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.