LETTER XXXVII.

_INTERNAL ANATOMY AND PHYSIOLOGY OF INSECTS._

SENSATION.

Having given you this full account of the _external_ parts of insects, and their most remarkable variations; I must next direct your attention to such discoveries as have been made with regard to their _Internal Anatomy and Physiology_: a subject still more fertile, if possible, than the former in wonderful manifestations of the POWER, WISDOM and GOODNESS of the CREATOR.

The vital system of these little creatures, in all its great features, is perfectly analogous to that of the vertebrate animals. _Sensation_ and _perception_ are by the means of _nerves_ and a _common sensorium_; the _respiration_ of air is evident, being received and expelled by a particular apparatus; _nutrition_ is effected through a _stomach_ and _intestines_; the analogue of the _blood_ prepared by these organs pervades every part of the body, and from it are secreted various peculiar substances; _generation_ takes place, and an intercourse between the sexes, by means of appropriate _organs_; and lastly, _motion_ is the result of the action of _muscles_. Some of these functions are, however, exercised in a mode apparently so dissimilar from what obtains in the higher animals, that upon a first view we are inclined to pronounce them the effect of processes altogether peculiar. Thus, though insects respire _air_, they do not receive it by the _mouth_, but through little orifices in the _sides_ of the body; and instead of _lungs_, they are furnished with a system of air-vessels, ramified _ad infinitum_, and penetrating to every part and organ of their frame; and though they are nourished by a fluid prepared from the food received into the stomach, this fluid, unlike the blood of vertebrate animals, is _white_, and the mode in which it is distributed to the different parts of the system, except in the case of the true _Arachnida_, in which a circulation in the ordinary way has been detected, is altogether obscure.

In order that you may more clearly understand the variations that occur in insects, and in what respects they differ amongst themselves, and from the higher animals, in the vital functions and their organs, I shall consider them as to their organs of _sensation_, _respiration_, _circulation_, _nutrition_, _generation_, _secretion_, and _muscular motion_.

* * * * *

_Organs of Sensation._--The nervous system of animals is one of the most wonderful and mysterious works of the CREATOR. Its pulpy substance is the _visible_ medium by which the governing principle[1] transmits its commands to the various organs of the body, and they move instantaneously--yet this appears to be but the conductor of some higher principle, which can be more immediately acted upon by the mind and by the will. This principle, however, whatever it be, whether we call it the nervous _fluid_, or the nervous _power_[2], has not been detected, and is known only by its effects. The system of which we are speaking may therefore be deemed the foundation and root of the animal, the centre from which emanate all its powers and functions.

Comparative anatomists have considered the nervous system of animals as formed upon _four_ primary types, which may be called the _molecular_, the _filamentous_, the _ganglionic_, and _cerebro-spinal_[3]. The _first_ is where invisible nervous molecules are dispersed in a gelatinous body, the existence of which has only been ascertained by the nervous irritability of such bodies, their fine sense of touch, their perceiving the movements of the waters in which they reside, and from their perfect sense of the degrees of light and heat[4]. Of this description are the infusory animals, and the _Polypi_. The nervous molecules in these are conjectured to constitute so many ganglions, or centres of sensation and vitality[5]. The _second_, the filamentous, is where the nervous system consists of nervous threads radiating from the mouth, as in the _Radiata_, or star-fish and sea-urchins[6]. The _third_, the ganglionic, is where the nervous system consists of a series of ganglions connected by nervous threads or a medullary chord, placed, except the first ganglion, below the intestines, from which proceed nerves to the various parts of the body. This system may be considered as divisible into two--the _proper ganglionic_, in which it is ganglionic with the ganglions arranged in a series with a double spinal chord. This prevails in the classes _Insecta_, _Crustacea_, _Arachnida_, &c., and the _improper ganglionic_, in which it is ganglionic with the ganglions dispersed irregularly, but connected by nervous threads, as in the _Mollusca_[7]. In the _fourth_, the cerebro-spinal, the nervous tree may be said to be double, or to consist of _two_ systems--the first taking its origin in a brain formed of two hemispheres contained in the cavity of the head, from which posteriorly proceeds a spinal marrow, included in a dorsal vertebral column. These send forth numerous nerves to the organs of the senses and the muscles of the limbs. The second consists of two principal ventral chords, which by their ganglions, but without any direct communication, anastomose with the spinal nerves and some of those of the brain, and run one on each side from the base of the skull to the extremity of the _sacrum_. This system consists of an assemblage of nervous filaments bearing numerous ganglions, from which nervous threads are distributed to the organs of nutrition and reproduction[8]. Its chords are called the _great sympathetic_, the _intercostal_, or _trisplanchnic_ nerves[9]. While the first of these two systems is the messenger of the will, by means of the organs of the senses connects us with the external world, and is subject to have its agency interrupted by sleep or disease[10]; the latter is altogether independent of the will and of the intellect, is confined to the internal organic life, its agency continues uninterrupted during sleep, and is subject to no paralysis. While the former is the seat of the intellectual powers, the latter has no relation to them, but is the focus from whence _instincts_ exclusively emanate: from it proceed spontaneous impulses and sympathies, and those passions and affections that excite the agent to acts in which the will and the judgement have no concern[11].

It is probable, though the above appear to exhibit the _primary_ types of nervous systems, that others exist of an _intermediate_ nature, with which future investigators may render us better acquainted[12]: but as our business is solely with that upon which _insects_ in this respect have been modelled, without expatiating further in this interesting field, I shall therefore now confine myself to them.

We have before seen[13] that the nervous system of insects belongs to the _ganglionic_ type: but it requires a more full description, and this is the place for it. It originates in a small brain placed in the head, and consisting almost universally of two lobes, sometimes extremely distinct. It is placed over or upon the _œsophagus_ or gullet, and from its posterior part proceeds a double nervous chord, which embracing that organ as a collar dips below the intestines, and proceeds towards the anus, forming knots or ganglions at intervals, in many cases corresponding in number with the segments of the body, and sending forth nerves in pairs, the ramifications of which are distributed to every part of the frame. In the perfect insect the bilobed ganglion of the head or the brain is usually of greater volume than in the larva, and the ganglions of the spinal chord are fewer, which gives a more decided character of _centricity_ to the whole nervous system[14]. This may be considered more particularly with respect to its _substance_ and _colour_; its _tunics_, and _parts_.

I. _Substance and Colour._--The nervous apparatus of insects is stated by those who have examined it most narrowly, though consisting of a cortical and medullary part, the latter more delicate and transparent than the former, to be less tender and less easy to separate than the human brain[15]. It has a degree of tenacity, and does not break without considerable tension; in general, it is clammy and flabby, and under a microscope a number of minute grains are discoverable in it, and when left to dry upon glass, it appears to contain a good deal of oil, which does not dry with the rest[16]. That of the ganglions differs from the substance of the rest of the spinal chord, in being filled with very fine aërial vessels, which are not discoverable in the latter[17]. With regard to _colour_, Lyonet states that the chords of the spinal marrow in the larva of the great goat-moth are of a blueish gray, and have some transparence[18]; Malpighi and Swammerdam observed that the cortical part of the ganglions of that of the silk-worm and the hive-bee had a reddish hue, while the medullary part was white[19]; Cuvier relates that the brain and the third ganglion in _Hypogymna dispar_, with us a scarce moth, differed in colour from all the rest, being quite white, while the others were more or less tinted, and examined under a lens appeared variegated by reddish sinuous markings, resembling blood vessels as they are seen in injected glands[20].

II. _Tunics._--The coats that inclose the various branches of the nervous system in insects seem analogous to those of vertebrate animals. The first thing that strikes the eye, when these parts in a recent subject are submitted to a microscope, is a tissue of very delicate vessels, which ramify beyond the reach of the assisted sight; these are merely air-vessels or _bronchiæ_ derived originally from the _tracheæ_ of the animal: but besides these is an exterior and an interior tunic; the first corresponding with the _dura mater_ of anatomists; and the other, which is the most delicate and incloses the cortical and medullary parts, with the _pia mater_[21].

III. _Parts._--The nervous system of insects consists of the _brain_; the _spinal marrow_ and its _ganglions_; and the _nerves_.

i. _Brain_.[22] Linné denied the existence of a _brain_ in insects, and most modern physiologists seem to be of the same opinion. A part however, analogous to this important organ--at least in its situation, and in its emission of nerves to the principal organs of the _senses_, in which respect it certainly differs very materially from the upper cervical ganglion, which Dr. Virey regards as its analogue[23]--is certainly to be found in them; and as Messrs. Cuvier and Lamarck distinguish this part by the name of _brain_, we may continue to call it by that name without impropriety. The _brain_ of insects, then, is distinguished from the succeeding ganglions of the spinal chord by its _situation_ in the head, the middle of the internal cavity of which it occupies, and by being the only ganglion _above_ the œsophagus. It is usually small, though in some cases larger than they are[24]. It consists of two lobes, more or less distinct and generally of a spherical form. In _Oryctes nasicornis_ and _Pontia Brassicæ_ the lobes are separated both before and behind[25]; while in the larva of _Dytiscus marginalis_, but not in the imago, in which there are two large hemispheres separated by a furrow, the brain is undivided[26]. Cuvier mentions the larva of a saw-fly in which this part is formed of _four_ nearly equal spherical bulbs[27]: in the Scorpion (to judge by the figure of Treviranus[28]) the two lobes represent an equilateral triangle, the exterior angle of which terminates in several lesser spherical bulbs; in _Acrida viridissima_, _Nepa cinerea_, _Clubiona atrox_, and the common Louse, the lobes are pear-shaped[29].

ii. _The spinal marrow and its ganglions_[30]. From the _posterior_ part of the brain of insects, but in the ground and water beetles (_Eutrech_in_a_ and _Eunech_in_a_) from its _sides_ below[31], issue two chords which diverging embrace the _œsophagus_, and dipping below it and the intestines,--a situation they maintain to the end of their course,--and in their further progress uniting at intervals and dilating into several knots or ganglions, compose their spinal marrow. This part is so named, from a supposed analogy to the spinal marrow of vertebrate animals, which however admits of some degree of doubt; yet, since it mixes the functions of that organ with those of the great sympathetic nerves, the denomination is not wholly improper, and may be retained. Though this chord is usually _double_ when it first proceeds from the brain, and surrounds the _œsophagus_ like a collar, yet in some insects it may be called a _single_ chord. This is the case with that of the common louse, in which Swammerdam could perceive no opening for the transmission of the part just named[32]; if he was not mistaken in this, the brain, as well as the rest of the spinal marrow in that animal, would be below the intestines; from the figures of Treviranus it should seem that the spiders, at least _Clubiona atrox_, are similarly circumstanced[33]; in the cheese-maggot, which turns to a two-winged fly (_Tyrophaga Casei_), the chord is also single, but it has a small orifice through which the gullet passes[34]. At the union of the chords in other cases below that organ, a knot or ganglion is usually formed, and an alternate succession of internodes and ganglions commonly follows to the end. The internodes also may generally be stated to consist of a _double_ chord, though in many cases the two chords unite and become one, or are distinguished only by a longitudinal furrow, and even where they are really distinct and separable, in the body of the insect they lie close together[35]. In the rhinoceros beetle (_Oryctes nasicornis_) and _Acrida viridissima_ &c. _all_ the internodes consist of a double chord[36]; but in many other insects numerous variations in this respect occur.--Thus in the stag-beetle the _last_ internode is single[37]; in the caterpillar of the cabbage butterfly (_Pontia Brassicæ_) the _five first_ are double, and the _six last_ single[38]; in that of the great goat-moth (_Cossus ligniperda_) the _three first_ only are double, but the others terminate in a fork[39]; in the cockroaches (_Blatta_) the _four first_, in _Hydrophilus piceus_ the _three first_, and in _Eristalis tenax_ the _two first_ only are double, the rest being all single[40]. A singular variation takes place in _Hypogymna dispar_; _all_ the internodes are single, except the _second_, the chords of which at first are separate, and afterwards united[41]; and, to name no more, in _Clubiona atrox_ there is only _one_ internode, which is single, with a longitudinal furrow[42]. In some, as in the louse, the grub of _Oryctes nasicornis_, and the cheese-maggot, there are no internodes, the spinal marrow being formed of knots separated only by slight or deep constrictions[43].

I must next say something of the _ganglions_[44]. Lyonet has observed that, in the caterpillar of the great goat-moth, these in one respect differ remarkably from the chords that connect them; in the latter the air-vessels or bronchiæ only cover the _outside_ of the tunic, while in the former they enter the _substance_ of the ganglion, which is quite filled with their delicate and numberless branches[45]. Every ganglion may be regarded in some degree as a centre of vitality or little brain[46], and in many cases, as well as the brain, they are formed of two lobes[47]. I shall now consider them more particularly as to their _station_, _number_, and _shape_.

1. With regard to the first head, their _station_, they are most commonly divided between the trunk and abdomen; but in some cases, as in _Hydrophilus piceus_ and _Acrida viridissima_, the _first_ ganglion is in the _head_[48]; in others, as in the louse, the water-scorpion, and the grub of the rhinoceros-beetle, they are confined to the _trunk_, their functions in the abdomen being supplied by numerous radiating nerves[49]; in others again, as in the scorpion, they are all _abdominal_. The ganglions vary also in their situation with respect to each other. Thus in some, as in the larva of the Chamæleon-fly (_Stratyomis Chamæleon_), they are so near as to appear like a string of beads[50]; in that of the ant-lion (_Myrmeleon_) the two ganglions of the trunk are separated by an interval from those of the abdomen, which are so contiguous as to resemble the rattle of the rattle-snake[51]. In others the internodes are longer, and the ganglions occur at nearly equal intervals, as in the larva of the _Ephemeræ_[52]; but in the majority they are unequal in length: thus in the scorpion the three first ganglions are the most distant[53]; in the hive-bee the third and fourth[54]; and in the spider the last[55].

2. The ganglions also in different species, and often in the same insect in its different states, vary in their _number_. Thus in the grub of the rhinoceros-beetle the whole spinal marrow appears like a _single_ ganglion divided only by transverse furrows[56]; in the water-scorpion there are _two_[57]; in the louse there are _three_[58]; in the rhinoceros-beetle there are _four_[59]; _five_ in the stag-beetle[60]; _seven_ in the hive-bee and some _Lepidoptera_[61]; _eight_ in the grub of the stag-beetle[62]; _nine_ in the great _Hydrophilus_[63]; _ten_ in _Dytiscus_[64]; _eleven_ in the grub of the great _Hydrophilus_[65]; _twelve_ in the grub of _Dytiscus_ and the caterpillars of _Lepidoptera_[66]; _thirteen_ in the larva of _Æshna_[67]; and _twenty-four_ in _Scolopendra morsitans_[68]. You must observe that, generally speaking, the number of ganglions is less in the _imago_ than in the _larva_. With regard to the distribution of these knots to the different primary parts of the body, the following table will exhibit it, as far as I am acquainted with it, at one view. I omit those in which the ganglions are only in _one_ of these parts.

Head. Trunk. Abdomen.

_Acrida viridissima_ 1 3 6[69]

_Hydrophilus piceus_ 1 6 2

_Clubiona atrox_ 0 2 1

_Gryllotalpa vulgaris_ 0 2 7[70]

_Myrmeleon, Larva_ 0 2 8[71]

_Eristalis tenax_ 0 3 2[72]

_Apis mellifica_ 0 3 4

_Ephemera, Larva_ 0 3 7

_Æshna, Larva_ 0 6 7

3. I am next to say a few words upon the _shape_ of the ganglions. Most commonly it approaches to a _spherical_ figure, but in many instances, as I said before, they, as well as the brain, consist of _two_ lobes: they are, however, seldom all precisely of the same shape. In the _Dytisci_, and _Carabi_, the last is marked with a transverse furrow, which seems to indicate the reunion of two[73]; in the stag-beetle, the first ganglion is oval or elliptical, the second hexagonal; the third and fourth shaped like a crescent, and the last like an olive[74]; in the caterpillar of the great goat-moth the first is oblong and constricted in the middle, and the seven last are rhomboidal[75]; in the great _Hydrophilus_ the _second_, and in the silk-worm _all_ the ganglions are quadrangular[76]; in _Hypogymna dispar_ the _third_ is heart-shaped[77]; the great ganglion which forms the spinal marrow of the cheese-maggot is pear-shaped[78]; that of the grub of the rhinoceros-beetle is fusiform[79]; and in the scorpion all the ganglions are lenticular[80]. But the most remarkable in this respect are those of a spider (_Clubiona atrox_): in this insect the brain sits upon a bilobed ganglion of the ordinary form, which is immediately followed without any internode by another bilobed one, terminating on each side in four pear-shaped processes or fingers, which give it a very singular appearance[81].

iii. The _nerves_[82] of insects, as of other animals, are white filaments running from the brain and spinal marrow to every part of the body which they are destined to animate; and their numerous ramifications, when delineated, form no unpleasing picture[83]. In the caterpillar of the goat-moth the accurate Lyonet counted _forty-five_ pairs of them, and _two_ single ones, making in all _ninety-two_ nerves; whereas in the _human_ body anatomists count only _seventy-eight_[84]. From the brain issue several pairs, which go to the _eyes_, _antennæ_, _palpi_, and other parts of the mouth: sometimes those that render to the mandibles issue from the first ganglion, as in the larva of _Dytiscus marginalis_, the stag-beetle, &c.[85]; those both of _mandibles_ and _palpi_ in the great _Hydrophilus_[86]; and in _Blatta_ some which act also upon the _antennæ_[87].

The _optic_ are usually the most conspicuous and remarkable of the nerves. In some insects with large eyes, as many _Neuroptera_, _Hymenoptera_, and _Diptera_, their size is considerable; in the hive-bee they present the appearance of a pair of kidney-shaped lobes, larger than the brain[88]; in the dragon-flies, whose brain consists of two very minute lobes, these nerves dilate into two large plates of a similar shape, which line all the inner surface of the eyes[89]; in the stag-beetle they are pear-shaped, and terminate in a bulb, from which issue an infinity of minute nerves[90]; it is probable that this takes place in all cases, and that a separate nerve renders to every separate lens in a compound eye[91]; the optic nerve in _Dytiscus_ and _Carabus_ is pyramidal, with the base of the pyramid at the eye and the summit at the brain[92]; in _Eristalis tenax_ it is very large, cylindrical, and of a diameter equal to the length of the last-mentioned part, upon the side of which it is supported; it terminates in a very large bulb corresponding to the eye[93]: in _Scolopendra morsitans_ the optic nerves divide into four branches long before they arrive at the eyes, and in this insect the nerves which render to the antennæ are so thick as to appear portions of the brain, which they equal in diameter[94]. Swammerdam discovered in the grub of the rhinoceros-beetle and in the caterpillar of the silk-worm, a pair of nerves which he regarded as analogous to the _recurrent nerves_ in the human subject, and therefore he distinguishes them by the same name[95]: they issue from the lower surface of the brain, or that which rests on the _œsophagus_, and at first go towards the mouth, but afterwards turn back, and uniting form a small ganglion; this produces a single nerve, which passing below the brain follows the œsophagus to the stomach, where it swells into another ganglion, from which issue some small nerves that render to the stomach, and one more considerable which accompanies the intestinal canal, producing at intervals lateral filaments which lose themselves in the tunics of that tube[96]. Lyonet afterwards discovered these nerves in the caterpillar of the goat-moth[97], and Cuvier in other insects[98].

The other nerves which issue from the brain exhibit no remarkable features. Those which originate in the spinal marrow are mostly derived from the ganglions, and are sometimes interwoven with the muscles, as the woof with the warp in a piece of cloth[99]; those from the three or four first commonly rendering to the muscles of the legs, wings, and other parts of the _trunk_, and those from the remainder to the _abdomen_. After their origin they often divide and subdivide, and terminate in numerous ramifications that connect every part of the body with the _sensorium commune_. A _pair_ of nerves is the most usual number that proceeds from each side of a ganglion[100]; but this is by no means constant, since in the louse, the hive-bee, and several other insects, only a _single_ nerve thus proceeds[101]; and in the larva of _Ephemeræ_, while _two_ pairs issue from the _six first_ ganglions, only a _single_ one is emitted by the _five last_[102]. In the spinal marrow of the rhinoceros-beetle, both larva and imago, the nerves consist of simple filaments which diverge like rays in all directions[103]: the same circumstance distinguishes the cheese-maggot, only some of the nerves appear to branch at the end[104]: in the louse, the last ganglion sends forth posteriorly three pairs of nerves which render to the abdomen[105]. Sometimes, though rarely, nerves originate in the _internodes_ of the spinal marrow. Cuvier indeed has asserted that in invertebrate animals _all_ the nerves spring from the ganglions, and never immediately from the spinal marrow; but Swammerdam, in describing those of the silk-worm, mentions and figures four pairs as proceeding from the four anterior internodes, excluding the first[106]; and at the same time he gives it as his opinion, that all the nerves in insects really originate from the marrow itself, and not from the ganglions, which he asserts are of a different substance, and are inclosed in the marrow for the sake of giving it greater firmness[107]. In this opinion, however, he seems singular[108]. Those remarkable nerves described by Lyonet under the name of _spinal bridle_ (_bride épinière_) also take their origin, not from the ganglions, but from a bifurcation of the spinal marrow. Of these, in the caterpillar of the goat-moth there are _ten_, the first issuing from the bifurcation of the internode between the fourth and fifth ganglions, and the remainder from the succeeding ones. After approaching the succeeding ganglion, these nerves form a pair of branches that diverge nearly at right angles from the bridle, and producing several lesser branches, lose themselves in the sides of the animal[109]. Besides the nerves above mentioned, two generally issue from the posterior part of the last ganglion, diverging in opposite and oblique directions: some of these render to the parts of generation; and in the silk-worm, and probably other species, the innermost pair is perforated for the passage of the _vasa deferentia_[110].

After duly considering this general outline of the nervous system of insects, the question will continually occur to you,--is then what you have called the _brain_ the _sensorium commune_ of these animals, in the same manner as it is in those with warm blood? To this query a negative must be returned. In the latter, the brain is the common centre to which, by means of the nerves and spinal marrow, all the sensations of the animal are conveyed, and in which all its perceptions terminate. The nerves and spinal marrow are merely the _roads_ by which the sensations travel; and if their communication with the brain, by any means be cut off at the neck, the whole trunk of the animal becomes paralytic, evidently proving that the organ by which it feels is the brain. This, however, is so far from being the case in insects, that in them, if the head be cut off, the remainder of the body will continue to give proofs of life and sensation longer than the head: both portions will live after the separation, sometimes for a considerable period; but the largest will survive the longest, and will _move_, _walk_, and occasionally even _fly_, at first almost as actively without the head, as when united to it. Lyonet informs us, that he has seen motion in the body of a wasp _three_ days after it had been separated from the head; and that a caterpillar even _walked_ some days after that operation; and when touched, the headless animal made the same movements as when intire[111]. Dr. Shaw has observed--an observation confirmed in Unzer's _Kleine Schriften_,--that if _Geophilus electricus_ be cut in two, the halves will live and appear vigorous even for a _fortnight_ afterwards; and what is more remarkable, that the _tail_ part always survives the _head_ two or three days[112]. The _sensorium commune_ of insects, therefore, does not, as in the warm-blooded animals, reside in the brain alone, but in the spinal marrow also. It was on this account probably that Linné denied the existence of a _brain_ in insects, regarding it merely as the first ganglion of the spine.

Cuvier and other modern physiologists, from the ganglionic structure of this organ, are of opinion that it is not the analogue of the _cerebro-spinal_ system of vertebrate animals, but rather of their _great sympathetic_ nerves. Indeed, considering solely the _external_ structure of the nervous system of insects, a great resemblance strikes us between it and these nerves; for besides its general ganglionic structure, there is also in them an _upper_ ganglion in the neck, seemingly corresponding with what we have named the brain of insects, from which the nervous chord dips to the lower part of the neck, where it forms a _second_ ganglion, which appears to correspond with what we have considered as their second ganglion[113]. We may observe, however, that at least in one respect there is even an _external_ resemblance between the brain of insects and that of vertebrate animals:--it most commonly consists, as has been stated, like them, of two lobes, often very distinct; a circumstance which not unfrequently distinguishes the other ganglions[114], and is not borrowed from the ganglions of the great sympathetics. With respect to the internal structure of the ganglions and spinal marrow of insects, we know little to build any theory upon, except that the internal substance of the former is filled with air-vessels; at least so Lyonet, as has been already observed, found in the goat-moth, while only the tunics of the latter are covered by them. Taking the above resemblance to the brain of vertebrates into consideration, there appears ground for thinking that the nervous system of insects, like some of their articulations[115], is of a _mixed_ kind, combining in it both the cerebro-spinal and the ganglionic systems; and this will appear further if we consider its _functions_.

That learned and acute physiologist Dr. Virey, assuming as an hypothesis, that the structure of the system in question is simply ganglionic, and merely analogous to the sympathetic system of vertebrate animals, has built a theory upon the assumption, which appears evidently contradicted by facts. Because, as he conceives after Cuvier, insects are not gifted with a real brain and spinal marrow, he would make it a necessary consequence that they have no degree of _intellect_, no memory, judgement or free will; but are guided in every respect by instinct and spontaneous impulses,--that they are incapable of instruction, and can superadd no acquired habits to those which are instinctive and inbred[116]. This consequence would certainly necessarily follow, was their nervous system perfectly analogous to the sympathetic of warm-blooded animals. But when we come to take into consideration the _functions_ that in insects this system confessedly discharges, we are led to doubt very strongly the correctness of the assumption. Now in these animals the system in question not only renders to the nutritive and reproductive organs, which is the principal function of the great sympathetic nerves in the vertebrates; but by the common organs maintains a connexion with the external world, and acquires ideas of things without, which in them is a function of the cerebral system: from the same centre also issue those powers which at the bidding of the will put the limbs in action, which also belongs to the cerebral system. That insects have memory, and consequently a real brain, has been before largely proved, as also that they have that degree of intellect and judgement which enables them to profit by the notices furnished by their senses[117]. What can be the use of eyes,--of the senses of hearing, smelling, feeling, &c. if they are not instructed by them what to choose and what to avoid? And if they _are_ thus instructed--they must have sufficient intellect to apprehend it, and a portion of free will to enable them to act according to it. With regard to the assertion that they are incapable of instruction, or of acquiring new habits; few or no experiments have been tried with the express purpose of ascertaining this point: but some well-authenticated facts are related, from which it seems to result that insects may be taught some things, and acquire habits not instinctive. They could scarcely be brought from their wild state, and domesticated, as bees have been so universally, and both ants and wasps occasionally[118], without some departure from the habits of their wild state; and the fact of the corsair-bees, that acquire predatory habits before described[119], shows this more evidently: but one of the most remarkable stories to our purpose upon record, is that of M. Pelisson, who, when he was confined in the Bastile, tamed a spider, and taught it to come for food at the sound of an instrument. A manufacturer also in Paris, fed 800 spiders in an apartment, which became so tame that whenever he entered it, which he usually did bringing a dish filled with flies but not always, they immediately came down to him to receive their food[120].

All these circumstances having their due consideration and weight, it seems, I think, most probable, that as insects have their communication with the external world by means of certain organs in connexion with their nervous system, and appear to have some degree of intellect, memory, and free will, all of which in the higher animals are functions of a cerebral system, and at the same time in other respects manifest those which are peculiar to the sympathetic system,--it is most probable, I say, as was above hinted, that in their system _both_ are _united_.

I must bespeak your attention to a circumstance connected with the subject of this letter, which merits particular consideration: I mean the gradual change that takes place in the nervous system when insects undergo their metamorphoses; so that, except in the _Orthoptera_, _Hemiptera_, and _Neuroptera_ Orders, in which no change is undergone, the number of ganglions of the spinal chord is less in the imago than in the larva. There seems an exception indeed to this rule in the case of the rhinoceros-beetle, in the larva of which there is only _one_ ganglion, while in the imago there are _four_[121]. But as this one ganglion occupies the whole spinal marrow, it is really of greater extent than the four of the imago; so that even in this case there is a concentration of the cerebral pulp. In some cases, as in _Dytiscus marginalis_, and _Hydrophilus piceus_[122], the imago has only _one_ ganglion less than the larva, but more generally it loses _four_ or _five_. Dr. Herold has traced the gradual changes that take place in the spinal marrow of the common cabbage-butterfly (_Pontia Brassicæ_), from the time that it has attained its full size to its assumption of the imago. Of these I shall now give you some account.

In the full-grown caterpillar, besides the brain there are _eleven_ ganglions, the chords of the four first internodes being double, and the rest single: from each ganglion proceed two pairs of nerves, one from each side. In this the lobes of the brain form an angle with each other[123]. In two days the double chords mutually recede, so as to diminish the interval between the ganglions, and the single ones have become curved: thus the length of the spinal marrow is shortened about a _fourth_, and the fourth and fifth ganglions have made an approach to each other[124]. On the eighth day, when the insect has assumed the pupa but remains still in the skin of the caterpillar, the flexure of the internodes is much increased; the first ganglion is now united to the brain, and the fourth and fifth have joined each other, though they are still distinct; the spinal marrow has now lost considerably more than a _third_ of its length[125]. On the fourteenth day, the internodes, except the double ones, have become nearly straight again; the fourth and fifth ganglions have coalesced so as to form one, and the sixth and seventh have each lost their pairs of nerves[126]. Shortly after this, these last ganglions have nearly disappeared, and the chords of the three first internodes have again approached each other[127]. The next change exhibited is the absorption of the first ganglion by the brain, the union of the chords of the first internode, which is now straight, the approximation of the second and third ganglions, and the enlargement of the one formed by the union of the fourth and fifth, at the expense perhaps of the sixth and seventh, which have now intirely disappeared, and in their place is a very long internode. These united ganglions retain the pairs of nerves they had when separate[128]. Just before the assumption of the _imago_, the direction of the lobes of the brain becomes horizontal, the second and third ganglions unite, and the internode between the third and fourth is shortened[129]. Lastly, when the animal is become a butterfly, the second and third ganglions have coalesced, and are joined to that formed by the union of the fourth and fifth; a short isthmus or rather constriction, with an orifice, being their only separation: each of these united ganglions send forth laterally four pairs of nerves[130]. In his figure, Dr. Herold has not represented the orifice for the passage of the gullet, but doubtless one exists, which for an animal that imbibes only fluid food is probably very minute. In _Hypogymna dispar_, we learn from Cuvier, this orifice is of that description, and of a triangular shape[131].

It can admit of no reasonable doubt that one of the principal intentions of these changes is to accommodate the nervous system to the altered functions of the animal in its new stage of existence, in which the antennæ, eyes, and other organs of the senses, as well as the limbs and muscles moving them, and the sexual organs, being very different from those of the larva, and if not wholly new, yet expanded from minute germs to their full size, may well demand corresponding changes in the structure of the nervous system by which they are acted upon.

But are these changes also concerned, as Dr. Virey conjectures, in producing that remarkable alteration which usually takes place between the _instincts_ of the larva and imago? In order to answer this question, it will be requisite first to quote the ingenious illustration with which this able physiologist elucidates his ideas on this point. "The more readily," he observes, "to comprehend the action of instinct, let us compare the insect to one of those hand-organs in which a revolving cylinder presents different tunes noted at its surface, and pressing the keys of the pipes of the organ, gives birth to all the tones of a song: if the tune is to be changed, the cylinder must be pulled out or pushed in one or more notches, to present other notes to the keys. In the same manner let us suppose that nature has impressed or engraved certain determinations or notes of action, fixed in a determinate series in the nervous system and the ganglions of the caterpillar, by which alone she lives, she will act according to a certain sequence of operations; and, so to speak, she will sing the air engraven within her. When she undergoes her metamorphosis into a butterfly, her nervous system being, if I may so express myself, pulled out a notch, like the cylinder, will present the notes of another tune, another series of instinctive operations; and the animal will even find itself as perfectly instructed and as capable of employing its new organs, as it was to use the old ones. The relations will be the same; it will always be the play of the instrument[132]."

This illustration is doubtless at the first glance very striking and plausible: but a closer examination will, I think, show, that, as in so many other instances in metaphysical reasoning, when fanciful analogies are substituted for a rigid adherence to stubborn facts, it is satisfactory only on a superficial view, and will not stand the test of investigation; and as this is a question intimately connected with what I have advanced on the subject of instinct in a former letter, I must be permitted to go somewhat into detail in considering it.

To prove his position, Dr. Virey ought at least to be able to show that, whenever a change takes place in the instincts of insects in their different states of larva and imago, a corresponding change takes place in the external structure of the nervous chord. But what are the facts? In three whole orders, viz. _Orthoptera Hemiptera_, and _Neuroptera_, as mentioned above[133], the structure of the nervous chord is _not_ changed; and yet we know that many tribes of these orders acquire instincts in their imago state altogether different from those which directed them in their state of larvæ. A perfect _Locust_, for instance, acquires the new instincts of using its wings; of undertaking those distant migrations of which so many remarkable instances were laid before you in a former letter[134]; and, if a female, of depositing its eggs in an appropriate situation. But if such striking changes in the instinct of these tribes can be effected without any perceptible alteration in the structure of the nervous chord, it is contrary to the received rules of philosophical induction to refer to this alteration the changes in the instincts of other tribes where it is found. Is it not far more probable that this alteration has in fact no connexion with the changes of instinct, but is solely concerned with those remarkable changes in the organs of sense and motion, which occur in the larva and imago states of the orders in which it is observed? In a common caterpillar, the form of the body, the legs, the eyes, and other organs of the senses, all strikingly differ from those of the imago; whereas, with the exception of the acquisition of new wings, a perfect locust differs little from its larva: so that we may reasonably expect a corresponding change, such as we find it, in the structure of the nervous chord of the lepidopterous insect, not called for in that of the neuropterous species, in which accordingly it does not take place.

This reasoning, in opposition to Dr. Virey's theory, that the changes of instinct depend on the altered structure of the nervous system, becomes greatly strengthened when we advert to the higher classes of animals, which surely in any investigation of the nature of instinct ought to be closely kept in view; for the faculty, though often less perfect in them than in insects, is still of the _same kind_, and may consequently be expected to follow the same general laws. In a young swallow, for example, all its instincts are not developed at once any more than in an insect. The instinct which leads it to migrate does not appear for some months after its birth, and that of building a nest still later. But we have not the slightest ground for believing that these new instincts are preceded by any change in the structure of the great sympathetic nerve, or of any other portion of the nervous system: and the same may be said as to the sexual instincts developed in quadrupeds some years subsequent to their birth. If, then, these remarkable changes in the instinct of the higher classes of animals can take place independently of any visible change in the nerves, what substantial reason can be assigned why they may not also in the class of insects?

On the whole, I think you will agree with me, that there is nothing in Dr. Virey's hypothesis which should lead me to alter the opinion I have already so strongly expressed in a former letter[135], as to the insufficiency of the mechanical theories of instinct hitherto promulgated, adequately to explain _all_ the phenomena; and unless they do this they are evidently of small value. Such theories as I have there adverted to may often seem to be supported by a few insulated facts, but with others, far more numerous, they are utterly at variance; and, to omit many other instances, I am strongly inclined to doubt the possibility of satisfactorily explaining the _variety_ of instincts exercised by a bee[136], or the _extraordinary_ development of new ones in particular circumstances only[137], on any merely mechanical grounds.

And after all, even suppose it could be demonstratively shown that _every_ instinct is as clearly dependent on secondary causes, as I have formerly admitted that _some_ doubtless seem to be, yet what would this teach us as to the essential nature of instinct? We have advanced indeed a step; but still, as I have before observed in referring to the theories of Brown and Tucker, we have only placed the world upon the tortoise, and instinct, as to its _essence_, which is what we want to detect, is as mysterious as ever: just as, though we can clearly prove that the mind is acted upon by the senses, yet this throws no light upon the essential nature of the mind, which we are forced to admit is inscrutable, as if to teach us humility, and prevent our vainly fancying, that though allowed to discover some of the arcana of nature, we shall ever be able to penetrate into her inmost sanctuaries.

That Dr. Virey should regard instinct in insects as purely mechanical was the natural consequence of his denying them any portion of intellect; but his opinion cannot I think be consistently assented to, if it be the fact, as I have just shown[138], that they are not wholly devoid of the intellectual principle. Whatever is merely mechanical, must, under similar circumstances, always act precisely in the same way. An automaton once constructed, whilst its machinery remains in order, will invariably perform the same actions; and Des Cartes, when he had constructed his celebrated female automaton, imagined that he had irrefragably proved his principle, that brutes are mere machines. But if, instead of losing himself in the wilds of metaphysical speculation, he had soberly attended to facts, he would have seen that the instinct of animals can be modified and counteracted by their intellect, and consequently cannot be regarded as simply mechanical. Though the instinctive impulse of an empty stomach powerfully impel a dog to gratify his appetite, yet, if he be well tutored, the fear of correction will make him abstain from the most tempting dainties: and in like manner a bee will quit the nectary of a flower, however amply replenished with sweets, if alarmed by any interruption. The ants on which Buonaparte amused himself with experiments at St. Helena, though they stormed his sugar-basin when defended by a fosse of water, controlled their instinct and desisted when it was surrounded with vinegar[139]: and in the remarkable instance communicated to Dr. Leach by Sir Joseph Banks, the instinct of a crippled spider so completely changed, that from a sedentary web-weaver it became a hunter[140]. There is evidently, therefore, no analogy between actions strictly mechanical and instincts, which, though they may often seem to be excited by mechanical causes, are liable to be restrained or modified by the connexion of the instinctive and intellectual faculties[141]; and while we are ignorant how this connexion takes place, it is obviously impossible to reason logically on the subject.

In thus denying that any existing _mechanical_ theory of instinct is satisfactory, I by no means intend to assert that instinct is purely _intellectual_. I have already given you my opinion[142], that it is not the effect of any immediate agency of the Deity; nor am I prepared to assent to the doctrine of a writer, who has in some respects written ably on the subject in question, who says, that "the Divine Energy does in reality act not _immediately_, but _mediately_, or through the medium of _moral_ and _intellectual influences_ upon the nature or consciousness of the creature, in the production of the various, and in many instances truly wonderful, actions which they perform[143]." The same objection applies to this as to so many other metaphysical theories, that it is not adequately supported by _facts_; and all theories not so supported are injurious to science in proportion as their plausibility is greater, by leading the student to relax in that observation of nature and attentive study of the instincts of animals, on which alone sound hypothesis on this subject can be ultimately founded.

I shall conclude these remarks on the nature of instinct with a few observations as to the circumstances in which insects may be supposed to be guided by this faculty, and those in which _intellect_ seems to direct them. The bee, when it takes its flight to a field where flowers abound, is governed by intellect in the use of its senses; for these are given to it as _guides_: and when it arrives there, they direct it to the flowers, and enable it to ascertain which contains the treasures it is in search of; but having made this discovery, its instinct teaches it to imbibe the nectar and load its hind legs with pollen.--Again: its senses, aided by memory, enable it to retrace its way to the hive, where instinct once more impels it in its various operations. So that when we ascribe a certain degree of intellect to these animals, we do not place them upon a par with man; since all the most wonderful parts of their economy, and those manipulations that exceed all our powers, we admit not to be the contrivance of the animals themselves, but the necessary results of faculties implanted in their constitution at the first creation by their MAKER. I may further repeat, that the mere fact of being endowed with the external organs of sense, proves a certain degree of intellect in insects. For if in all their actions they were directed merely by their instinct, they might do as well without sight, hearing, smell, touch, &c. but having these senses and their organs, it seems to me a necessary consequence, that they must have a sufficient degree of intellect, memory, and judgement, to enable them advantageously to employ them.

There is this difference between intellect in man, and the rest of the animal creation. Their intellect teaches them to follow the lead of their senses, and make such use of the external world as their appetites or instincts incline them to,--and _this is their wisdom_; while the intellect of man, being associated with an immortal principle, and being in connexion with a world above that which his senses reveal to him, can, by aid derived from heaven, control those senses, and bring under his instinctive appetites, so as to render them obedient to the το ἡγεμονικον, or governing power of his nature: AND THIS IS HIS WISDOM.

I am, &c.

FOOTNOTES:

[1] Το Ἡγεμονικον.

[2] See Hooper's _Medical Dictionary_, under _Nervous Fluid_, and Mr. Sandwith's useful _Introduction to Anatomy and Physiology_, 83.

[3] _N. Dict. d'Hist. Nat._ xvi. 305--.

[4] Cuv. _Anat. Comp._ ii. 362. Compare MacLeay _Hor. Entomolog._ 215--.

[5] _N. Dict. d'Hist. Nat._ _ubi. supr._

[6] Cuv. _Anat. Comp._ ii. 360. MacLeay _Hor. Ent._ 201.

[7] _N. Dict. d'Hist. Nat._ xvi. 306. MacLeay _Hor. Ent._ 200--.

[8] _Ibid._ 307. The great sympathetic nerves in _fishes_ are said to have no ganglions. Cuv. _ubi. supr._ 297.

[9] They are called _trisplanchnic_ because they render to the _three_ cavities of the _viscera_:--viz. the thoracic, the abdominal and the pelvic. _N. Dict. d'Hist. Nat._ xxii. 524. 527.

[10] In _Hemiplegia_, &c.

[11] _N. Dict. d'Hist. Nat._ xvi. 307.

[12] Thus in the _Molluscæ_ there must be a great difference in this respect, since in some of these the brain or cerebral ganglion has been cut off with the head, and another reproduced. _Ibid._ xvi. 306. Comp. v. 391.

[13] VOL. III. p. 29.

[14] Comp. PLATE XXX. FIG. 1. and 6. and Carus. _Introd. to Comp. Anat._ i. 64.

[15] Lyonet _Anatom._ 100.

[16] _Ibid._ 101.

[17] Lyonet _Anatom._ 100. In man and the vertebrate animals, the medullary pulp is every where homogeneous; under the microscope it appears to consist of a number of minute conglomerated globules. M. Vauquelin has analysed it, and found it to contain, of water 80 parts; of albumen in a state of demicoagulation 7·0; of phosphorus 1·50; of osmazone 1·12; of a white and transparent oily matter 4·53; of a similar red do. 0·75; of a little sulphur and some salts 5·15. _N. Dict. d'Hist. Nat._ xxii. 531--.

[18] _Anat._ 99.

[19] Malpigh. _de Bombyc._ 20. Swamm. _Bibl. Nat._ i. 224. a.

[20] _Anat. Comp._ ii. 348.

[21] Lyonet _Anat._ 100. _t._ iv. _f._ 6. Sandwith _Introd._ 59--.

[22] PLATE XXI. FIG. 1. 7. 8. _a._

[23] _N. Dict. d'Hist. Nat._ xxii. 527.

[24] _Ibid._ v. 591.

[25] Cuv. _Anat. Comp._ ii. 318. Swamm. _Bibl. Nat._ _t._ xxix. _f._ 7. Herold _Schmetterl._ _t._ ii. _f._ 1-10. _a._

[26] Cuv. _Ibid._ 322. 337.

[27] Cuv. _Anat. Comp._ 324.

[28] _Arachnid._ _t._ i. _f._ 13. _m.m._

[29] Cuv. _ubi supr._ 343. 346. Treviranus _Arachnid._ _t._ v. _f._ 45. _a._ PLATE XXI. FIG. 8. _a._

[30] Ibid. FIG. 1. _b.b._

[31] Cuv. _ubi supr._ 337.

[32] PLATE XXI. FIG. 8. Swamm. _Bibl. Nat._ i. 36. b.

[33] _Arachnid._ _t._ v. _f._ 45.

[34] Swamm. _ubi supr._ _t._ xliii. _f._ 7.

[35] _Ibid._ 112. a.

[36] Cuv. _Anat. Comp._ ii. 337. 343--.

[37] _Ibid._ 336.

[38] Herold _Schmetterl._ _t._ ii. _f._ 1.

[39] Lyonet _Anat._ 98.

[40] Cuv. _ubi supr._ 342. Gaede N. _Act. Acad. Cæs._ XL. ii. 323. Cuv. _Ibid._ 351.

[41] Cuv. _ubi. supr._ 348.

[42] Treviranus _Arachnid._ _t._ v. _f._ 45.

[43] PLATE XXI. FIG. 7. 8. Swamm. _Bibl. Nat._ _t._ xliii. _f._ 7.

[44] PLATE XXI. FIG. 7. 8. _c._

[45] Lyonet _Anat._ 100.

[46] _N. Dict. d'Hist. Nat._ xxii. 522--.

[47] Lyonet _ubi supr._ _t._ ix. _f._ 1-4.

[48] Cuv. _Anat. Comp._ ii. 339. 343.

[49] PLATE XXI. FIG. 7.

[50] Swamm. _ubi supr._ _t._ xl. _f._ 5. Cuvier (ii. 332.) accuses Swammerdam of representing the spinal marrow in this grub as producing nerves only on _one_ side; whereas he expressly states (ii. 50. b.) that a considerable number spring on each side from the eleven ganglions, but that to avoid confusion he had omitted some.

[51] Cuv. _ubi supr._ 325.

[52] Swamm. _Bibl. Nat._ _t._ xv. _f._ 6.

[53] Treviran. _Arachnid._ _t._ l. _f._ 13. 1-4.

[54] Swamm. _ubi supr._ _t._ xxii. _f._ 7.

[55] Treviran. _ubi supr._ _t._ v. _f._ 45.

[56] PLATE XXI. FIG. 7.

[57] Cuv. _Anat. Comp._ ii. 346.

[58] PLATE XXI. FIG. 8.

[59] Cuv. _ubi supr._ 337.

[60] _Ibid._ 335--.

[61] Cuv. _ubi supr._ 348.

[62] _Ibid._ 320--.

[63] _Ibid._ 340--.

[64] _Ibid._ 338--.

[65] Gaede _ubi supr._

[66] Cuv. _ubi supr._ 323--. 327--. Mr. Bauer (_Phil. Trans._ 1824. _t._ ii. _f._ 1.) has figured only _seven_, excluding the brain, in that of the silk-worm, and Malpighi (_De Bombyc._ _t._ vi. _f._ 2.) _ten_,--Swammerdam (_Bibl._ Nat. _t._ xxviii. _f._ 3.) however has _twelve_.

[67] _Ibid._ 326.

[68] _Ibid._ 352.

[69] _Ibid._ 343--.

[70] _Ibid._ 345.

[71] _Ibid._ 325--.

[72] _Ibid._ 351.

[73] Cuv. _ubi supr._ 339.

[74] _Ibid._ 335--.

[75] Lyonet _Anat._ 190.

[76] Cuv. _ubi supr._ ii. 340. Malpigh. _de Bombyc._ _t._ vi. _f._ 2.

[77] Cuv. _Ibid._ 348.

[78] Swamm. _Bibl. Nat._ _t._ xlviii. _f._ 7.

[79] Cuv. _Ibid._ 319.

[80] _N. Dict. d'Hist. Nat._ xxx. 420.

[81] Treviran, _Arachnid._ _t._ v. _f._ 45. _m._

[82] PLATE XXI. FIG. 1. 7. 8. _d._

[83] Lyonet _ubi supr._ _t._ x. _f._ 5. 6.

[84] _Ibid._ 192.

[85] Cuv. _ubi supr._ 323. 335.

[86] _Ibid._ ii. 339.

[87] _Ibid._ 342.

[88] Swamm. _Bibl. Nat._ _t._ xxii. _f._ 6. _m.m._

[89] Cuv. _ubi supr._ 350.

[90] _Ibid._ 335.

[91] VOL. III. p. 495--. Lyonet. _Anat._ 581.

[92] Cuv. _ubi supr._ 337.

[93] Cuv. _ubi supr._ 351.

[94] _Ibid._ 352.

[95] Cuvier (_Ibid._ 319.) seems not to have been aware that Swammerdam was the first discoverer of these nerves, since he attributes their name to Lyonet.

[96] _Bibl. Nat._ i. 138. b. _t._ xxviii. _f._ 2. _a_, _b_, _c_. _f._ 3. _g_.

[97] _Ubi supr._ 578.

[98] _Ubi supr._ 320. 339, &c.

[99] Cuv. _ubi supr._ 349.

[100] Lyonet _Anat._ _t._ ix. x.

[101] PLATE XXI. FIG. 8. Swamm. _Bibl. Nat._ _t._ xxii. _f._ 6.

[102] _Ibid._ _t._ xv. _f._ 6.

[103] PLATE XXI. FIG. 7.

[104] Swamm. _ubi supr._ _t._ xliii. _f._ 7. _h_, _h_.

[105] PLATE XXI. FIG. 8.

[106] In Mr. Bauer's figure (_Philos. Trans._ 1824. _t._ ii. _f._ 1.) no less than _eighteen_ pairs of nerves are represented as issuing from the internodes; but it should seem as if in the specimen from which his figure was taken, several of the ganglions, perhaps from some injury received in the dissection, had become obliterated, while their nerves remained: yet still, even making allowance for these, many pairs will appear to take their origin from the spinal chord.

[107] Comp. Cuv. _Anat. Comp._ ii. 102-123.; with Swamm. Expl. of PLATES XXXII. _t._ xxviii. _f._ 3. _k._

[108] Malpighi seems, however, to agree with him. _De Bombyc._ _t._ vi. _f._ 1.

[109] Lyonet _ubi supr._ 201. _t._ ix. _f._ 1, 2. n. 1, 2. &c.

[110] Swamm. _ubi supr._ 1. 139. a. _t._ xxviii. _f._ 3. _s_, _s_.

[111] In Lesser _Insecto-theol._ ii. 84. note *.

[112] _Linn. Trans._ ii. 8. Aristotle had observed this vitality of insects, and that that of the myriapods is greatest. _Hist. Animal._ _l._ iv. _c._ 7. _De Respiratione_, _c._ 3. _Reptiles_ have also this faculty. _N. Dict. d'Hist. Nat._ xxix. 161.

[113] Cuv. _Anat. Comp._ ii. 283--. These are named "the upper and lower cervical ganglions."

[114] Lyonet _Anat._ _t._ ix. x. PLATE XXI. FIG. 1. _a._ _b._

[115] VOL. III. p. 663. 670.

[116] _N. Dict. d'Hist. Nat._ ii. 47--. v. 592. xvi. 308--.

[117] VOL. II. p. 519--. 507--.

[118] Huber _Fourmis_, 260--. Reaum. vi. 172--.

[119] VOL. II. p. 204.

[120] _N. Dict. d'Hist. Nat._ ii. 279--.

[121] Cuv. _Anat. Comp._ ii. 319. 337.

[122] Ibid. ii. 322, 323--; 338. 339--.

[123] PLATE XXX. FIG. 1.

[124] Ibid. FIG. 2.

[125] PLATE XXX. FIG. 3.

[126] Herold _Schmett._ _t._ ii. _f._ 6.

[127] Ibid. _t._ ii. _f._ 7.

[128] PLATE XXX. FIG. 4.

[129] Ibid. FIG. 5.

[130] Ibid. FIG. 6.

[131] _Anat. Comp._ ii. 348.

[132] _N. Dict. d'Hist. Nat._ xvi. 313. Comp. i. 420.

[133] See above, p. 23.

[134] VOL. I. p. 217--.

[135] VOL. II. p. 461.

[136] VOL. II. p. 493.

[137] Ibid. p. 503.

[138] See above, p. 21.

[139] Antommarchi's _Last Days of Napoleon_.

[140] _Linn. Trans._ xi. 393.

[141] VOL. II. p. 509.

[142] VOL. II. p. 463, 5.

[143] _Zoological Journal_, n^o. i. 5.