LETTER XLV.

_SENSES OF INSECTS._

At first one would think that the _senses_ of insects might be described in very few words, and scarcely afford matter for a separate letter; but when we find that physiologists are scarcely yet agreed upon this subject, and that the use of some of their organs, which appear to be organs of sensation, has not yet been satisfactorily ascertained--we shall not wonder that it requires more discussion than at the first blush we were aware of. In treating on this head I shall first say something on the senses in _general_, and then confine myself to those of insects.

Touch, taste, smell, hearing, and sight, I need not tell you, is the usual enumeration of the senses: but as the term includes every means of communication with the external world, the list perhaps might be increased; and there is ground for thinking that the number _seven_, so signalized as a _sacred_ number[1025], may also here have place. Dr. Virey, an eminent physiologist, whose sentiments on various subjects I have before noticed with approbation[1026], appears to be of opinion that there are really _seven_ senses; which he divides into those that are altogether _physical_, and those that are more connected with the _intellect_. The first of these divisions contains _four_ senses,--touch, love, taste, and smell;--the second _three_,--hearing, sight, and the internal sense of thought, or the brain[1027]. That he is right in adding _love_ to the list seems to me evident, because it is as distinct from _touch_, as _smelling_ and _taste_ are. With regard to the other, though it may be expected that there should be a transitive sense connecting the intellect (if I may so speak) with the external organ of sense, and as a medium by which the former can receive the notices of the external world furnished by the latter; yet it seems improper to make the _entire brain_ itself a _sense_. We know that the agent between the common sensory and the sense is the consciousness or _perception_ of the impression. "Seeing we may see and not _perceive_, and hearing we may hear and not _understand_." The picture may be painted upon the retina of the eye, the sound may strike upon the tympanum of the ear; but neither the one nor the other be received by the intellect, unless the internal power or faculty of perception be in action and mediate between them. This is what I mean by the _internal sense_, which, to use a term of Mr. W. S. MacLeay's[1028], is _osculant_ between intellect and sense, or forms the transit from one group of powers to the other.

Of the ordinary senses, _sight_ holds the first rank: it can dart to the region of the stars, and convey by the perceiving sense, to the sensory, ideas of innumerable objects. Next in rank is _hearing_, which can receive sounds from a great distance; but the ideas it remits are confined only to one object, the variations of tones. In the other organs the sensitive power is much more confined. There is another difference between the intellectual and physical senses:--the former are the only ones that receive and convey sensations of the beautiful and sublime; of harmony and discord,--the latter, though they minister more to our sensual enjoyments, add little to our intellectual; and therefore too devoted an indulgence in them debases our nature, and levels us with the brutes, which use their eyes and ears only for information, not for pleasure[1029].

In man the ordinary five senses are usually in their greatest perfection, although in some animals particular senses have a greater range. The Vertebrates in general are also gifted with the same number, though there are some exceptions. But in the _Invertebrates_ they are seldom to be met with all together in the same object. The _Cephalopods_ have no _smell_. Several _Gasteropods_ can neither _hear_ nor _see_. The animals of bivalve shells have neither eyes, nor ears, nor smell; and the zoophytes and the races below them have, it is affirmed, only the single sense of _touch_, which in them is so extremely delicate as to be acted upon even by _light_[1030].

Not so our insects. These, there is good reason to believe, possess all the ordinary senses. That they can _see_, _touch_, _taste_, and _smell_, no one denies. Linné and Bonnet, however, thought them deprived of _hearing_[1031]; but numerous observations prove the contrary. That they hear in their _larva_ state, is evident from facts stated by the latter physiologist. He found that the sound of his voice evidently affected some caterpillars; which he attributes, but surely without reason, to the delicacy of their sense of touch: at another time, when some caterpillars of a different species were moving swiftly, he rang a small bell; upon which they instantly stopped and moved the anterior part of their body very briskly[1032]. That they possess this faculty in their _imago_ state is confirmed still more strongly by facts. I once was observing the motions of an _Apion_ under a pocket microscope: on seeing me it receded. Upon my making a slight but distinct noise, its antennæ started: I repeated the noise several times, and invariably with the same effect. A _Harpalus_, which I was holding in my hand, answered the sound in the same manner repeatedly. Flies, I have observed, at brisk and distinct sounds move all their legs; and spiders will quit their prey and retire to their hiding places. Insects that live in society give notice of intended movements, or assemble their citizens for emigration by a certain _hum_[1033]. But the most satisfactory proof of the hearing of these animals is to be had from those _Orthoptera_ and _Hemiptera_ whose males are vocal. Brunelli kept and fed several males of _Acrida viridissima_ (a grasshopper with us not uncommon) in a closet, which were very merry, and continued singing all the day; but a rap at the door would stop them instantly. By practice he learned to imitate their chirping: when he did this at the door, at first a few would answer him in a low note, and then the whole party would take up the tune and sing with all their might. He once shut up a male in his garden, and gave the female her liberty; but as soon as she heard the male chirp, she flew to him immediately[1034].

But although physiologists are for the most part agreed that insects have the ordinary five senses of vertebrate animals, yet a great variety of opinions has obtained as to their external organs; so that it has been matter of doubt, for instance, whether the _antennæ_ are for smell, touch, or hearing; and the _palpi_ for smell, taste, or touch. Nor has the question, as it appears to me, been satisfactorily decided: for though it is now the most general opinion that the primary use of antennæ is to _explore_ as _tactors_, yet by the most strenuous advocates of this opinion they are owned not to be _universally_ so employed; so that granting this to be _one_ of their principal functions, yet it seems to follow that there may be _another_ common to them all, which of course would be their _primary_ function. We are warned, however, not to lay any stress upon the argument to be drawn from analogy; and told that we might as well dispute about the identity of the nose of a man, the proboscis of the elephant, the horn of the rhinoceros, the crest of the cock, or the beak of the toucan[1035]. But this is merely casting dust in our eyes: for though three of these are _nasal_ organs, bearing _nostrils_; the two others have no relation to the question, the horn of the rhinoceros and the crest of the cock being merely _appendages_, and have no more analogy to the nose and nostrils, which co-exist with them, than they have to the eyes or ears. I have on a former occasion observed, that a gradual change sometimes takes place in the functions of particular organs; but still, generally speaking, this observation regards _secondary_ functions--the _primary_ usually remaining untouched. We may say, for instance, with regard to the primary use of the _legs_ of animals, that it is locomotion; while the secondary is either walking, running, jumping, flying, or swimming, according to the circumstances and nature of the animal. Thus the _fore-legs_ of the _Mammalia_, in _birds_ become _wings_, and both pair in _fish_ are changed to _fins_. Observe, I do not say _always_ and invariably, but in most cases,--that analogous parts have analogous uses, at least as far as _primary_ uses are concerned. When, therefore, we cannot have demonstrative evidence concerning the function of an organ discoverable in any animal, we may often derive satisfactory probable arguments from the analogies observable in their structure compared with that of other animals, concerning the nature of whose organs we have no doubt. In fact, the chief evidence we have with regard to the office of the organs of sense in the animals immediately below ourselves, is that of analogy;--because _we_ see with our eyes, hear with our ears, &c., we conclude, with reason, that _they_ do the same.

In inquiring therefore into what may be the most general use of the antennæ of insects, I shall endeavour to discover whether there is any part in the higher animals to which they may be deemed to exhibit any analogy. And here I must refer you to what I have said on a former occasion upon the present subject; where I made it evident, I hope, that the great bulk of the parts and organs of insects, in this particular differing from the majority of Invertebrates, are, some in one respect, some in another, and some in many, really analogous to those of the higher animals[1036]; and that a great many of them, though varying in their structure, have the same functions. Thus the analogues of the _eyes_ of Vertebrates are for _seeing_; of the _jaws_ for _masticating_; of the _lips_ for _closing_ the _mouth_; of the _legs_ for _walking_, &c. We have seen also very recently, that a similar analogy, more or less strongly marked, holds also in their internal organs[1037]; so that it may be safely affirmed, that if all the invertebrate insects, though gifted with numerous peculiarities, present the most striking picture of those animals that have an internal skeleton, and more particularly of the _Mammalia_,--we may assume it as a probability, the above circumstances being allowed their due weight, that where facts do not prove the contrary, the function of analogous organs is more or less synonymous, though perhaps the structure and _modus operandi_ may be different.

In the letter lately referred to, I observed that the antennæ of insects are analogous to _ears_ in Vertebrates[1038]. Their _number_ corresponds; they also stand out from the head; and what has weighed most with me, unless they are allowed as such, no other organ can have any pretension to be considered as representing the ear. If we reflect, that in every other part and organ, the head of insects has an analogy to that of _Mammalia_, we must regard it as improbable that these prominent organs should not also have their representative. Admitting then that they are the analogues of ears, it will follow, not as demonstratively certain, but as probable, that their _primary_ function may be something related to hearing. I do not say direct _hearing_, or that the vibrations of sound are communicated to the sensorium by a complex structure analogous to that of the internal ear in _Mammalia_--but something _related_ to hearing. I conceive that antennæ, by a peculiar structure, may collect notices from the atmosphere, receive pulses or vibrations, and communicate them to the sensorium, which, though not precisely to be called hearing, may answer the same purpose. From the _compound_ eyes that most of them have, the sense of seeing in insects must be very different from what it is in vertebrate animals; and yet we do not hesitate to call it _sight_: but since antennæ, as we shall see, apparently convey a _mixed_ sensation, I shall have no objection, admitting it as their primary function, to call it after Lehmann _Aëroscepsy_[1039]. I lately related some instances of _sound_ producing an effect on the _antennæ_ of insects: I will now mention another that I observed, still more remarkable. A little moth was reposing upon my window; I made a quiet, not loud, but distinct noise: the antenna nearest to me immediately moved towards me. I repeated the noise at least a dozen times, and it was followed every time by the same motion of that organ; till at length the insect being alarmed became more agitated and violent in its motions. In this instance it could not be _touch_; since the antenna was not applied to a surface, but directed towards the quarter from which the sound came, as if to listen. Bonsdorf made similar observations, to which Lehmann seems not disposed to allow their proper weight[1040]. It has been used as an argument to prove that antennæ are primarily _tactors_, or instruments of _touch_, that _Fœnus Jaculator_, before it inserts its ovipositor, plunges its _antennæ_ into the hole forming the nidus of the bee, to the grub of which it commits its egg[1041]. But had those who used this argument _measured_ the antennæ and the ovipositor of this ichneumon, they would have discovered that the latter is thrice the length of the former: and as these insects generally insert it so that even part of the abdomen enters the hole, it is clear that the antenna cannot _touch_ the larva; its object therefore cannot be to explore by that sense. Others suppose that by these organs it _scents_ out the destined nidus for its eggs; but Lehmann has satisfactorily proved that they are not _olfactory_ organs. We can therefore only suppose, either that by means of its antennæ it _hears_ a slight noise produced by the latent grub, perhaps by the action of its mandibles; or else that by its motions it generates a _motion_ in the atmosphere of its habitation, which striking upon the antennæ of the _Fœnus_, are by them communicated to its sensory. A similar disproportion is observable between the antennæ and ovipositor of _Pimpla Manifestator_, before signalized[1042]. Bees, when collecting honey and pollen, first insert the organs in question into the flowers which they visit; but, as I have more than once observed, they merely insert the _tip_ of them. If anthers are bursting, or the nectar is exuding, these processes probably are attended by a slight noise, or motion of the air within the blossom, which, as in the last case, affects, without immediate contact, the exploring organs.

If the _structure_ of antennæ be taken into consideration, it will furnish us with additional reasons in favour of the above hypothesis, with regard to their primary function. We shall find that these organs, in most of those insects which take their food by suction, are usually less gifted with powers of motion, than they are in the mandibulate tribes; so that in the majority of the Homopterous _Hemiptera_ and _Diptera_, as is generally acknowledged, they cannot be used for _touch_. Under this view, they may be divided into _active_ antennæ and _passive_ antennæ: of the former, the _most_ active and versatile are those of the _Hymenoptera_. By means of them, as was before observed[1043], their gregarious tribes hold converse, and make _inquiry_--frequently without _contact_--in the pursuit and discharge, if I may so speak, of the various duties devolved upon them by PROVIDENCE. Amongst active antennæ, some are much more _complex_ in their structure than others--a circumstance which is often characteristic of the _male_ insect[1044]: but if we examine such antennæ, we shall find that their most _sensitive_ parts cannot come in _contact_ with the earth or other bodies for exploring their way; but having thus a greater surface exposed to the action of the atmosphere, they have more points to receive vibrations, or any pulses or other notices communicated to it. It is thus, probably, that in their flights, when they approach within a certain distance, they discover the station of the other sex. Even the plumose antennæ of male gnats may in some respects thus be acted upon. In the Lamellicorn beetles, the knob of these organs in both sexes consists of laminæ, the external ones on their outside, of a corneous substance; while their internal surface, and the inner laminæ--which are included between them, as an oyster between the valves of its shell--are covered with nervous papillæ. If you examine the proceedings of one of these little animals, you will find before it moves from a state of repose that its antennæ emerge, and the laminæ diverge from each other; but that it does not apply them to _surfaces_ to explore its way, but merely keeps them _open_ to receive notices from the atmosphere. Even _simple_ antennæ are often employed in this way, as well as for touch. I once noticed a species of _Leptocerus_, a trichopterous genus, in which these organs are very long, that was perched upon a blade of grass; its antennæ vibrated, and it kept moving them from side to side in the air, as if thus by aëroscepsy it was inquiring what was passing around it. Dr. Wollaston has an observation bearing so precisely upon this question, and in general so extremely similar to what is here advanced, that I must copy it for your consideration. "Since there is nothing in the constitution of the atmosphere," says he, "to prevent vibrations much more frequent than any of which we are conscious, we may imagine that animals like the _Grylli_, whose powers appear to commence nearly where ours terminate, may have the faculty of hearing still sharper sounds, which at present we do not know to exist; and that there may be other insects, hearing nothing in common with us, but endued with a power of exciting, and a sense that perceives, vibrations indeed of the same nature as those which constitute our ordinary sounds, but so remote, that the animals who perceive them may be said _to possess another sense, agreeing with our own solely in the medium by which it is excited_, and possibly wholly unaffected by these slower vibrations of which we are sensible[1045]." That insects, however, hear nothing in common with us, is contrary to fact; at least with respect to numbers of them. They hear our sounds, and we theirs; but their hearing or analogous sense is much nicer than ours, collecting the slightest vibratiuncle imparted by other insects, &c. to the air. In inquiring how this is done, it may be asked--How know we that every joint of some antennæ is not an acoustic organ, in a certain sense distinct from the rest? We see that the eyes of insects are usually compound, and consist of numerous distinct lenses;--why may not their external ears or their analogues be also multiplied, so as to enable them with more certainty to collect those fine vibrations that we know reach their sensory, though they produce no effect upon our grosser organs? I propose this merely as conjecture, that you may think it over, and reject or adopt it, in proportion as it appears to you reasonable or the contrary; and in the hope that some anatomist of insects, who, to the sagacity and depth of a Cuvier and a Savigny adds the hand and eye of a Lyonet, may give to the world the results of a more minute dissection and fuller investigation of the antennæ of these animals, than has yet been undertaken.

But besides receiving notices from the atmosphere, of sounds, and of the approach or proximity of other insects, &c., the antennæ are probably the organs by which insects can discover alterations in its state, and foretel by certain prognostics when a change of weather is approaching. Bees possess this faculty to an admirable degree. When engaged in their daily labours, if a shower is approaching, though we can discern no signs of it, they foresee it, and return suddenly to their hives. If they wander far from home, and do not return till late in the evening, it is a prognostic to be depended upon, that the following day will be fine: but if they remain near their habitations, and are seen frequently going and returning, although no other indication of wet should be discoverable, clouds will soon arise and rain come on. Ants also are observed to be excellently gifted in this respect: though they daily bring out their larvæ to sun them, they are never overtaken by sudden showers[1046]. Previously to rain, as you well know, numberless insects seek the house; then the _Stomoxys calcitrans_, leaving more ignoble prey, attacks us in our apartments, and interrupts our studies and meditations[1047]. The insects of prey also foresee the approach of wet weather, and the access of flies, &c. to places of shelter. Then the spiders issue from their lurking-places, and the ground-beetles in the evening run about our houses. Passive antennæ, which are usually furnished with a terminal or lateral bristle, and plumose and pectinated ones, seem calculated for the action of the _electric_ and other fluids dispersed in the atmosphere, which in certain states and proportions may certainly indicate the approach of a tempest, or of showers, or a rainy season, and may so affect these organs as to enable the insect to make a sure prognostic of any approaching change: and we know of no other organ that is so likely to have this power. I say _electric_ fluid, because when the atmosphere is in a highly electrified state, and a tempest is approaching, is the time when insects are usually most abundant in the air, especially towards the evening; and many species may then be taken, which are not at other times to be met with: but before the storm comes on, all disappear, and you will scarcely see a single individual upon the wing. This seems to indicate that insects are particularly excited by electricity[1048].--But upon this head I wish to make no positive assertion, I only suggest the probability of the opinion[1049].

From all that has been said, I think you will be disposed to admit that the primary and most universal function of the antennæ is to be the organs of a sense, if not the same, at least analogous to _hearing_, and answering the same end; something perhaps between it and touch. In some, however, as has been found in the _Crustacea_, an organ of hearing, in the ordinary sense, may exist at the base of the antennæ, which may act the part in some measure of the external ear, and collect and transmit the sound to such organ[1050].

That numerous antennæ, as a _secondary_ function, explore by _touch_, is admitted on all hands, and therefore I need not enlarge further upon this point; but shall proceed to inquire whether insects do not possess some other peculiar organs that are particularly appropriated to this sense. First, however, I must make some _general_ observations upon it. Of all our senses, _touch_ is the only one that is not _confined_ to particular organs, but dispersed over the whole body: insects, however, from the indurated crust with which they are often covered, feel sensibly, it is probable, only in those parts where the nerves are exposed, by being covered with a thinner epidermis, to external action. Not that they cannot feel at all in their covered parts; for as we feel sufficiently for walking, though our feet are covered by the thick sole of a boot or shoe, so insects feel sufficiently through the crust of their legs for all purposes of motion. Besides, the points that are covered by a thinner cuticle are often numerous; so that touch, at least in a _passive_ sense, may be pretty generally dispersed over their bodies; but _active_ or exploring touch is confined to a few organs, as the _antennæ_, the _palpi_, and the _arms_. The two last I shall now discuss.

Various opinions have been started concerning the use of the _palpi_. Bonsdorf thought that they were organs of _smell_; Knoch, that this sense was confined to the _maxillary_ ones, and that the _labial_ ones were appropriated to _taste_[1051]: but the most early idea, and that from which they derive their present name of palpi (_feelers_), is, that they are organs of active _touch_; and this seems to me the most correct and likely opinion. Cuvier, himself a host, has embraced this side of the question[1052], and Lehmann also admits it[1053]. The following observations tend to confirm this opinion. The palpi of numerous insects when they walk, are frequently, or rather without intermission, applied to the surface on which they are moving--this you may easily see by placing one upon your hand; which seems to indicate that they are _feelers_. In the _Araneidæ_ they are used as _legs_; and by the males at least, as _exciting_ if they be not really _genital_ organs[1054]. In the _Scorpionidæ_ they answer the purpose of _hands_: besides being usually much shorter than antennæ, they are better calculated to assist an insect in threading the dark and tortuous labyrinths through which it has often to grope its way, and where antennæ cannot be employed. I have noticed that _Hydrophili_--in which genus the _palpi_ are longer than the antennæ--when they swim, have their antennæ folded; while the former are stretched out in front, as exploring before them. As these are attached to the under-jaws and under-lip, we may suppose they are particularly useful to insects in taking their food; and upon this occasion I have often observed that they are remarkably active. I have seen _Byturus tomentosus_, a beetle which feeds upon pollen, employ them in opening anthers; and the maxillary pair appear to me to assist the maxillæ in holding the food, while the mandibles are at work upon it.

The _arms_ or fore-legs of some insects are also organs of _active_ touch, being used, as we have seen, for cleaning the head, digging, repairing their dwellings, and the like[1055]. By the _Ephemeræ_, which have very short antennæ, the fore-legs, when they fly, are extended before the head, parallel with each other and quite united--probably to assist in cutting the air. The _Trichoptera_ use their antennæ for the same purpose.

* * * * *

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.

FOOTNOTES:

[1025] VOL. III. p. 15. note^a.

[1026] Ibid. 58--. See above, p. 26.

[1027] _N. Dict. d'Hist. Nat._ xxx. 584.

[1028] _Hor. Entomolog._ 37.

[1029] _N. Dict. d'Hist. Nat._ xxx. 584--.

[1030] Cuv. _Anat. Comp._ ii. 362.

[1031] _Syst. Nat._ i. 535. Bonnet _Œuvr._ ii. 36.

[1032] _Ibid._

[1033] VOL. II. p. 162.

[1034] Lehmann _De Sens. Extern. Animal. Exsang._ 22--.

[1035] _Ibid._ _De Antenn. Insect._ ii. 79.

[1036] VOL. III. p. 43--.

[1037] See above, p. 1--.

[1038] VOL. III. p. 46.

[1039] _De Antenn. Insect._ ii. 65--.

[1040] _De Antenn. Insect._ ii. 42.

[1041] _Ibid._ 26.

[1042] See above, p. 218.

[1043] VOL. II. p. 64, 198--.

[1044] VOL. III. p. 319--.

[1045] _Philos. Trans._ 1820. 314.

[1046] Lehmann _De Usu Antenn._ ii. 66--.

[1047] VOL. I. p. 48, 110.

[1048] Compare what is said above (p. 141) with respect to bees.

[1049] See, for further arguments, Lehmann _ubi supr._ c. ix.

[1050] Marcel de Serres thinks he has discovered an organ of hearing in most insects, but he does not state its situation. _Mém. du Mus._ 1819. 99. Treviranus, with regard to the _Blattina_, suspects it to be situated between the eye and the base of the antennæ, perhaps alluding to the spot noticed above. (VOL. III. p. 505.) Carus, who mentions the above, says, "Is it not reasonable to ask if the sense of hearing may not reside in the membrane which connects the antennæ with the head?" _Introd. to Comp. Anat._ i. 80--.

[1051] Lehmann _De Sens. Extern. Anim. Exsang._ De Olfactu.

[1052] Cuv. _Anat. Comp._ ii. 675.

[1053] _Ubi supr._

[1054] Marcel de Serres says they are connected with _testes_ seated in the trunk (_Mem. du Mus._ 1819. 95); but Treviranus denies this (_Arachnid._ 36--. _t._ iv. _f._ 33).

[1055] VOL. II. p. 361--. III. p. 544--.

[1056] Lehmann _De Sens. Extern. &c._ De Olfactu.

[1057] Lehmann _ubi supr._ &c. 27.

[1058] _Ibid._ and _De Usu Antenn._ ii. 24--. Cuv. _Anat. Comp._ ii. 675.

[1059] Lehmann _De Usu Antenn._ ii. 28.

[1060] _Ibid._ 31.

[1061] _Ibid._ 35--.

[1062] VOL. III. p. 475--.

[1063] _N. Dict. d'Hist. Nat._ xxiii. 210.

[1064] _Ubi supr._

[1065] See above, p. 63. Sprengel _Commentar._ 14--.

[1066] Huber _Abeilles_ ii. 375--. Dr. Bevan in his interesting work on the _Honey-Bee_ adopts the opinion here stated with respect to the organ of smell in that animal. 265, 303.

[1067] VOL. I. p. 352--.

[1068] VOL. III. p. 480--.

[1069] VOL. III. p. 454--.

[1070] _Introd. to Comp. Anat._ i. 76. The part he alludes to, is figured PLATE VI. FIG. 4. a. g´.

[1071] _Ibid._ This membrane likewise represents the Nose and Rhinarium in that fly.

[1072] VOL. III. p. 481.

[1073] Cuv. _Anat. Comp._ ii. 682--.