Another sense of which the organ seems uncertain is that of smelling, and various and conflicting opinions have been circulated concerning it. Christian thought that insects smell distant objects with their antennæ, and near ones with their palpi[1056]. Comparetti has a most singular opinion. He supposes in different tribes of insects that different parts are organs of smell: in the Lamellicorns he conjectures the seat of this sense to reside in the knob of the antennæ; in the Lepidoptera in the antlia; and in some Diptera and Orthoptera in certain frontal cells[1057]. At first sight, one of the most reasonable opinions seems to be that of Baster, adopted by Lehmann, and which has received the sanction of Cuvier[1058],—that the spiracles are organs of smell as well as of respiration. Lehmann has adduced several arguments in support of this opinion. Because we both respire and smell with our nostrils, he concludes that neither the antennæ nor any other part of the head of insects can serve for smell, since they are not the seat also of respiration; and that there can be no smell where the air is not inspired[1059]. Again, because nerves from the ganglions of the spinal chord terminate in bronchiæ near the spiracles, they must be for receiving scents from those openings. Though it was necessary, in the higher animals, that the organ of scent should be near the mouth, because they are larger than their food; yet the reverse of this being the case with insects, which often even reside in what they eat, it is to them of no importance where their sense of smelling resides[1060]. By exposing antennæ, by means of an orifice in a glass vessel, to the action of stimulant odours, they appeared quite insensible to it: but he does not name the result of any experiment in which he exposed the mouth to this action; nor at all distinctly how the insect was affected when the spiracles were exposed to it[1061].
But though some of these arguments appear weighty, there are others, I think, that will more than counterbalance them, making it probable that the seat of this sense is in the head, either in its ordinary station at the extremity of what I call the nose, between it and the upper-lip, or under those parts. That the nose corresponds with the so-named part in Mammalia, both from its situation and often from its form, must be evident to every one who looks at an insect[1062]; and when we further consider the connexion that obtains between the senses of smell and taste, how necessary it is that the seat of the one should be near that of the other, and that it really is so in all animals in which we certainly know its organ[1063]; we shall feel convinced that the argument from analogy is wholly in favour of the nose, and may thence consider it as probable that the sense in question does reside there. Lehmann seems to be of opinion, because an insect is usually smaller than what it feeds upon, that it makes no difference whether it smells with its head or with its tail: but one would think that a flying insect would be more readily directed to its object by smelling with the anterior part of the body than with the posterior; and that a feeding one would also find it more convenient in selecting its food. As to the argument,—that smell must be the necessary concomitant of the respiratory openings, and that there can be no smell where the air is not inspired,—this seems asserting more than our knowledge of these animals will warrant: for the organs of the other senses, though the senses themselves seem analogous, are so different in their structure, and often in the mode in which they receive the impressions from external objects, that analogy would lead us to expect a difference of this kind also in the sense of smell. Besides, smell does not invariably accompany respiratory organs even in the higher animals,—for we breathe with our mouths, but do not smell with them. Cuvier says that the internal membrane of the tracheæ being soft and moist, appears calculated to receive scents[1064]. But here his memory failed him; for it is the external membrane alone that answers this description; the internal consisting of a spiral elastic thread, and seeming not at all fitted to receive impressions, but merely to convey the air[1065]. That nerves penetrate to the bronchiæ, does not necessarily imply that they are connected with the sense in question, since this may be to act upon the muscles which are every where distributed.
I shall now state some facts that seem to prove that scents are received by some organ in the vicinity of the mouth, and probably connected with the nose. M. P. Huber, desirous of ascertaining the seat of smell in bees, tried the following experiments with that view. These animals, of all ill scents, abominate most that of the oil of turpentine. He presented successively to all the points of a bee's body, a hair-pencil saturated with it: but whether he presented it to the abdomen, the trunk, or the head, the animal equally disregarded it. Next, using a very fine hair-pencil, while the bee had extended its proboscis, he presented the pencil to it, to the eyes and antennæ, without producing any effect; but when he pointed it near the cavity of the mouth, above the insertion of the proboscis, the creature started back in an instant, quitted its food, clapped its wings, and walked about in great agitation, and would have taken flight if the pencil had not been removed. On this, it began to eat again; but on the experiment being repeated, showed similar signs of discomposure: oil of marjoram produced the same effect, but more promptly and certainly. Bees not engaged in feeding appeared more sensible of the impression of this odour, and at a greater distance; but those engaged in absorbing honey might be touched in every other part without being disturbed. He seized several of them, forced them to unfold their proboscis, and then stopped their mouth with paste. When this was become sufficiently dry to prevent their getting rid of it, he restored to them their liberty: they appeared not incommoded by being thus gagged, but moved and respired as readily as their companions. He then tempted them with honey, and presented to them near the mouth, oil of turpentine, and other odours that they usually have an aversion to; but all produced no sensible effect upon them, and they even walked upon the pencils saturated with them[1066].
These experiments incontestibly prove that the organ of scent in bees—and there is no reason to think that other insects do not follow the same law—is in or near the mouth, and above the proboscis. It remains, therefore, that we endeavour to discover its precise situation: and as insects cannot tell us, nor can we perceive by their actions, in what precise part the sense in question resides, the only modes to which we can have recourse to form any probable conjecture, are analogy and dissection. At first, the opinion noticed above, that the palpi are its organs, seems not altogether unreasonable; but as the argument from analogy, except as to their situation near the mouth, is not in favour of them, and there seems no call, were smell their function, for the numerous variations observable in their structure, I think we must consider them, as I have endeavoured to prove, rather as instruments of touch. Let us now inquire, whether there be not discoverable upon dissection, in the interior of the head of any insects, some organ that may be deemed, from its situation, under what we have called the nose and nostrils, the seat of the sense we are treating of. The common burying-beetle (Necrophorus Vespillo) is an insect remarkable for its acuteness of smell, which enables it to scent out and bury, as was formerly related to you[1067], the carcases of small animals. Take one of these insects, and kill it as formerly directed,—examine first its nose: in the middle of the anterior part you will see a subtrapezoidal space, as it were cut out and filled with a paler piece of a softer and more membranous texture. Next divide the head horizontally; and under the nose, and partly under this space, which I call the rhinarium or nostril-piece[1068], you will find a pair of circular pulpy cushions, covered by a membrane transversely striated with beautifully fine striæ. These are what I take to be the organs of smell, and they still remain distinctly visible in a specimen I have had by me more than fifteen years. A similar organ may be discovered in the common water-beetle (Dytiscus marginalis), but with this peculiarity, that it is furnished with a pair of nipples. I have before described an analogous part covered with papillæ, in Æshna viatica, and you will find it in other insects[1069]. Perhaps at first this part may seem merely a continuation of the palate; but if you consider the peculiarities in its structure just noticed, it is evidently a sensiferous organ; and as the sense of smell appears to reside in the head, this is its most probable seat. But by what channel scents act upon it,—whether they are transmitted through the pores of the part representing the nostrils, or received by the mouth,—I will not venture to assert positively: but from the circumstance of their being membranous in some insects remarkable for acute scent, as in Necrophorus, Staphylinus, &c., there seems some ground for the former opinion, which receives further confirmation from an observation of an eminent Comparative Anatomist, M. Carus, with respect to Acrida verrucivora, in which under the nose and rhinarium, as appears from his description, he found some tracheæ, and two lobes of the cerebral ganglion, which caused him to regard this as the seat of the sense of smell[1070]. He also tells us that Rosenthal, in the blue-bottle-fly (Musca vomitoria) places the sense of smell partly in a delicately folded membrane observable in its head[1071]. As the sense of smell in these little beings is extremely acute, as well as their hearing, the perception of odours may reach their sensory through the above pores; and even those in the hard rhinarium of an Anoplognathus may receive and transmit them; and besides the upper-lip and nose are often united by membrane, perhaps representing the rhinarium, as in Goerius, &c.[1072] which may facilitate such transmission.
That insects taste, no one hesitates to believe, though some have supposed the palpi to be the organ of that sense; but as they have a tongue, as we have shown, we may with Cuvier conclude, that one of its primary functions is to taste their food[1073]. I shall not therefore launch out further upon this head.
I have now placed before you a picture, or rather sketch, of the insect world. And whether we regard their general history and economy, their singular metamorphoses, the infinite varieties and multiplicity of their structure both external and internal, and their diversified organs both of sense and motion—I think you will be disposed to own, that in no part of his works is the hand of an Almighty and All-wise Creator more visibly displayed, than in these minutiæ of creation; that they are equally worthy of the attention and study of the Christian Philosopher with any of the higher departments of the animal kingdom; and that all praise is due to Him, for placing before our eyes, for our entertainment and instruction, such a beautiful moving picture of little symbols and agents, perpetually reflecting his glory and working his will.
I am, &c.