De Geer has recorded a very singular fact which deserves your notice. An Ichneumon, appropriated to one of theTortrices, had deposited its eggs in two of their caterpillars; each produced a considerable number; but those that emerged from one were allfemales, and those from the other,males[989]. He observed a similar fact take place withMisocampus Puparum[990]. One might conjecture from this circumstance, that as in the queen-bee[991], so in these Ichneumons, the eggs producing the two sexes were arranged separately in the ovaries. Reaumur has related, that in one instance three or four males were produced to one female; and in another four or five females to one male[992].
But though the great majority of insects are subject to thisScolechiasis[993]in their larva state, yet sometimes they are not attacked by theIchneumontill they have becomepupæ. Of this kind is one just mentioned (M. Puparum), which commits its eggs to the chrysalis of the butterfly of the nettle,Vanessa Urticæ: the moment this caterpillarquits its skin to assume that state, while it is yet soft they pierce it and confide to it their eggs[994]. De Geer and others have supposed that this same Ichneumon attacks theCocciandCoccinellæ[995]; but this probably is an erroneous supposition.Cryptus Compunctoralso attacks the pupæ of butterflies[996].
If we consider the great purpose ofProvidencein giving being to this tribe of destroyers—the keeping of insects within their proper limits,—we may readily conceive that this purpose is more effectually answered by destroying them in theirpreparatorythan in theirultimatestate, since at that time the laying of their eggs and a future progeny could not so effectually be prevented;—this will account for there being few or no Ichneumons appropriated to them in their latter state.
The next tribe of insect parasites are to be found in theDipteraOrder. The species that has been particularly noticed as such is theTachina Larvarum; its larva is polyphagous, laying its eggsuponthe bodies of caterpillars of different kinds. Sometimes a pair is placed on the first segment, sometimes on the head itself, and sometimes near the anus. These eggs are very hard, convex, of an oval figure, polished and shining like a mirror. They are fixed so firmly that if you attempt to remove them with a penknife the skin comes off with them. When hatched, they enter the body and feed on the interior, and, undergoing their metamorphosis within it, do not emerge till they enter their perfect state. The caterpillar thus attacked lives long enough to spin its cocoon, when it dies[997]. Sometimes, however, these animalsquit their prey sooner. Reaumur saw a grub of one of theMuscidæcome out of a caterpillar, and then become a pupa, which was so large that he wondered how it could have been contained in the animal it had quitted[998].
We have now done with those parasites that produce in insects the disease I have calledScolechiasis[999]: the rest, which belong to theApteraOrder, will afford us examples both ofPhthiriasisandAcariasis[1000].
I begin with thefirst. Mr. Sheppard once brought me a specimen of a bird-louse (Nirmus) which he took upon a butterfly (Vanessa Io): and should such a capture be more than once repeated, it would afford acertaininstance of thefirstof these diseases amongst insects;—but most probably the specimen in question had dropped from some bird upon the butterfly. The only remaining animal belonging to the apterous hexapods that is parasitic on insects, is by many supposed to be the larva of a giant-beetle (Meloe Proscarabæus). I have before alluded to this animal[1001], and shall now resume the subject. Gœdart, Frisch, and De Geer, observed that it deposited in the earth one or two considerable masses, containing an infinite number of very minute orange-coloured eggs adhering to each other, which in about a month were hatched, and produced a number of small hexapods distinguished by two pairs of anal setæ and a proleg, by means of which they could move readily upon glass, as I have myself seen: these little animals precisely corresponded with one found by the latter author uponEristalis intricarius; and when that fly was placed amongst them, they immediately attached themselves to it, so as to leave no doubt of their identity[1002]. A congenerous species had been detected upon wild bees, and described by Linné under the name ofPediculus Apis. De Geer is so thoroughly to be depended upon for his veracity and accuracy of observation, that we cannot suppose there is any incorrectness in his statement. If the mass of eggs be, as he represents it, of the size of a hazel-nut, it must have been the product of a very large insect: in confirmation of this opinion it may be further observed, that the larva of the kindred genusCantharisagrees with it in having anal setæ, though it appears to differ in having only two conspicuous segments in the trunk[1003]. Those which infest wild bees make their first appearance uponacridplants, which theMeloelikewise feeds upon; from whence with wonderful agility they leap upon theAndrenæ, &c. that visit these flowers. Strong, however, as all these facts appear, still we cannot help exclaiming with the illustrious Swede last named, Who could ever have imagined that the larva of this greatbeetlewould be found upon the body offlies,—and we may add, orbees? Who could ever imagine that it would feed like abird-louseand resemble it so closely? that in the insertion of its palpi it should exhibit a characterexclusivelybelonging to that tribe[1004]? Another circumstance seems to indicate that these hexapods at the time that they take their station in bees or flies are perfect insects—they do not vary in size, at least not materially. Where, we may also ask, if they are to becomelarge beetles, where do they take their principal growth? It cannot be as parasites on the little bees or flies that they are usually found upon; they must soon desert them, and like their kindred blister-beetles, as is most probable, have recourse to vegetable food. What an anomalyin rerum natura! It is much to be wished that some skilful insect-anatomist would carefully dissect theMeloe; or perhaps by digging round the roots of the ranunculuses and other acrid plants the larva of that beetle might be discovered in a later stage of growth, and so this mystery be cleared up. I should observe here, that Scopoli has described three parasites asPediculi; viz.P. rostratus,coccineus, andCerambycinus; the first of which Fabricius has adopted under the name ofP. Gryllotalpæ, but which are all evidently hexapodAcarina[1005].
Acariasisseems a disease almost as universal amongst insects asScolechiasis; with this difference however, thatAcarimost commonly take their station upon them in theirperfectstate. You have doubtless often observed the common dung-beetles (Geotrupes) covered on the underside of their body with small mites (Gamasus Coleoptratorum) which look as if they were engaged in suction—they are often so numerous that no part is uncovered; they also attack other beetles[1006], and are sometimes found on humble-bees. They are easily disturbed, run with great swiftness, and may often be seen in hotbeds and fermenting dung prowling in search of the stercorarious beetles. But the most remarkable insect of this kind is theUropoda vegetans: it derives its nutriment from the insects it assails not by itsmouth, butby means of a longanal pedicleby which it is attached to them. De Geer found these in such numbers upon a species ofLeptura, that its whole body was almost covered with them; they hung from the legs and antennæ in bunches, and gave the animal a most hideous and disgusting appearance. Under this load of vermin it could scarcely walk or move, and all its efforts to get rid of them were in vain: many were attached to its body and to each other by their anal pedicles, but others had cast them off and were walking about. When put into a glass with earth, they began to abandon their prey, so that in a few days it was quite freed from its plagues. He found that these parasites lived long in alcohol[1007].
If you inquire—How are these mites originally fixed by their pedicles? it seems most probable, that as theHemerobii, when they lay their eggs, know how to place them upon a kind of footstalk, so the parentUropodahas the same power; and this pedicle appears to act the part of an umbilical chord, conveying nutriment to the fœtus not from aplacenta, but from the body of the insect to which it is attached; till having thus attained a certain maturity of growth and structure, it disengages itself and becomes locomotive. Many eggs of the aquaticAcarina(Hydrachna, &c.) are also furnished with a short pedicle by which they are fixed toDytisciand other water insects. De Geer found some of this description on the underside of the water-scorpion, so thickly set as to leave no void space: they were oval, of a very bright red, and of different sizes on different individuals; whence it was evident that they grow when thusfixed: when hatched or released—for perhaps they may be regarded as fœtuses in theiramniosrather than eggs—they cease to be parasitical. Let us admire on this occasion, (piously observes this great Entomologist,) the different and infinitely varied means by which theAuthorof Nature has endowed animals, particularly insects, for their propagation and preservation: for it is a most extraordinary sight to see eggs grow, and pump as it were their nutriment from the body of another living animal[1008]. As these mites are fixed to thecrustas well as its inosculations, they must have some means of forcing their nutriment through its pores.
Another insect, remarkable for its resemblance in some respects to the scorpion—called in this country the book-crab (Chelifer cancroides), from its being sometimes found in books—occasionally is parasitic upon flies, especially the common blue-bottle-fly (Musca vomitoria). They adhere to it very pertinaciously under the wings; and if you attempt to disturb them, they run backwards, forwards, or sideways, with equal facility.
Spiders also are infested by mites. Mr. Briggs once found a very smallTheridion, to the thorax of which were attached four oblong bright scarlet mites, each of which was as large as the thorax itself. He afterwards met with another spider still smaller, attacked by two of these swoln parasites, one of which appeared to him nearly equal to the spider in size. This mite was probably eitherLeptus Phalangii, orAstoma parasiticum.
2. We now come to a perfectly distinct tribe of insect parasites, which belong to that section or order ofintestinalwormswhich Rudolph has denominatedEntozoa nematoidea, and LamarckVers rigidules[1009]. To this tribe belong theGordiusof Linné and theFilariaof modern zoologists, which from the experiments and observations of De Geer, Dr. Matthey, &c. appear to have been too hastily separated, being really congenerous, and living indifferently in water and in the intestines of insects and other animals[1010]. To this genus belong the guinea-worm (Gordius medinensis[1011]), theFuria infernalis, and several others that are found in various vertebrate animals. These little worms have been discovered in insects of almost every Order; and their attack generally produces the death of the animal, though they appear not to devour those parts that are essential to life[1012]. I once took a specimen ofPœcilus azureus, and upon immersing it in boiling water I was surprised to see what at first I mistook for an intestine, thrust itself forth; but upon a nearer inspection, to my great surprise I found it was one of these worms, thicker than a horse-hair and of a brown colour. Mr. W. S. MacLeay also once found one inAbax Striola. It still remains in my specimen, making it appear as if it had a long tail. De Geer long ago found these worms in grasshoppers[1012]; but Dr. Matthey has given the fullest account of one which infestedAcrida viridissima. A friend of his noticing one of these insects which had not strength enough to leap and could scarcely even walk,being struck with the circumstance, caught the animal, upon which its hind legs were immediately detached from it. His surprise was greatly increased when he saw issue from its body a cylindrical worm about two feet and a half in length. Upon being called, Dr. M. soon recognised it for aGordiusorFilaria; and on his putting it into water, it moved in it with great velocity, twisting its long and slender body in all directions. Upon opening the body of the grasshopper, nothing appeared within it but the intestine shrunk up to a thread. A few days after, another was brought, which appeared in full vigour, but its abdomen was enormously distended, and from it another worm was extracted, which remained without motion rolled in a spiral direction: intending to preserve this in spirits of wine—as it had become flat he first immersed it in water, that it might recover if possible its cylindrical form. Upon immersion a movement took place in the animal, and it gradually recovered its plumpness; but it still remained without motion, as if dead, for nearly five days, when another living specimen being brought and placed with it, as soon as water was poured on them, the seemingly dead one began to show by a slight oscillation in its extremities that life was not extinct in it. Fresh water being poured upon it, at the end of the day it had recovered all its strength and agility. He afterwards often repeated the same experiment with a similar result[1013]. From this account it appears that theGordiusorFilariahas a property resembling that of theVibrio Tritici, so well described and so beautifully figured by M. Bauer[1014], of apparently dying and being resuscitated by immersion in water.How long it can retain this property remains to be ascertained.
De Geer states that he had seen them of the length of two feet[1015]; but they vary considerably in this respect. In ants, in which Gould detected them, he states their length to be not more than half an inch[1016]. In caterpillars, which they sometimes infest, they are longer; in that ofNotodonta Ziczac, De Geer found one three inches and a half long[1017]; and Rösel three, of six inches, in that ofDeilephila Euphorbiæ[1018]; and inPhalangium cornutum, according to Latreille, they extend to more than seven inches[1019]. In the larva of aPhryganeaL. the author first named found one which was more than a foot long, corresponding exactly with theGordius aquaticusof Linné; being forked at one extremity, brown above, gray below, and black at each end[1020]. These animalsappearto die as soon as they leave the body[1021]they have preyed upon; except this happens in water, when their activity has no repose. In this element they give their bodies every possible inflexion, often tying themselves in knots in various places, interlacing and twisting themselves in a hundred different ways; so that when confined in the body of an insect, from their extreme suppleness and power of contortion they find sufficient space wherein to pack their often enormous length[1022]. Linné makes one of their habitats clay; and Mr. W. S. MacLeay finds them very common at Putney in clay at the bottom of pools.
Dr. Matthey asks—How does theGordiusget intoAcrida viridissima[1023]? And De Geer—Why do they die after having quitted a caterpillar? and where do they perpetuate their species[1024]? These questions, without further observations, cannot easily be answered. However, it may be supposed that carnivorous insects, such asHarpali, &c. may swallow them when found apparently dead in clay, where the water has been evaporated, or when they have been ejected by other insects; and they may revive in their bodies, as Dr. Matthey found them to do in water. It is not difficult to conjecture that the larvæ ofPhryganeæmay meet with them when young in the water, and sometimes unluckily swallow them with their food. Why they become as dead when they emerge from their prey we cannot at present conjecture; but no doubt to answer some wise purpose;—in rainy seasons they probably revive and get into little hollows full of rain-water. Upon De Geer's last question—How they perpetuate their species—at present I can offer no conjecture.
I am, &c.
At first one would think that thesensesof 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 ingeneral, 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 numberseven, so signalized as asacrednumber[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 arereallysevensenses; which he divides into those that are altogetherphysical, and those that are more connected with theintellect. The first of these divisions containsfoursenses,—touch, love, taste, and smell;—the secondthree,—hearing, sight, and the internal sense of thought, or the brain[1027]. That he is right in addingloveto the list seems to me evident, because it is as distinct fromtouch, assmellingandtasteare. 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 theentire brainitself asense. We know that the agent between the common sensory and the sense is the consciousness orperceptionof the impression. "Seeing we may see and notperceive, and hearing we may hear and notunderstand." 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 theinternal sense, which, to use a term of Mr. W. S. MacLeay's[1028], isosculantbetween intellect and sense, or forms the transit from one group of powers to the other.
Of the ordinary senses,sightholds the first rank: itcan dart to the region of the stars, and convey by the perceiving sense, to the sensory, ideas of innumerable objects. Next in rank ishearing, 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 theInvertebratesthey are seldom to be met with all together in the same object. TheCephalopodshave nosmell. SeveralGasteropodscan neitherhearnorsee. 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 oftouch, which in them is so extremely delicate as to be acted upon even bylight[1030].
Not so our insects. These, there is good reason tobelieve, possess all the ordinary senses. That they cansee,touch,taste, andsmell, no one denies. Linné and Bonnet, however, thought them deprived ofhearing[1031]; but numerous observations prove the contrary. That they hear in theirlarvastate, 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 theirimagostate is confirmed still more strongly by facts. I once was observing the motions of anApionunder 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. AHarpalus, 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 certainhum[1033]. But the most satisfactory proof of the hearing of these animals is to be had from thoseOrthopteraandHemipterawhose males are vocal. Brunelli kept and fed several males ofAcrida viridissima(a grasshopper with us not uncommon) in a closet, whichwere 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 theantennæare for smell, touch, or hearing; and thepalpifor 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 toexploreastactors, yet by the most strenuous advocates of this opinion they are owned not to beuniversallyso employed; so that granting this to beoneof their principal functions, yet it seems to follow that there may beanothercommon to them all, which of course would be theirprimaryfunction. 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 merelycasting dust in our eyes: for though three of these arenasalorgans, bearingnostrils; the two others have no relation to the question, the horn of the rhinoceros and the crest of the cock being merelyappendages, 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 regardssecondaryfunctions—theprimaryusually remaining untouched. We may say, for instance, with regard to the primary use of thelegsof 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 thefore-legsof theMammalia, inbirdsbecomewings, and both pair infishare changed tofins. Observe, I do not sayalwaysand invariably, but in most cases,—that analogous parts have analogous uses, at least as far asprimaryuses 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;—becausewesee with our eyes, hear with our ears, &c., we conclude, with reason, thattheydo the same.
In inquiring therefore into what may be the most generaluse 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 theeyesof Vertebrates are forseeing; of thejawsformasticating; of thelipsforclosingthemouth; of thelegsforwalking, &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 theMammalia,—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 andmodus operandimay be different.
In the letter lately referred to, I observed that the antennæ of insects are analogous toearsin Vertebrates[1038]. Theirnumbercorresponds; they also stand out from thehead; 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 ofMammalia, 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 theirprimaryfunction may be something related to hearing. I do not say directhearing, or that the vibrations of sound are communicated to the sensorium by a complex structure analogous to that of the internal ear inMammalia—but somethingrelatedto 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 thecompoundeyes 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 itsight: but since antennæ, as we shall see, apparently convey amixedsensation, I shall have no objection, admitting it as their primary function, to call it after LehmannAëroscepsy[1039]. I lately related some instances ofsoundproducing an effect on theantennæof 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 immediatelymoved 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 betouch; 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 primarilytactors, or instruments oftouch, thatFœnus Jaculator, before it inserts its ovipositor, plunges itsantennæinto the hole forming the nidus of the bee, to the grub of which it commits its egg[1041]. But had those who used this argumentmeasuredthe 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 cannottouchthe larva; its object therefore cannot be to explore by that sense. Others suppose that by these organs itscentsout the destined nidus for its eggs; but Lehmann has satisfactorily proved that they are notolfactoryorgans. We can therefore only suppose, either that by means of its antennæ ithearsa slight noise produced by the latent grub, perhaps by the action of its mandibles; or else that by its motions it generates amotionin the atmosphere of its habitation, which striking upon the antennæ of theFœnus, are by them communicatedto its sensory. A similar disproportion is observable between the antennæ and ovipositor ofPimpla 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 thetipof 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 thestructureof 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 HomopterousHemipteraandDiptera, as is generally acknowledged, they cannot be used fortouch. Under this view, they may be divided intoactiveantennæ andpassiveantennæ: of the former, themostactive and versatile are those of theHymenoptera. By means of them, as was before observed[1043], their gregarious tribes hold converse, and makeinquiry—frequently withoutcontact—in the pursuit and discharge, if I may so speak, of the various duties devolved upon them byProvidence. Amongst active antennæ, some are much morecomplexin their structure than others—a circumstance which is often characteristicof themaleinsect[1044]: but if we examine such antennæ, we shall find that their mostsensitiveparts cannot come incontactwith 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 tosurfacesto explore its way, but merely keeps themopento receive notices from the atmosphere. Evensimpleantennæ are often employed in this way, as well as for touch. I once noticed a species ofLeptocerus, 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 soprecisely 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 theGrylli, 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 saidto 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 toenable 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 theStomoxys 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 theelectricand 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 sayelectricfluid, 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 disposedto 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 tohearing, and answering the same end; something perhaps between it and touch. In some, however, as has been found in theCrustacea, 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 asecondaryfunction, explore bytouch, 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 somegeneralobservations upon it. Of all our senses,touchis the only one that is notconfinedto 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 soleof 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 apassivesense, may be pretty generally dispersed over their bodies; butactiveor exploring touch is confined to a few organs, as theantennæ, thepalpi, and thearms. The two last I shall now discuss.
Various opinions have been started concerning the use of thepalpi. Bonsdorf thought that they were organs ofsmell; Knoch, that this sense was confined to themaxillaryones, and that thelabialones were appropriated totaste[1051]: but the most early idea, and that from which they derive their present name of palpi (feelers), is, that they are organs of activetouch; 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 arefeelers. In theAraneidæthey are used aslegs; and by the males at least, asexcitingif they be not reallygenitalorgans[1054]. In theScorpionidæthey answer the purpose ofhands: besides being usually much shorter than antennæ, theyare 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 thatHydrophili—in which genus thepalpiare 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 seenByturus 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.
Thearmsor fore-legs of some insects are also organs ofactivetouch, being used, as we have seen, for cleaning the head, digging, repairing their dwellings, and the like[1055]. By theEphemeræ, 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. TheTrichopterause their antennæ for the same purpose.
Another sense of which the organ seems uncertain is that ofsmelling, and various and conflicting opinions have been circulated concerning it. Christian thought that insects smelldistantobjects with theirantennæ, andnearones with theirpalpi[1056]. Comparetti has a most singularopinion. He supposes in different tribes of insects that different parts are organs of smell: in theLamellicornshe conjectures the seat of this sense to reside in theknobof theantennæ; in theLepidopterain theantlia; and in someDipteraandOrthopterain certainfrontal 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 thespiraclesare 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 forsmell, since they are not the seat also ofrespiration; 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 themouthto 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 thenose, between it and the upper-lip, or under those parts. That the nose corresponds with the so-named part inMammalia, 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 itsheador with itstail: but one would think that aflyinginsect would be more readily directed to its object by smelling with theanteriorpart of the body than with theposterior; and that afeedingone would also find it more convenient in selecting its food. As to the argument,—thatsmellmust be thenecessaryconcomitant of therespiratoryopenings, and that there can be no smell where theairis not inspired,—this seems asserting more than our knowledge of these animals will warrant: for the organs of theothersenses, though the senses themselves seem analogous, are so different in their structure, and often in the mode inwhich 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 notinvariablyaccompany respiratory organs even in the higher animals,—for webreathewith ourmouths, but do not smell with them. Cuvier says that theinternalmembrane of the tracheæ being soft and moist, appears calculated to receive scents[1064]. But here his memory failed him; for it is theexternalmembrane alone that answers this description; theinternalconsisting 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 themouth, and probably connected with thenose. M. P. Huber, desirous of ascertaining the seat of smell inbees, 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 itnear the cavity of the mouth, above the insertionof 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 infeedingappeared 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 themouth, and above the proboscis. It remains, therefore, that we endeavour to discover itsprecisesituation: and as insects cannot tell us, nor can we perceive by their actions, in what precise part the sense in questionresides, 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 therhinariumor nostril-piece[1068], you will find a pair of circular pulpy cushions, covered by a membrane transversely striated with beautifully fine striæ.Theseare 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 discoveredin the common water-beetle (Dytiscus marginalis), but with this peculiarity, that it is furnished with a pair ofnipples. I have before described an analogous part covered with papillæ, inÆshna 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 beingmembranousin some insects remarkable for acute scent, as inNecrophorus,Staphylinus, &c., there seems some ground for theformeropinion, which receives further confirmation from an observation of an eminent Comparative Anatomist, M. Carus, with respect toAcrida verrucivora, in which under thenoseandrhinarium, 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 anAnoplognathusmay receive and transmit them; and besides the upper-lip and nose are often united by membrane, perhaps representing therhinarium, as inGoerius, &c.[1072]which may facilitate such transmission.
That insectstaste, no one hesitates to believe, though some have supposed the palpi to be the organ of that sense; but as they have atongue, as we have shown, we may with Cuvier conclude, that one of its primary functions is totastetheir 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 anAlmightyandAll-wise Creatormore 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.