogen that enters into its composition, it possesses a character of animalization more marked than any other animal substance; and its elements are so approximated in the blood, that the slightest stagnation causes them to coagulate: and the muscles are without doubt, in the living subject, the only organs that can separate this matter from the mass of blood, and appropriate it to themselves[809]. The _primary_ bundles of muscles are formed of the simple fibres, and the _secondary_ are the result of an aggregation of the primary. The smaller bundles are not always exactly parallel to each other, but must in many cases diverge more or less, to produce those variations in shape observable in the muscles themselves: there are intervals therefore between the bundles, which in some animals are filled by a cellular substance[810]. Probably much of this statement will apply in most instances to the muscles of _insects_, but we may conclude that the globules that form them are infinitely smaller[811]. Lyonet has given some interesting observations with regard to those of the caterpillar of the _Cossus_: he describes them as of a soft transparent substance, capable of great extension, covered and filled by silver tubes of the _bronchiæ_, penetrated by the nerves, and containing oily particles. Each muscle was enveloped in membrane, and was composed of many parallel bands, consisting of bundles of fibres enveloped likewise in separate membranes. The fibres themselves, (but it is doubtful whether he arrived at the ultimate term of muscular fibre,) in a favourable light and under a good magnifier, appeared to be twisted spirally[812]. In spiders the muscles seemed to him to consist of _two_ substances, the one soft and the other hard, the last forming a kind of stiff twisted filament[813]. A muscle thus composed of different bundles of fibres may be stated as to its _parts_, in _insects_, to consist of base, middle, and apex: the _base_ is that part by which they are fixed to any given point of the internal surface of the crust, or of one of its processes, which serves as their fulcrum; the _apex_ is that part by which they are fixed, either mediately or immediately, to the organ to be moved; and the _middle_ is the remainder of the muscle. We usually discover in them no inflation of the middle corresponding with the _belly_ of the muscles in vertebrate animals; they occasionally, however, terminate in a _tendon_, as those of the thighs and legs; but these tendons are of a different nature from the fibrous ones of warm-blooded animals; for they are hard, elastic, and without apparent fibres: the fleshy ones of the muscle envelope them, and are inserted in their surface[814].

iii. _Shape._ The muscles of insects are usually _linear_, with 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.

The alternate rush of air from the abdomen into the alitrunk, and from the atmosphere into the abdomen, is attended by the constriction or expansion of that part as it rises or falls in flight[890], which seems to require the action of constrictor and laxator muscles.

iv. The _Viscera_. Having before had occasion sufficiently to notice the muscles by which the systole and diastole of the _dorsal vessel_ of insects is maintained[891], I shall now only mention those that are _woven_ round their alimentary canal, by which the peristaltic motion of that organ, causing its contractions and the propulsion of its contents, takes place. One would at first think that a view of the _intestines_ of any animal could under no circumstances afford any very pleasing spectacle to the eye of any but a scientific spectator; but any _lady_ who is fond of going to Disons to be tempted with an exhibition of fine lace, would experience an unexpected gratification could she be brought to examine those of a caterpillar under a microscope: with wonder and delight she would survey the innumerable muscular threads that in various directions envelope the gullet, stomach, and lower intestines of one of these little animals; some running longitudinally, others transversely, others crossing each other obliquely, so as to form a pattern of rhomboids or squares; others again, surrounding the intestine like so many rings, and almost all exhibiting the appearance of being woven, and resembling fine lace,--one pattern ornamenting one organ; another, a second; and another, a third. This will suffice to give some idea of this part of the muscular structure of these little animals[892].

Lyonet counted the muscles contained in the body of the caterpillar of the _Cossus_. In the head he found 228; in the body, 1647; and enveloping the intestines, no less than 2186; which, after deducting 20 that are common to the gullet and the head, gives a total of 4061[893]. In the human subject only 529 have been counted[894]: so that this minute animal has 3532 muscles more than the Lord of the creation!

* * * * *

The muscles of the _Arachnida_ seem less numerous than those of insects. In the _Scorpionidea_ they appear to be robust, formed of simple straight fibres, of a whitish gray colour: a muscular web, rather strong, clothes the _parietes_, but rarely adheres to them, of the abdomen, and envelopes the _viscera_, with the exception of the lungs, and probably of the heart. The dorsal part of this web gives birth to seven pairs of filiform muscles, which traverse the liver, and are attached to a muscular riband which, passing above the lungs, runs the whole length of the ventral _parietes_. These muscles when exposed to view resemble extended cords. The abdominal segment preceding the tail is filled with a powerful muscular mass which moves that organ[895]. Treviranus discovered two longitudinal muscles in _Scorpio europæus_, running from the breast to the tail, which above and below each gill were connected by another running transversely across the heart, thus forming a quadrangular area in which the gills are situate[896]. The heart appears to be moved by muscles not very dissimilar to those of the _Cossus_[897], as is likewise that of the _Araneidea_; in _Clubiona atrox_ the wider part of this organ is muscular, and incloses a considerable cavity[898]. In this tribe the muscles of the abdomen, the skin of which is soft and unfit to act as a lever to them, are attached to a cartilage, and thus their action is better sustained[899].

Having thus laid before you all of importance that I can collect with regard to the apparatus of muscles discoverable in insects, I shall next say something upon a few other points connected with that subject. When I enlarged upon their _motions_, I related a few instances of the extraordinary power of that apparatus[900] in _leaping_ ones; but this power is not confined to that circumstance. The _flea_, not more remarkable for its compressed form, enabling it to glide between the hairs of animals, and its elastic coat of mail, by which it can resist the ordinary pressure of the fingers, than for its muscular strength, has attracted notice on this account from ancient times. Mouffet relates that an ingenious English mechanic, named Mark, made a golden chain of the length of a finger, with a lock and key, which was dragged by a flea;--he had heard of another that was harnessed to a golden chariot, which it drew with the greatest ease[901]. Another English workman made an ivory coach with six horses, a coachman on the seat with a dog between his legs, a postillion, four persons in the coach, and four lacqueys behind--which also was dragged by a single flea. At such a spectacle one would hardly know which most to admire, the strength and agility of the insect, or the patience of the workman. Latreille mentions a flea of a moderate size dragging a silver cannon on wheels, that was twenty-four times its own weight, which being charged with powder, was fired without the flea appearing alarmed[902]. Many caterpillars are accustomed to extend their bodies from a twig, supported merely by the four hind feet, in one fixed attitude, either in an oblique, horizontal, or vertical direction, either upwards or downwards, and that for hours together. We may conceive what prodigious muscular force must be exerted upon this occasion, by reflecting that the most expert rope-dancer, though endued with the power of grasping with his feet like a bird with its claws, could not maintain himself in a horizontal position even for an instant. Bradley asserts that he has seen a stag-beetle carry a wand half a yard long and half an inch thick, and fly with it several yards[903]. Some insects have the faculty of resisting pressure in a wonderful degree. If you take a common dung-chafer (_Geotrupes_) in your hand and press it with all your strength, you will find with what wonderful force it resists you; and that you can scarcely overcome the counteraction, and retain the insect in your hand: was it not for this quality, the grub of the gad-fly must be crushed probably in passing through the anal sphincter of the horse[904]. But that of _Eristalis tenax_ affords a more surprising instance of this power of counteraction:--an inhabitant of muddy pools, it has occasionally been taken up with the water used in paper-making, and strange to say, according to Linné, has resisted without injury the immense pressure given to the surrounding pulp[905]; like _leather-coat Jack_ mentioned by Mr. Bell[906], who, from a similar force of muscle, could suffer carriages to drive over him without receiving any injury. Almost as remarkable is the state of extreme relaxation into which the muscles of some larvæ fall, when their animation is suspended; and the revived tension to which a subsequent resumption of the vital powers restores them. Bonnet having suspended the animation of the caterpillar of _Sphinx Ligustri_ by keeping it submerged, squeezed it between his fingers, until it had wholly lost its cylindrical form and was as flat and supple as the empty finger of a glove; yet in less than an hour the very same caterpillar became as firm, as compact, as cylindrical, and in short, as well, as though it had never been submitted to treatment so rough[907].

It is fortunate that animals of a large size, as has been well remarked, especially noxious ones, have not been endowed with a muscular power proportionable to that of insects. A _cockchafer_, respect being had to their size, would be _six_ times stronger than a _horse_; and if the _elephant_, as Linné has observed, was strong in proportion to the _stag-beetle_, it would be able to pull up rocks by the root, and to level mountains[908]. Were the _lion_ and the _tiger_ as strong and as swift for their magnitude as the _Cicindela_ and the _Carabus_, nothing could have escaped them by precaution, or withstood them by strength. Could the _viper_ and the _rattlesnake_ move with a rapidity and force equivalent to that of the _Iulus_ and _Scolopendra_, who could have avoided their venemous bite? But the CREATOR in these little creatures has manifested his Almighty POWER, in showing what he could have done had he so willed; and his GOODNESS in not creating the higher animals endued with powers and velocity upon the same scale with that of insects, which would probably have caused the early desolation of the world that he has made. From this instance we may conjecture, that after the resurrection, our bodies by a change in the structure and composition of their muscular fibre--for we know that their locomotive powers and organs, as far as the muscle is concerned, will then be of a very different nature[909]--may become fitted for motions and a potent agency of which we have now no conception.

This wonderful strength of insects is doubtless the result of something peculiar in the structure and arrangement of their muscles, and principally their extraordinary power of contraction, excited by the extent of their respiration: for animals that respire but little, as the fœtus in the womb and the pullet in the egg, have very little contractile muscular power[910]. To get some idea from facts of this extraordinary contractile power in insects,--extract the sting of a bee or a wasp, with its muscles, which appear to be attached to powerful cartilaginous plates[911], and you will find it continue for a long time to dart forth its spicula, almost as powerfully as when moved by the will of the animal. A still more extraordinary instance of irritability is exhibited by the _antlia_, or instrument of suction of the butterfly. If this organ, which the insect can roll up spirally like a watchspring or extend in a straight direction, be cut off as soon as the animal is disclosed from the chrysalis, it will continue to roll up and unroll itself as if still attached to its head: and if after having apparently ceased to move for three or four hours it be merely touched, it will again begin to move and resume the same action. This surprising irritability and contractility of muscle doubtless depends upon the peculiar structure of the antlia, which is composed of an infinite number of horny rings, acted upon by muscles, more numerous probably than those which move the trunk of the elephant. The motion only ceases when the muscles become dry and rigid.

* * * * *

I have already, under another head[912], considered the _annual_ sleep, or winter state of torpidity of insects, during which an intermission for the most part of muscular motion and action takes place. I shall now make a few observations with respect to their _diurnal_ sleep, which may very properly have its place in the present letter. That insects, usually so incessantly busy and moving in every direction, require their intervals of repose, seems to call for no proof. We see some that appear only in the _day_, and others only in the _night_, others again only at certain hours; which leads to the conclusion, that when they withdraw from action and observation, it is to devote themselves to rest and sleep. The cockchafer flies only in the evening; but if you chance to meet with it roosting in a tree in the earlier part of the day, you will find it perfectly still and motionless, with its antennæ folded and applied to the breast:--we cannot indeed say that its eyes are shut; for as insects have no eyelids, that sign of sleep can never be found in them. Again, if a Lepidopterist goes into the wood to capture moths in the day-time, he finds them often perched on the lichens that cover the north side of the trunk of a tree, with their wings and antennæ folded, and themselves without motion, and insensible of his approach and their own danger. Thus it was that I captured that rare insect the lobster-moth (_Stauropus Fagi_) in the New Forest. Some, however, have asserted that the caterpillar of the silkworm, except when they moult, never intermits feeding day or night, and consequently does not sleep: but the accuracy of this statement, both from analogy and observation, admits of great doubt. Malpighi informs us that these caterpillars for an hour and more, twice a day, remain immoveable with their heads bent down as though asleep, and even if disturbed, resume again the same inactive posture[913]; and other larvæ in great numbers certainly seem to have regular intermissions from eating of considerable duration: those called Geometers, for hours together remain motionless projected from a twig, to which they adhere by their posterior prolegs alone; and the processionary caterpillars make only _nightly_ sorties from their nests, passing the day in inaction and repose[914]. Bees have been often seen by Huber, when apparently wearied with exertion, even in the middle of the day, to insert the half of their bodies into an empty cell, and remain there, as if taking a nap, without motion for half an hour or longer[915]; and at night they regularly muster in a state of sleep-like silence. Mr. Brightwell once observed an individual living specimen of _Haltica concinna_, which appeared to remain motionless on the same spot of a wall for three successive days.

Before concluding these remarks on the Internal Anatomy and Physiology of Insects, I shall explain to you, as you will probably feel inclined occasionally to pursue the subject, the best mode of _dissecting_ them.--By far the most useful dissecting instruments for this purpose are very fine-pointed and sharp _scissors_, as these will enable you to divide the integument and separate other parts with much less risk of injuring their delicate structure than any knife. These scissors are what Swammerdam chiefly used; and he had some so extremely small and fine, that he was necessitated to employ a lens when he sharpened them. If to these be added a sharp and fine-pointed _knife_ or two, some _needles_ fixed in handles, also fine-pointed--(you will find them more convenient than any other instrument for detaching minute parts and fibres,) a pair of fine and accurately adjusted _pliers_, and an assortment of camel's-hair _brushes_,--you will be nearly set up as an Entomological dissector. You will still, however, require a small dissecting table, with a projecting and moveable arm for lenses of various descriptions, so as to admit both the hands to be employed upon the subject under examination; and for this purpose probably no contrivance can be better adapted than that of Lyonet, of which the figure in Adams _On the Microscope_ will convey a better idea than any description[916].

Previously to dissecting any insect, it must be killed by plunging it into boiling water, which is recommended by Lyonet, or spirits of wine or of turpentine; and it is often useful to let _larvæ_ remain a few days in the latter, by which means the vessels become firmer and stronger. The parts of _pupæ_ become much more distinct if they are boiled for a few minutes: and the same mode may be adopted in the examination of spiders.

The most convenient mode of proceeding, which was that also of Lyonet, is to dissect the insect in water, or, to avoid putridity, in diluted spirits,--if small, upon a concave glass, to which it should be fastened by means of a little melted wax; if larger, in the bottom of a common chip box, surrounded with a border of wax to retain the fluid. The integuments of the insect, being carefully divided longitudinally with scissors, should if flexible be turned back, and fixed by small pins stuck in by a fine pair of pliers, while the skin at the same time is stretched by another. After making such observations as present themselves without further dissection, the viscera must be cautiously extracted, washing away the fat which surrounds them with spirits of turpentine, in which it is soluble, applied by camel's-hair pencils. After separation they may conveniently be examined by putting them into water, and gently shaking them so as to cause the parts to unfold. If endowed with the patience of Swammerdam, you may even arrive at injecting these minute parts with wax or coloured fluids, conveyed by delicate glass tubes having one end as fine as a hair, which he also employed to fill the viscera with air; and afterwards drying them in the shade, and anointing them with oil of spike in which a little resin had been dissolved, he succeeded in preserving them. If it is not convenient to finish the dissection of an insect at once, it should be covered with spirits of wine. Swammerdam found a mixture of spirits and distilled vinegar very useful for keeping caterpillars previously to dissecting them, as it consolidated the parts[917].

* * * * *

And now having brought to a close my long wanderings in this ample and intricate field, and having threaded, as well as my slender powers and limited knowledge enabled me, the infinite turnings and convolutions of this Dædalean labyrinth--the _Anatomy_ and _Physiology_ of insects,--will you not own that the volume of wonders I have laid before you proves irrefragably that, though these minims of nature apparently rank so low in the scale of being, yet in their structure, instead of being, as might be expected, more simple, they are infinitely more complex and highly wrought than those animals that are placed the nearest to ourselves? the CREATOR in the latter doing every thing by a beautiful _simplicity_; while in the former, the more to magnify his power and skill, because they afford no apparent space for it, by a wonderfully curious and intricate _multiplicity_: and whether we study the one or the other, we shall in both trace the footsteps of that adorable LOVE which has shown attention to the comfort and well-being of the lowest insect, as well as of the highest of his creatures.

I am, &c.

FOOTNOTES:

[801] VOL. II. LETTER XXII. VOL. III. LETTERS XXXIV.-XXXVI.

[802] VOL. II. p. 280, 295--, 306, 310--. &c.

[803] _Philos. Trans._ 1818. 174. _t._ viii. _f._ 4-6.

[804] See above, p. 150--.

[805] _Schmetterl._ 105.

[806] _Philos. Trans._ 1819. 172, 174, 187.

[807] _Anat. Comp._ i. 90.

[808] _Philos. Trans._ 1819. 175.

[809] Cuv. _ubi supr._ 90--.

[810] Cuv. _Ibid._ i. 89--.

[811] See above, p. 85.

[812] Lyonet _Anat._ _t._ iv. _f._ 3.

[813] _Ibid._ 93--.

[814] Cuv. _Anat. Comp._ i. 134.

[815] Chabrier _Sur le Vol des Ins._ c. i. 445.

[816] PLATE XXI. FIG. 6. _a._

[817] De Geer iv. _t._ xv. _f._ 11. _m n, o p._

[818] Lyonet _Anat._ 93.

[819] Lyonet _Anat._ _t._ xiii. _f._ 1, 2.

[820] Ramdohr _Anat._ _t._ v. _f._ 1. _e._ _f._ 3.

[821] Chabr. _ubi supr._ 440--.

[822] _Ibid._ 442, &c.

[823] _N. Dict. d'Hist. Nat._ xxii. 80.

[824] VOL. III. p. 663, 670. See above p. 21.

[825] Chabrier _Sur le Vol des Ins._ c. i. 446.

[826] VOL. III. p. 411.

[827] _Ubi supr._ 437, 439.

[828] PLATE XXII. FIG. 11, 12. c. Chabrier _ubi supr._ c. iii. _t._ xi. viii. _f._ 9. S. D. _i, k._ c. i. 440--.

[829] PLATE XXII. FIG. 11, 12. c. Chabrier _Sur le Vol des Ins._ c. iii. _t._ xi. viii. _f._ 9. S. D. _i, k._ c. i. 440--.

[830] Cuv. _Anat. Comp._ i. 94--.

[831] _N. Dict. d'Hist. Nat._ xxii. 80.

[832] _Ubi supr._ 101--.

[833] VOL. I. p. 67.

[834] _Anat. Comp._ i. 432--.

[835] _Anat._ _t._ vii. _f._ 2. left hand.

[836] _Ibid._ right hand.

[837] _Ibid._ 115--.

[838] Cuv. _ubi supr._

[839] VOL. III. p. 135--.

[840] _Anat. Comp._ i. 447.

[841] VOL. III. p. 366. PLATE XXVII. FIG. 1, 4. n´.

[842] Ibid. FIG. 3. n´.

[843] PLATE XXVII. FIG. 1. _a._

[844] VOL. III. p. 367--, 541, 584. PLATE XXII. FIG. 7. Cuv. _ubi supr._ 448.

[845] PLATE XXVII. FIG. 5. _a._

[846] _Anat. Comp._ i. 136.

[847] De Geer iv. _t._ xv. _f._ 11. _o, p._

[848] Marcel de Serres _Comparaison, &c._ 3--.

[849] _Ibid._ 4.

[850] _Ibid._ 5.

[851] PLATE XXII. FIG. 11. _h´_.

[852] VOL. III. p. 579.

[853] PLATE XXII. FIG. 6. VOL. III. p. 585--.

[854] Cuv. _Anat. Comp._ i. 436. PLATE XXI. FIG. 6.

[855] Ibid. _a, b._ Lyonet _Anat._ 37.

[856] Cuv. _ubi supr._ 458--. VOL. III. p. 368, 378, 382.

[857] Cuv. _ubi supr._ 459.

[858] Chabr. _Sur le Vol des Ins._ c. i. 441.

[859] Chabr. _Sur le Vol des Ins._ c. i. 415.

[860] _Ibid._

[861] _Ibid._ c. iii. 344. _t._ viii. _f._ 8, 9.

[862] _Ibid._ c. i. 440.

[863] _Ibid._ 444.

[864] _Ibid._ 445. c. iii. 359.

[865] _Ibid._ c. ii. 332. c. iii. 359.

[866] _Ibid._ c. i. 445.

[867] _Ibid._ c. iv. 78.

[868] Chabr. _Sur le Vol des Ins._ c. i. 415, 442. c. iv. 80.

[869] _Ibid._ c. i. 442.

[870] _Ibid._ 439--.

[871] Chabrier _Analyse_, 28. The latter part of this passage is copied from a MS. note of the author's in my copy.--W. K.

[872] Chabrier _Analyse_, Ibid. _Sur le Vol des Ins._ c. i. 445. VOL. III. p. 617.

[873] _Analyse_ ubi supr.

[874] _Sur le Vol des Ins._ c. i. 448, c. ii. 336.

[875] VOL. III. p. 579.--

[876] Chabr. _Ibid._ c. i. 443. ii. 316, 332.

[877] Chabr. _Sur le Vol des Ins._ c. ii. 333.

[878] _Ibid._ 332. PLATE XXII. FIG. 11, 12. c. A cupuliform process is also observable at the side of the metaphragm. Ibid. FIG. 10. a.

[879] Chabr. _Ibid._ c. iv. _t._ xi.-4. _f._ 14.

[880] _Ibid._ c. i. 445. xi.-8. _f._ 8, 9.

[881] Chabr. _Sur le Vol des Ins._ c. ii. 336. note 1. VOL. III. p. 292--.

[882] Chabr. _Ibid._ c. i. 447.

[883] See above, p. 66--.

[884] See above, p. 73--.

[885] Chabrier _Sur le Vol des Ins._ c. i. _Addend._ 298.

[886] See above, p. 178--.

[887] VOL. III. p. 700--.

[888] Chabr. _ubi supr._ c. i. 422.

[889] Cuv. _Anat. Comp._ i. 451.

[890] Chabr. _Analyse_ 25. _Sur le Vol des Ins._ c. i. 423, 452. _Addend._ 301.

[891] See above, p. 83.

[892] Lyonet _Anat._ _t._ xiii. _f._ 1, 2.

[893] Lyonet _Anat._ _t._ xiii. 188--, 584.

[894] _Ibid._ 189.

[895] _N. Dict. d'Hist. Nat._ xxx. 421.

[896] _Arachnid._ 9. _t._ i. _f._ 7. _r._

[897] _Ibid._ _o._

[898] _Ibid._ 10.

[899] _Arachnid._ 45. _t._ iii. _f._ 31. _m, n, q, r, t._

[900] VOL. II. p. 309--.

[901] Mouffet _Theatr._ 275.

[902] _N. Dict. d'Hist. Nat._ xxviii. 249.

[903] _Phil. Acc. of Works of Nat._ 144.

[904] Clark in _Linn. Trans._ iii. 309.

[905] _Fn. Suec._ 1799.

[906] _Anatomy of Expression in Painting_, 170.

[907] Bonnet _Œuvr._ ii. 124.

[908] _N. Dict. d'Hist. Nat._ xxii. 81.

[909] 1 Cor. xv. 50--.

[910] _N. Dict. d'Hist. Nat._ ubi. supr.

[911] Swamm. _Bibl. Nat._ _t._ xviii. _f._ 2. _l, m, n, o._ Reaum. v. _t._ xxix. _f._ 7. _m, n, o, p, q._

[912] VOL. II. LETTER XXVI.

[913] _De Bombyc._ 5.

[914] Reaum. ii. 185--.

[915] VOL. II. p. 186.

[916] _t._ vi. _f._ 3.

[917] These directions for dissecting are chiefly taken from Swammerdam, _Life_ xiv.-- and Lyonet _Anat._ 7--.