title="[Pg 21]"/> 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