An Introduction to Entomology: Vol. 4 or Elements of the Natural History of the Insects

_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].