| 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.