M. Cuvier regards the hairs of insects as merely a continuation of the epidermis, with which they fall when the animal changes its skin[1161]; but this will apply only to the hairs of larvæ: for the hairs of perfect insects in many cases are implanted in a pore, and pass through epidermis or crust to the membrane that lines it, in which they terminate.
v. We are now to consider thecompositionof the integument; under which term I would include the different layers of which it consists, and its articulation.
1. With respect to thefirstof these circumstances, thelayersof which the integument consists, it seems to exhibit some, although not an exact, analogy with theskin, rather than theskeleton, of the vertebrate animals[1162]. In these last, the skin is stated to consist offourlayers. Of these the exterior one is theepidermis, or scarf-skin: under this is therete mucosum, or mucous tissue, which gives its colour to the skin; next follows thepapillary tissueformed by the extremities of the nerves, and in which the sense oftouchprincipally resides; the last and innermost layer is theskin proper, or leather, calledDermis,Derma, orCorium[1163].Twoof these layers M. Cuvier assigns to insects. They have, he observes, in every state, a trueepidermis[1164]; and in their state of larva he finds that the infinite variety of colours that so adorn many of them is produced by a mucous substance observable between the epidermis and the muscles[1165]: this seems analogous to therete mucosum. To this, dried and mixed with their horny substance, he attributes also the colours of the perfect insect: "for," says he, "when theLepidopteraare in the chrysalis, the little coloured scales which are to ornament their wings, are then in a state of mucosity similar to that which is found under the skin of the caterpillar. The colours of theArachnida," he goes on, "are also due to this mucosity: it is discoverable under the skin, and has the appearance of minute glandular points of which the shades vary considerably. But in theColeoptera, and many other Orders, thecolours of the skin are mixed in its horny tissue, nearly as those of theTestaceaare in their calcareous shells"[1166]. In the perfect insects, therefore, in most cases, we may consider theepidermisandrete mucosumas together forming the exterior and coloured integument of insects—that part which in the table, since it is not properly anepidermis, I have distinguished by the name ofExoderma.
The learned author just quoted has observed nothing under the skin of white-blooded animals that he regards as analogous tonervous papillæ[1167]. In some parts of insects, as in the lamellæ of the antennæ of thePetalocera, and the extremities of the joints, especially the last, of many palpi, there is however an appearance of them; and it seems reasonable to suppose that where the sense oftouchresides, there must, even in insects, be something of apapillary tissue.
With regard to the innermost integument of the vertebrate animals,—theleather, or real skin,—this learned comparative anatomist finds nothing analogous to it in the integuments of insects[1168]; but as he does not notice it, he appears to have overlooked the substance that lines the outer crust, orexoderma, in theColeopteraand most other orders. This is not always easily detected; but it may generally be discovered by breaking, or rather tearing (not cutting), after having cleared away the muscles, any part of the body of an insect. It is always very visible on the under side ofelytra[1169], but is not discoverableintegmina. It appears to consist, in many cases, of several layers of a whitish membrane, and generally breaks into fibres. In some elytra of the largerDynastidæ, towards the sides the exterior layer is separated from the rest by a kind of cellular substance. The fibrous structure of this inner skin (which I call theEsoderma) seems to give it some affinity to the skin of vertebrate animals[1170]. In many parts of the body, however, it appears to be merely a thin pellicle. A medical friend, to whom I showed specimens of it, thinks it a kind of cellular membrane.
2. A few words are next necessary with regard to thearticulationof the integument, or the mode by which the several pieces of which it and its members consist, are united to each other. In some, as in several of the parts of the head, the occiput, vertex, temples, cheeks, &c.—the line of distinction is merely imaginary; in others an impressed line separates a part from its neighbours, as is the case with thenoseinVespa, &c. the head in theArachnida. But in the majority of instances the parts are separated by a suture, or form a real joint. The kinds of articulation observed by anatomists in vertebrate animals do not all occur in insects, and they seem to have some peculiar to themselves. Thus, for instance, they have no propersuture; for though they exhibit the appearance both of theharmonicandsquamose(ecailleuseCuv.) sutures[1171], yet these parts being all limited bytheesoderma, or skin, above noticed as lining the integument, and all admitting a degree of motion more or less intense, rather afford examples, as the case may be, of other kinds of articulation[1172]. Again, they have no properEnarthrosis, or ball and socket; though the anterior coxæ of the Capricorn-beetles (CerambyxL.) approach very near to this kind of articulation, as will be shown more fully in another place. The inosculating segments or rings, which distinguish the abdomen, and sometimes other parts of insects, are an example of a kind of articulation not to be met with in theVertebrata. The ginglymous articulation, in which the prominences of the ends of two joints are mutually received by their cavities, and which admits only of flexion and extension, often prevails in the limbs, &c. of insects; but in many cases the joints are merely suspended to each other by a ligament or membrane; and, in fact, the integument of insects, with regard to its articulation, even where the joints ginglymate, may be said in general to consist of pieces connected by the internal ligament, membrane, or skin that lines it; for even in the legs, where the ginglymous articulation is sometimes very remarkable and complex, as will be shown to you hereafter, the joints are also connected by this substance, as you may see if you examine the legs of any Coleopterous insect.
Thenumberof articulations or pieces that form the integument and its members in these animals, varies greatly in different tribes, genera, &c. Thus, in the common louse (Pediculus humanus) they scarcely reach fifty, while in some cockroaches (Blatta) they amount to more than eight times that number.
Having premised these observations on the external anatomy of the body in general, in the remainder of the present letter I shall confine myself to the consideration of theheadand itsparts.
I. TheHeadof insects, as the principal seat of the organs of sensation, must be regarded in them, as well as in the vertebrate animals, as thegoverningpart of the body. It may be considered with respect to itssubstance,figure,composition,superficies,proportion,direction,articulationwith thetrunk,motions—and more particularly as to itspartsandappendages.
i. With regard to itssubstance—the head may be said in general to be thehardestpart of the crust: and it is so for very good reasons. In the first place, as it has to make way for the rest of the body when the animal moves, it is thereby best fitted to overcome such resistance as may be opposed by the medium through which it has to pass; in the next, as it bears the organs of manducation, it was requisite that it should be sufficiently firm and solid to support their action, which is often upon very hard substances; and besides this, as no motion of its partsinter se, as in the case of the trunk, is requisite to facilitate the play of its organs, a thin integument was not wanted.
ii. The most general law relative to thefigureor shape of the head seems to be, that it should approach to that of an equilateral triangle, with its angles rounded, and the vertex being the mouth; and that the vertical diameter should be less than the horizontal, whether longitudinal or transverse. But the infractions of this law are numerous and various. Thus, in some insects an isosceles triangle is represented by the head, the length being greater than the breadth; in others, instead of being flat it is compressed, so that the horizontal diameter is less than the vertical; in others, again, it is orbicular, or round and depressed; in others nearly spherical: occasionally it is rather cylindrical. In many instances it is very long; in others the width exceeds the length. Though often narrowest before, in some cases the reverse takes place. Its anterior end is often attenuated into a long or short snout or rostrum, and its posterior into a long or short neck. Its contour, though usually regular, is sometimes either cut into lobes, or scooped out into sinuosities. But to enumerate minutely all the variations of form which take place in the head of insects would be endless; I shall therefore proceed to the next particular.
iii. Thecompositionof the head is very simple; for, exclusive of its organs, it consists only of a single piece or box, without suture or segment, with an aperture at the end below to receive the instruments of manducation, others for the eyes and stemmata when present, and also for the antennæ. In theArachnida, &c., in which the head is not separated from the thorax, it is merely a plate, the under-side or cavity of which is occupied and filled by the above instruments.
iv. With regard to itssuperficies, the head of insects is generally more or less uneven, though in some cases it presents no inequalities. In many of the Lamellicorn tribes, and a few other individuals, in one sex at least, as has been before observed[1173], it is armed with long horns, or prominent tubercles; it is often covered with numerous puncta, or pores; and some of its parts, as the nose, after-nose, &c. in particular groups, marked out by an impressed line[1174]. In manyHymenoptera,Diptera, &c. its upper surface is convex, and the lower concave; in others both surfaces are convex.
v. It is the most general rule, as to itsproportion, that it shall be smaller than either trunk or abdomen; but in some instances, as in the S. American ant,Atta megacephala, it is much larger than either.
vi. By thedirectionof the head, I mean its inclination with respect to the prothorax. The most natural direction, or that which obtains most generally, is for it to form an angle more or less obtuse with the part just mentioned. This seems to obtain particularly inColeoptera; but in some, asMylabris, it is inflexed, forming an acute angle with it. In the HeteropterousHemiptera(CimexL. &c.) it is generally in the same line with the body, or horizontal; and in manyDipterait is vertical.
vii. We now come to a circumstance which will detainus longer, namely, itsarticulation with the trunk, or rather with its anterior segment, theprothorax.—M. Cuvier makestwoprincipal kinds of articulation of the head upon the prothorax, in one of which the points of contact are solid, and the movement subordinate to the configuration of the parts; in the other, the articulation is ligamentous, the head and the thorax being united and kept together by membranes.
1. Thefirstof these kinds of articulation—that by the contact of solid parts—takes place, he says, infourdifferent ways. "In the most common conformation, in the part that corresponds to the neck, the head bears one or two smooth tubercles, which receive corresponding cavities of the anterior part of the prothorax observable in the Lamellicorn and Capricorn beetles. In this case the head can move backwards, and the mouth forwards and downwards. Thesecondmode of solid articulation takes place when the posterior part of the head is rounded, and turns upon its axis in a corresponding cavity of the anterior part of the prothorax; as may be seen inCurculio,Reduvius, &c. The axis of motion is then at the centre of articulation, and the mouth of the insect moves equally backwards and forwards, upwards and downwards, to right and left.—Thethirdsort of articulation, by solid surfaces, takes place when the head, truncated posteriorly, and presenting a flat surface, is articulated, sometimes upon a tubercle of the thorax, and sometimes upon another flat and corresponding surface, as in almost all theHymenopteraand the majority of theDiptera. The disposition of thefourthkind of articulation allows the head only the movement of the angular hinge (le seul mouvement de charnière angulaire).The only examples at present known are in some species ofAttelabusF. The head of these insects terminates behind in a round tubercle, received in a corresponding cavity of the thorax: the lower margin of this cavity has a notch, and permits the movement of the head only in one direction[1175]."
2. Thesecondkind of articulation, the ligamentous, he affirms takes place only inorthopterousand someneuropterousinsects: "The head in this kind of articulation is only impeded in its movements towards the back, because it is stopped there by the advance of the prothorax; but below it is quite free. The membranes or ligaments extend from the circuit of the occipital cavity to that of the anterior part of the prothorax, which gives a great extent to the movement[1176]."
When I consider the well-deserved celebrity of the great author whose words I have here quoted, and the great and useful light that the genius and learning which conducted his patient labours and researches have thrown over every department of comparative anatomy,—a science he may be almost said to have founded,—I feel the most intire reluctance to differ in any thing from an authority so justly venerable to all lovers of that interesting study. But, however great my diffidence and hesitation to express an opinion at all opposed to his, the interests of truth and science require that I should state those particulars in which my own observations, made upon a careful examination of various insects of every Order, have led to results in some respects different from theabove. "Aliquando bonus dormitat Homerus;" and if the Genius of Comparative Anatomy ever nodded, it sometimes happened when he was examining the structure of insects. An instance of this with regard to the mouth of the bee has been noticed by Mr. Savigny[1177]; and indeed it is not wonderful that in so extensive an undertaking, in which the number of examples to be examined upon every branch of his subject must be immense, that he did not always scrutinize minutely those that seemed less important. Every writer on every department of Natural History, especially where the objects of research, as in the insect world, are so infinite in number, will be liable to such mistakes; but these will meet with due allowance from every candid mind—
"Hanc veniam damus, petimusque vicissim:"
"Hanc veniam damus, petimusque vicissim:"
and I shall express my trust that you will overlook any errors of mine; and doubtless I shall not be free from them—
"————-Quas aut incuria fudit,Aut humana parum cavit natura——"
"————-Quas aut incuria fudit,Aut humana parum cavit natura——"
The two kinds of articulation of the head which our learned author has stated as the principal ones, will, I think, be found upon examination not so widely distant as his expressions seem to indicate; for in fact in all insects, as well as theOrthoptera, this part issuspendedby a membrane or ligament which unites the margins of the occipital cavity with those of the anterior one of the prothorax: thus forming all round some protection to theorgans that are transmitted from the head through the latter part to the rest of the body. Though the head in mostOrthopterais not partly received into the cavity of the prothorax, as is the case in the OrderColeoptera, but is rather suspended to it, yet in some instances, for example in the mole-cricket (Gryllotalpa vulgaris), it is partially inserted.
Again: when, in hisfirstmode of articulation by contact of solid parts, he speaks of one or two smooth tubercles of the neck, with their corresponding cavities in the prothorax, as forming the most common conformation, you would expect to find examples of this in very many insects; yet upon a close examination, unless inOryctes nasicornis[1178], and perhaps in others of theDynastidæMacLeay, you would scarcely meet with any thing that could be called a tubercle and its corresponding cavity in the neck or prothorax of any Lamellicorn or Capricorn beetle that you might chance to examine. You would find, indeed, that the occiput was usually smooth and very slippery, as if lubricated; that in its margin were one or two notches (Myoglyphides), with muscles attached to them; that in the former of these tribes, theLamellicorns, it projected on each side so as to form a more or less prominent angle; and that the throat (jugulum) was very convex, and lodged in a cavity of the lower margin of the prothorax: but further appearances of tubercles &c. you would in vain look for even in thistribe; and in theCapricornsyou would find that the general conformation in this respect belonged to our learned author'ssecondmode of solid articulation, resembling that of the majority of the weevils (CurculioL.), in which the head has no projecting angles or tubercles, or other elevation, but is received usually into the circular cavity of the prothorax.
Histhirdmode of this articulation, that of theHymenopteraandDiptera, is so peculiar, that it ought to be considered as aprimarykind; since in this the head movesuponthe prothorax as upon a pivot, and has a kind of versatile motion.
With regard to hisfourthmode, which from his description appears to be that ofApoderusOliv., he allows motion to the head only inonedirection, observing that the lower margin of the prothoracic cavity has a notch. But M. Latreille calls the articulation of the head in this genus anEnarthrosis[1179], which admits motion in every direction; and if you examine the common species (A. Coryli), you will find that the prothorax has a sinus taken out of its upper margin, as well as out of its lower one—which at any rate will allow a motionupwards.
I merely mention these little inaccuracies, with due diffidence, as some apology for giving you a different and at least a more popular and general view of this part of my subject, which I shall now proceed to state to you. It seems to me most convenient for the Entomologist, and most consonant to nature, to divide insects, with respect to the articulation of the head with the trunk, into threeprimarysections, each admitting one or more subdivisions.
1. Thefirstconsists of those whose head inosculates more or less in the anterior cavity of the chest; and whose articulation, therefore, seems to partake in a greater or less degree of the ball and socket (Enarthrosis). The head, however, is often capable of being protruded from this cavity. If you take into your hand any commonHarpalusthat you may find under a stone, you will see, if pressed, that it can shoot forth its head, so as to be entirely disengaged from the prothorax: a neck of ligament intervening between them[1180]: of course this power of protruding the head enables the animal to disengage it at its will from the restriction imposed upon its motions by the surrounding margin of the prothoracic cavity. To this section belong all theColeoptera, the HeteropterousHemiptera(CimexL., &c.), and some of theNeuroptera(Raphidia,Semblis, &c.).—It may be further divided intotwosubsections—those, namely, whose head inosculates in the prothorax by means of aneck: as for instance Latreille'sTrachelides,Apoderus, and theStaphylinidæ, amongst the beetles; theReduviadæamongst theHeteropterousinsects, andRaphidiain theNeuroptera; and those whose head inosculates in the prothorax without the intervention of a neck; as, thePetalocera, the aquatic beetles (Dytiscus,Hydrophilus, &c.), and most of the genusCurculioL. in the first of these orders, the great body of theCimicidæin the second, andSemblis,Corydalis, &c. in the third.
2. Thesecondsection consists of those insects whose head does not inosculate in the chest, but is merely suspendedto it by ligament or membrane. To this belong most of the tribes of theOrthopteraOrder, with the exception of theMantidæ, theDermaptera, theHomopterous Hemiptera, and such of theApteraas have the head distinct from the prothorax.—This section admits of atriplesubdivision: those, namely, whose head iswhollycovered by the shield of the prothorax, as inBlattaL.; those whose head ispartlycovered by it, asGryllotalpa, and otherGryllina; and those whose head is quite free, not being at all impeded in its motion by the prothorax, as theDermaptera,Nirmus,Pediculus, &c.
3. Thethirdsection consists of those whose head is truncated posteriorly, and flat or concave, with a very small occipital aperture, and is attached to a neck of the prothorax upon which it turns, or is merely suspended to that part. This includes theLepidoptera,Hymenoptera,Diptera, theLibellulina, &c. in theNeuroptera, and theMantidæin theOrthoptera.Threesubsections at least, if not more, present themselves in this section: the first is, of those whose head is united to the prothorax, without the latter forming anyneck. To this belong theLepidoptera,Trichoptera? Thesecondis of those the upper side of whose thoracic neck isligamentous; and here you may range most of theHymenoptera. Thethirdis of those in whom it is acontinuationof the ordinaryintegument. In this subsection theDiptera,LibellulinaandMantidæwill find their place. In this last section the head appears to turn upon the thorax as upon a pivot.
Before I finish what I have to say on thearticulationof the head, I must direct your attention to theanalogiesthat hold in this respect between the different tribes. Thus theColeopteraare analogous to theHeteropterous Hemiptera; theOrthoptera, with the exception of theMantidæ, to theHomopterous Hemiptera; theMantidæto theLibellulina; theLepidopterato theTrichoptera; theHymenopterato theDiptera, with a slight variation, and probably others might be traced.
viii. A word or two upon themotionsof which the head of insects is capable. M. Cuvier, in the extracts lately laid before you, speaks of different powers of movement as the result of different configurations of the parts of the head. This probably is correct with regard to many cases; but a great deal will depend upon the power the insect has of protruding its head and disengaging its base from the restriction of the prothorax; for where, like theHarpali,Staphylini, &c. it is able to do this, it can probably move its head in every direction. It is only where the ligaments are less elastic, or allow of little tension, that its movements are confined; and few living insects have been sufficiently examined to ascertain how far this takes place. In those cases belonging to thethirdsection of articulations, in which the head movesuponthe thorax as upon a pivot, as is the case withHymenopteraandDiptera, the movement is nearly versatile. I have seen a fly turn its head completely round, so that the mouth became supine and the vertex prone; and from the form and fixing of the head, it should seem that those of theMantidæwere endued with the same faculty.
ix. Thepartsandappendagesof the head are now inthe last place to be considered. I shall begin with theMouth, or rather the orifice in which thetrophior organs of manducation are inserted. In some insects, as was before observed, they occupy all the under-side of the head, as in theArachnida,Myriapoda, &c; but in the great majority they fill an orifice in its anterior part, which in some instances, as inLampyris, theLepidoptera,CimexL.,Truxalis, appears to be nearly under the head; but in general it terminates that part, though it extends further below than above. InChermes, a Homopterous genus, the promuscis is stated to be in theAntepectus, and consequently themouth; but, as I shall endeavour to prove to you hereafter, this is a fallacy. In the males of the species ofCoccusthere is no mouth at all. In that of the elm (C. Ulmi) in lieu there are ten little shining points, arranged two before and two behind in a line, and three on each side in a triangle[1181]. It is to be observed that the orifice of which I am speaking is usually much smaller in those insects which take their food by suction, theHemiptera,Lepidoptera,Diptera, &c., than in the masticating tribes. With regard to the real mouth, or that through which the food enters, I have nothing at present to observe, except that it lies between the upper-lip and tongue, is sometimes covered by a valve, as inApis,Vespa[1182], &c., and is different in the masticators and suckers.
I shall next offer a few observationsseriatim, as they stand in the Table, upon the organs of manducation; which, to avoid circumlocution, instead ofInstrumentacibaria, the name Fabricius gave them, I shall calltrophiorfeeders. It is upon these parts, you are aware, that the system of the celebrated Entomologist just mentioned is founded; and could they always, or even for the most part, be inspected with ease, they would no doubt afford characters as various and discriminative as those of the vertebrate animals. Differences in these parts indicate a difference in the mode in which the animal takes its food, and often in the kind of food, and sometimes in its general economy and habits,—circumstances which are powerful and weighty in supporting the claim of any set of animals to be considered as forming a natural genus or group. Trifling variations, however, of these parts, unless supported by other characters and qualities, ought not to have much stress laid upon them, since, if we insist upon these, in some tribes almost every species might be made a genus.
With respect to theirtrophiin general, insects of late have been divided into two great tribes[1183],masticatorsandsuckers; thefirstincluding those that are furnished with instruments to separate andmasticatetheir food; namely, an upper- and under-lip (labrumandlabium), upper- and under-jaws (mandibulæandmaxillæ), labial and maxillary palpi, and a tongue (lingua): and thesecondthose in which these parts are replaced by an articulate or exarticulate machine, consisting of several parts and pieces analogous to the above, which pierce the food of the animal, and form a tube by which itsucksits juices. If, however, the mode in which insects take their food bestrictly considered, it will be found that in this view they ought rather to be regarded as formingthreetribes; for the great majority of theHymenopteraorder, and perhaps some others, though furnished with mandibles and maxillæ, never use them for mastication, but really lap their food with their tongue: these, therefore, might be denominatedlappers.
When a mouth is furnished with thesevenordinary organs used in taking the food and preparing it for deglutition—I mean the upper-lip and the two upper-jaws; the under-lip and the two under-jaws, including the labial and maxillary palpi; and the tongue—I denominate it aperfect mouth; but when it is deficient in any of these organs, or they exist merely as rudiments, or when their place is supplied by others, (which, though they may be analogous parts, have little or no connection with them in their use or structure,) I denominate it animperfect mouth. This last I would further distinguish, according to the nature of itstrophi, by other and more distinctive terms, as I shall presently explain to you.
1.Labrum[1184].—I shall first consider the organs present in aperfect mouth, beginning with theupper-lip(labrum). This part, which Fabricius sometimes confounded with thenose, miscalling itclypeus, is usually a moveable[1185]piece, attached by its base to the anterior margin of the part last named, and covering the mouth, and sometimes the mandibles, from above. In insects in their last state it is usually of a horny or shellysubstance; yet in some cases, as inCoprisandCetonia, beetles thatimbibe juices, it is membranous. Informand shape it varies greatly, being sometimes nearly square, at others almost round; in some insects representing a parallelogram, in others a triangle, and in many it is oblong. In some instances it is long and narrow, but more generally short and wide. It is often large, but occasionally very minute. In the majority it has an intiremargin, but it is not seldom emarginate or bilobed, or even dentate. Itssurfaceis commonly even, but it is sometimes uneven, sometimes flat, at others convex, and in a few species armed with a short horn or tubercle[1186]. As to itspubescence, it is often naked, but now and then fringed or clothed with down or hairs, or sprinkled with bristles. It consists in almost every instance of a single piece; but an exception to this occurs inHalictus, a little bee, in the females of which it is furnished with a slender appendage[1187].—Thedirectionof the upper-lip is various. It is rarely horizontal, or in the same line with the nose, often vertical; it sometimes forms an obtuse angle with the anterior part of the head, and occasionally an acute one, when it is more or less inflexed. Theuseof this part is ordinarily to close the mouth from above, to assist in retaining the food while undergoing the process of mastication; but in many Hymenopterous insects its principal use seems to be, to keep thetrophiproperly folded; and in some cases where it is inflexed, as in the leaf-cutter bees (MegachileLatr.), to defend its base, while the mandibles are employed, from injury by their action[1188].
2.Labium[1189].—On the under-side of the head, and opposed to the upper-lip, the mouth is closed by another moveable organ, concerning the nomenclature and analogies of which Entomologists have differed considerably. At the first view of it, this organ seems a very complex machine, since the under-jaws ormaxillæare attached to it on each side, and the tongue is often seen to emerge from it above, so as to appear merely a part of it; but as the former answer to the upper-jaws, and the latter is the analogue of the part bearing the same name in the vertebrate animals, I shall consider these as distinct andprimaryorgans, and treat of the under-lip (labium) of which I am now speaking, by itself. Linné takes no notice of this part, but his illustrious compatriot and cotemporary, De Geer, did not so overlook it: he appears to consider the whole apparatus, including themaxillæ, as thelabium[1190]; but sometimes he distinguishes the middle piece by that name[1191]; and the tongue, in the case of the stag-beetle, he denominates a proboscis (trompe)[1192]. In theHymenopterahe calls this parttongue,under-lip, andproboscis: but seems to prefer the last term[1193]. Fabricius originally regarded the whole middle piece as alabium[1194]; but afterwards (though his definition is not accurate, since he assigns thepalpito theligula, which he affirms is covered by thelabium—circumstances by no means universal inColeoptera) he considers this as consisting ofligulaandlabium[1195]. Latreille at first regardedtheligulaof Fabricius as thelabium, and called thelabiumof that author thementum[1196]; but afterwards he gave the name oflabiumto the whole middle piece of the lower apparatus of the mouth—calling the upper piece, with Fabricius, theligula, and retaining the denomination ofmentumfor the lower[1197].
If the circumstances of the case are duly considered, I think you will be convinced that the term under-lip, orlabium, should be confined to that part which the learned Dane so named. For I would ask, Which is the part on the under side of the head that is the antagonist, if I may so speak, of the upper-lip orlabrum? Is it not that organ which, when the mouth is closed, meets that part, and in conjunction with it shuts all in? Now in numerous insects, particularly the Lamellicorn beetles (ScarabæusandLucanusL.), this is precisely the case. In the Predaceous beetles, indeed, (Cicindela,Carabus,DytiscusL. &c.) the under-lip does not meet the upper, to close the mouth and shut in the tongue; neither can the tongue be said so to do, but only, from some circumstances connected with its manner of taking its food, it is requisite that the last-mentioned organ should not be retractile or covered; but its miscalledmentumis still the analogue of that part which helps to close the mouth in the former tribe. Should not this, therefore, which so often performs theoffice, be distinguished by thename, of a lip? Again, is it not rather incongruous to consider that organ which confessedly more or less performs the office of atongue, as a part of thelip? Though it often wears that appearance, yet I believe, if the matteris thoroughly and patiently investigated, it will be found that on their upper side its roots are attached to the interior of the upper side of the head, as well as on their lower side to thelabium; so that it may be regarded as common to the two lips, and therefore not properly considered as an appendage of the under-lip alone.
Having assigned my reasons for preferring the name given to the part in question by Fabricius, rather than that of Latreille, I shall next make my observations on the part itself. In many cases thelabium, or the middle piece of the lower oral apparatus, appears to consist oftwojoints: this you may see in the great water-beetle (Hydrophilus piceus), the burying-beetles (Necrophorus), the Orthopterous tribes[1198], theHymenoptera[1199], and others. In this case the upper or terminal piece is to be regarded as thelabium, and the lower or basal one (which Mr. MacLeay calls thestipes) as thementumor chin—at other times, as in some Lamellicorn beetles, the only separation is a transverse elevated line, or an obtuse angle formed by the meeting of the two parts, and very frequently there is no separation at all, in which case the whole piece, thementummerging in it, may be denominated thelabium.
With respect to itssubstance, thelabiumin most Coleopterous insects is hard and horny, inNecrophorusit is membranous, and thementumharder; inPrionus coriarius, our largest Capricorn-beetle, both are membranous; in the bee-tribes,ApisL., thelabiumrather resembles leather, while thementumis hard. Itssurfaceis often convex, sometimes plane, and sometimes evenconcave; as for instance inMelolontha Fullo, a rare chafer occasionally found on the coast of Kent. In some it is covered with excavated points; in others it is quite smooth. In numbers, as in the Predaceous beetles, bothlabiumandmentumare perfectly naked; in others, as in the common cockchafer, they are hairy; inGeniates barbatusKirby, another chafer in the male insect, thelabiumis naked, while thementum, which forms a piece distinct from that part, is covered with a dense rigid beard[1200]. Inshapethe wholelabiumvaries considerably, much more than thelabrum; for in addition to most of the forms I enumerated when I described that organ, which I shall not here repeat, you may meet with examples of many others. Thus, to instance in the Petalocerous tribes (ScarabæusL.), in some, as in theRutelidæ, thelabiumis urceolate, or representing in some degree the shape of a pitcher[1201]; in others it is deeply concave, and not a little resembles a basin or a bowl[1202]; this form is peculiar to thelabiumofCremastocheilusKnoch, a scarce North American beetle; in another related to this, but of an African type (GenuchusKirby MS.Cetonia cruentaF.), it is a trapezoid plate, which is elevated from the head, and hangs over the throat like a chin[1203]. In theHymenopterait is extremely narrow and long, and embraces the sides of the tongue, as well as covering it from below; so that it wears the appearance of a kind of tube[1204]. Generally speaking, the length of thelabiumexceeds its breadth; but in the Predaceous beetles the reverse ofthis takes place, it being very short and wide, and usually terminating towards the tongue in three lobes or teeth which form two sinuses varying in depth[1205].
Thementumtaken by itself affords no very striking characters to which I need call your attention: I shall only observe, that inHymenopterait is generally of a triangular shape[1206]; but before I proceed to consider the labial palpi, it will be proper to notice the remarkablelabiumofOrthopterousinsects, and of theLibellulina, between which there is no little analogy. At first you would imagine the terminal part of this organ in the former to be the analogue of the tongue, orligulaF.; as it is indeed generally regarded by modern Entomologists[1207]. It seems, like the tongue of theCarabiL.,Dytisci, &c., to be a distinct piece, which has below it bothlabiumandmentum; but when you look within the mouth, you will find a linguiform organ[1208], which evidently acts the part of a tongue, and therefore ought to have the name; and the piece just alluded to must either be regarded as the termination of the lip, or as an external accompaniment of the tongue, analogous, it may be, to theparaglossæin bees[1209]. In a specimen ofAcrida viridissima(LocustaF.) which I dissected, the tongue is as long as the appendage of the under-lip, and by its upper surface seems to apply closely to it. In theLibellulinaa similar organ is discoverable[1210], which on its upper-side terminates in thepharynx, like that of one of theHarpalidæbefore mentioned. In theOrthoptera, therefore, I regard thelabiumas consisting of three articulations, the upper onedivided into two, three, or more lobes[1211]; the intermediate one more directly answering to thelabiumof other insects, and the basal one ormentum. This organ in theLibellulinais of a different structure: it has only two articulations representinglabiumandmentum; but the former consists of three parallel pieces, the two exterior ones rising higher than the intermediate one, and at their inner angle having an acute sinus from which the palpi emerge; and the intermediate piece, which is longitudinally channelled, lapping over the inner side of the lateral pieces. From the angle of the covered part of these pieces, a subulate short horizontal horn points inwards towards the tongue, which it must keep from closing with thelabium[1212].
3.Palpi Labiales[1213].—The last-mentioned organs, thelabial palpi, next claim our attention; but before I advert particularly to them, it will be proper to premise a few words uponpalpi, or feelers, in general. These are usually jointed moveable organs, of a corneous or coriaceous substance, attached by ligaments to thelabiumandmaxillæ, and in theCrustaceaeven to themandibulæ. Their joints, which are usually more or less obconical, articulate also in each other by ligaments, with perhaps some little mixture of the ball and socket. Their ends, the last joint especially, seem furnished with nervouspapillæwhich indicate some peculiar sense, of which they are the instrument. What that sense is has not been clearly ascertained, and concerning which I shall enter more into detail in another place. Their motion seemsrestrained, at least in some, to two directions, towards and from the mouth. They were calledpalpior feelers, because the insect has been supposed to use them in exploring substances. There seem to be no organs in the vertebrate animals directly analogous to thepalpiof insects andCrustacea, unless, perhaps, thecirrithat emerge from the lips of some fishes, as the cod, red mullet, &c. which Linné defines as used in exploring (prætentantes). Whether thevibrissæ, miscalledsmellers, of some quadrupeds and birds have any reference to them, I will not venture to affirm; but the feline tribe evidently use their bristles as explorers, and they are planted chiefly in the vicinity of the mouth.
Having made these general remarks, I shall now confine myself to thelabial palpi. I call themlabialpalpi, because that term is in general use, and because in many cases they really do emerge from what I consider as thelabium, as in most of thechafers; but they might with equal propriety be denominatedlingualpalpi, since they sometimes appear to emerge from thetongueas in the stag-beetle (Lucanus Cervus). In some instances, as in the Predaceous beetles, they seem to be common to bothlabiumand tongue, being attached at their base on the upper side to the former, and on the under side to the latter. As to theirsituation: they emerge from thebaseof thelabiumin the locusts (LocustaLeach)[1214]; from itsmiddleinHister maximus[1215]; from itssummitinAmblyterusMacLeay[1216]; and from its lateral margin inDynastesMacLeay, &c. They consist of fromonetofourjoints; which, I believe, they never exceed. They vary inlength; though generally shorter than the maxillary palpi, yet in the ferocious tiger-beetles (CicindelaL.) they equal them in length; and in the hive-bee and humble-bees, and many other bees, they are considerably longer[1217]. The two first joints of these palpi, however, in thesebeesare different in their structure from the two last, being compressed and flat, or concave; and the two last joints, instead of articulating with the apex of the second, emerge from it below the apex: so that these two first joints seem rather elevators of the palpi than really parts of them[1218]. With respect to the relativeproportionsof their joints to each other: in some cases the first joint is the longest and thickest, the rest growing gradually shorter and smaller[1219]; in others, the second is the longest[1220]; in others, again, the third[1221], and sometimes the last[1222]; and often all are nearly of the same size and length[1223]. They are more commonlynaked, but sometimes either generally or partiallyhairy. Thus inCicindela, the last joint but one is usually planted with long snow-white bristles in a double series, while the rest of the joints have none; and inCoprisLatr. all of them are extremely hairy. Inshapethey do not vary so much as the maxillary palpi, being most frequently filiform or subclavate, and sometimes setaceous; the last joint varies more in shape than the rest, and is often remarkablylarge, triangular, and shaped like the head of a hatchet[1224]; and at others it resembles the moon in her first quarter[1225]. In the great dragon-fly, ordemoiselleif you prefer the gentler French name (ÆshnaF.) the labial palpi, which are without any visible joints, are terminated by a minute mucro or point[1226]. With regard to theirdirectionand flexure, they frequently, as in the instance just mentioned, turn towards each other, and lie horizontally upon the end of thelabium. Sometimes, as in theCicindelidæ, they appear to point towards the tail of the insect, the last joint rising, and forming an angle with the rest of the feeler. In other instances they diverge laterally from thelabium, the last joint turning again towards it at a very obtuse angle.
4.Mandibulæ[1227].—Having considered the analogues of thelipsin our little beings, I must next call your attention to the representatives of thejaws. The vertebrate animals, you know, are mostly furnished with a single pair of jaws, one above and the other below, in which the teeth are planted and which have a vertical motion. But insects are furnished withtwopair of jaws, a pair of upper-jaws and a pair of under-jaws, which have no teeth planted in them, and the motion of which is horizontal.—I shall begin with an account of the upper-jaws. These by modern Entomologists, after Fabricius, aredenominatedmandibles(mandibulæ): a term appropriated by Linné to the beaks of birds. The upper-jaws of insects this great naturalist namedmaxillæ—and not improperly, since the office of mastication is more peculiarly their office than that of the under-jaws, which Fabricius has distinguished by that name: as the termmandible, however, is generally adopted, I shall not attempt to disturb it.
Themandiblesclose the mouth on each side under thelabrumor upper-lip. They are generally powerful organs, of a hardsubstancelike horn; but in the Lamellicorn beetles of Mr. MacLeay's families ofScarabæidæandCetoniadæ, they are soft, membranous, and unapt for mastication. In Coleopterous insects they commonlyarticulatewith the head by means of certainapophysesor processes, of which in many cases there arethreediscoverable at the exterior base of the mandibles; one, namely, at each angle, and one in the middle. That on the lower side is usually the most prominent, and wears the appearance of the condyle of a bone: it is received by a corresponding deep socket (or cotyloid cavity) of the cheek, in which, being perfectly smooth and lubricous, it moves readily, but withoutsynovia, like arotulain itsacetabulum. The upper one projects from the jaw, forms the segment of a circle, and is concave also on its inner face. A corresponding more shallow, or, as anatomists speak, glenoid cavity of the cheek, where it meets the upper-lip, receives it, and the concave part admits a lubricous ball projecting from the cheek, upon which it turns[1228].This structure you will find in the stag-beetle, and some other timber-devourers. OtherColeopterahave only a process of a similar structure at each of the dorsal angles of the base of the mandible, the intermediate one being wanting; and the articulation does not materially differ, as far as I have examined them, in theHymenopteraandNeuroptera. In theOrthoptera, the structure approaches more nearly to that of the stag-beetle, since there arethreeprominences: it is thus well described by M. Marcel de Serres: "This articulation," says he, "takes place intwoways. At first, in the upper surface of the mandible, and at its base, may be observed two small prominences and a glenoid cavity; these prominences are received in two glenoid cavities excavated in the shell of the front, as the cavity of the mandible receives a small prominence of the same part. Below the mandible, and at its base, there is a kind of condyle, which has its play in a cotyloid cavity excavated in the shell of the temple, far below the eye, and at the extremity of the coriaceous integument of the head[1229]." Within the head in this order, at least inLocustaLeach, is a verticalseptum, which divides the head into two chambers, as it were, an occipital and a frontal, consisting of a concave triangular stem, terminating in two narrower concave triangular branches, so as to resemble the letter Y, and forming three openings, an upper triangular one, and two lateral subquadrangular ones, which last are the cavities that receive the base of the mandibles. This partition, which I would nameCephalophragma, doubtless affords a point of attachment to many of the muscles of the head. It doesnot appear to have been noticed, unless it be synonymous with theintermaxillary arcadeof Marcel de Serres[1230]. Probably a corresponding support to the muscles, &c. may exist, as we have seen it does inVespaL.[1231], in many other heads of the different Orders, which have not yet fallen under examination. Many mandibles, as those of the hornet &c., appear to be suspended to the cavity of the head on the inside by a marginal ligament sufficiently relaxed to admit of their play: those of theOrthoptera, M. Marcel de Serres informs us, are united to the head by means of two cartilages, the outermost being much the shortest, to which their moving muscles are attached. These he considers as prolongations of the substance of the mandible[1232]. The bottom of mandibles, when cleared of the muscles &c., inclines almost universally to a triangular form; but in some cases, as in the stag-beetle, it is nearly atrapezium. I cannot conclude this subject without noticing themotionsof the mandibles. What the author lately quoted has said with regard to those of theOrthoptera, will, I believe, apply equally well to all the mandibulate orders. "The articulation of mandibles with the skull appears to take place by two points solely; and as these parts only execute movements limited to a certain direction, they may be referred toginglymus[1233].—The movements of mandibles are limited to those from within outwards, and from without inwards[1234]." Whether they are restricted from any degree of vertical motion, has not yet been proved, as the jaws of vertebrate animals move horizontally aswell as vertically—so those of insects may have some motion vertically as well as horizontally; and it seems necessary for some of their operations that they should. I am not anatomist enough to speak with confidence on the subject, but the ball and socket articulation at the lower part of the mandible, and the curving one at the upper, though a kind ofginglymus, seems to imply a degree ofrotatorymovement, however slight.
I must next say something upon the general shape of these organs. Almost universally they incline to a triquetrous or three-sided figure, with their external surface convex, sometimes partially so, and their internal concave. Most frequently they are arched, curving inwards; but sometimes, as inPrionus octangularis[1235], a Capricorn beetle, and others of that genus, they are nearly straight; and inRhina barbirostris[1236], a most remarkable Brazilian weevil, their curvature is outwards. InPholidotus lepidotusMacLeay, andLucanus Elephas, two insects of the stag-beetle tribe, they are bent downwards; and inLucanus nebulosusK. (RyssonotusMacLeay) they turn upwards[1237]. They are usually widest at the base, and grow gradually more slender to the apex, but in the hornet (Vespa Crabro) the reverse takes place, and they increase in width from the base to the apex; and in the hive-bee, and others of that tribe, they are dilated both at base and apex, being narrowest in the middle; others are nearly of the same width every where. In those insects that use their mandibles principally for purposes connected with their economy, they are often more broadin proportion to their thickness, than they are in those which use them principally for mastication. In the locust tribes (LocustaLeach), they are extremely thick and powerful organs, and fitted for their work of devastation; but in the glow-worm (Lampyris), they are very slender and minute. In those brilliant beetles, theBuprestes, they are very short; but in the stag-beetles, and those giants in the Capricorn tribe, thePrioni, they are often very long[1238]. They either meet at the summit, lap over each other, cross each other, or are protended straight from the head; as you have doubtless observed in the stag-beetle, whose terrific horns are mandibles of this description. These organs are usuallysymmetrical, but in some instances they are not: thus inHister lævus, a kind of dung-beetle, the left hand mandible is longer than the right; inCreophilus maxillosusK. (StaphylinusL.), a common rove-beetle, in the left hand mandible the tooth in the middle is bifid, and in the right hand one intire; and inBolbocerusK. the mandible of one side, in some species the dexter, and in others the sinister, has two teeth, and the other none.
The next circumstance with respect to these organs which demands our attention, is theteethwith which they are armed. These are merely processes of the substance of the mandible, and not planted in it bygomphosis[1239], as anatomists speak, as they are in vertebrate animals. They have, however, in their interior, at the baseat least, in theOrthoptera, a coriaceouslaminathat separates them in some sort from the body of the mandible[1240]. Many insects, however, have mandibles without teeth; some merely tapering to a sharp point, others obtuse at the end, and others truncated[1241]. Of those that have teeth, some have them on the inside at the base, asManticora, an African tiger-beetle[1242]; others in the middle, asStaphylinus olens, a rove-beetle,Lethrus cephalotes, &c.[1243]; others at the end, as many weevils (CurculioL.)[1244]; others again on the back, as theRutelidæ, a tribe of chafers[1245], andLethrus, a beetle just named; others once more on the lower side of the base, in the form of a tooth or spine, as inMelitta spinigera, a species of wild-bee, and some of its affinities[1246]; and lastly, others on the upper side of the base in the form of a long tortuous horn, as in that singular waspSynagris cornutaF. before noticed as a sexual character[1247]. In the stag-beetle tribes (LucanusL.) these teeth are often elongated into short lateral branches, or a terminal fork[1248]. They are sometimes truncated, sometimes obtuse, and sometimes acute.
But with regard to theirkind, it will be best to adopt the ideas of M. Marcel de Serres; for though his remarks are confined to theOrthoptera, they may be applied with advantage to the teeth that arm the mandiblesof insects in general. He perceives an analogy between those of this Order and the teeth of quadrupeds; and therefore divides them intoincisiveor cutting,laniaryor canine, andmolaryor grinding teeth. He denominates thoseincisivesthat are broad, having in some degree the shape of a wedge, their external surface being convex, and their internal concave; whence they are evidently formed for cutting. Thelaniariesare those which have a conical shape, are often very acute, and in general the longest of any; and in some insects, as the carnivorousOrthoptera(and theLibellulina), they cross each other. Themolariesare the largest of all, and their purpose is evidently to grind the food. There is never only a single one to each mandible, while the number of the incisives and laniaries is very variable. As the molaries act the principal part in mastication, they are nearer the inner base of the mandible or point of support: they serve to grind the food, which has been first divided by the incisives or torn by the laniaries. The carnivorous tribes are destitute of them; in the omnivorous ones they are very small, and in the herbivorous ones they are very large[1249]. So that in some measure you may conjecture the food of the animal from the teeth that arm its mandibles. Ofincisiveteeth you may find an example in those that arm the end of the mandibles of most grasshoppers (Locusta), and of the leaf-cutter-bees (MegachileLatr.)[1250]; of thelaniaryor canine teeth, you will find good examples in the mandibles of the dragon-flies (Libellulina); the two external teeth of theapex of those of the leaf-cutter bees may be regarded as between the incisives and laniaries; and the pointed mandibles without teeth may be deemed as terminating in a laniary one[1251]. The lower part of the inner or concave surface of the mandibles of grasshoppers will supply you with instances of themolaryteeth, and the apex, also, of those of some weevils, asCurculio HancockiK.[1252]But the most remarkable example of a molary organ is exhibited by many of the Lamellicorn beetles, especially those that feed upon vegetables, whether flower or leaf.—Knoch, who indeed was the first who proposed calling mandibles according to their teeth, incisive, laniary, or molary, but who does not explain his system clearly, observed that the mandibles of someMelolonthæhave a projection with transverse, deep furrows, resembling a file, for the purpose of bruising the leaves they feed upon[1253]: and M. Cuvier, long after, observed that the larvæ of the stag-beetle have towards their base a flat, striated, molary surface; though he does not appear to have noticed it in any perfect insect[1254]. This structure, with the exception of theScarabæidæandCetoniadæ, seems to extend very generally through the above tribe; since it may be traced even inGeotrupes, the common dung-chafer,in which at the base of one mandible is a concave molary surface, and in the other a convex one, but without any furrows: a circumstance that often distinguishes those that have furrows.—In theDynastidæthe affinity of structure with theMelolonthidæ&c. is more pronounced, the furrows to which ridges in the other mandible correspond being reduced to one or two wide and deep ones; whereas in some of the latter tribe they are very numerous. These mandibles, in many cases, at their apex are furnished with incisive teeth to cut off their food, and with miniature mill-stones to grind it[1255]. The part here alluded to I call theMola.