Chapter XVI: Letter XXXIV: External Anatomy of Insects (1)

THE HEAD, AND ITS PARTS.

Before I confine my observations to the _head_ of insects, which I propose to consider separately in the present letter, I must premise a few words upon their _body_ in general, or rather its _crust_, or external integument. In this we may notice its _substance_, general _form_, _sculpture_, _pubescence_, and _composition_.

i. I have already noticed the _substance_ of this integument in the preparatory states of insects[1151]; I shall not, therefore, here repeat what I then said, but restrict myself chiefly to the consideration of it as it is found in their _last_ state, in which it is usually firmer than in their previous stages of existence. In this respect, however, it varies much in the different Orders, and even in the different genera of the same Order. In some Coleopterous insects, for instance, it is very hard, and difficult to perforate; while in others it is soft, flexible, and a pin easily passes through it[1152]. And in general, from a substance in hardness resembling horn or shell, it passes through the intermediate degrees of that of leather and parchment, almost to a thin membrane. Yet in all cases there is enough of rigidity and hardness to answer the principal uses of a skeleton--to afford, namely, a sufficient point of attachment for the muscles, and to support and defend the interior organization; so that the play and action of the vital and secretory systems may not be interrupted or impeded.

With respect to the principles which enter into the composition of this integument, very little seems to be known at present; but few insects having been submitted to a chemical analysis. The blister-beetle (_Cantharis vesicatoria_), from its importance in medicine, has, however, been more than once analysed; and though the products have not been very precisely stated, yet we find amongst them phosphate of lime, albumen, and some other usual components of the substance of vertebrate animals[1153]. But which of these products belong to the integument, and which to its contents, cannot be ascertained, without a separate process for each; which would not, I conceive, be very feasible. The substance, however, of the integument of insects, though we know not its precise contents, which probably vary in different genera, &c., appears not to be exactly of the nature of any of those substances after which it has usually been denominated: it is not properly analogous either to real _horn_, _shell_, _skin_, or _leather_, &c. This seems to result from the following circumstance:--Most of the excretions of vertebrate animals, as horn, skin (at least when tanned), feathers, wool, hair, &c. when exposed to the action of fire liquify, more or less, before they incinerate; emitting at the same time a peculiar and disagreeable scent: but upon applying this test to the parts of insects of the different Orders, I found, in every instance, that incineration took place without liquefaction, and was unaccompanied by that peculiar scent which distinguishes the others. Even the _claws_, which to the eye appear, as to their substance, exactly like those of _Mammalia_, birds, &c. burn without melting, and retain their form after red heat. That the insect integument is not _calcareous_ like that of the _Crustacea_, and the shells of _Molluscæ_, you may easily satisfy yourself, by immersing them in an acid test. I made this experiment upon portions of insects of several of the Orders, in an equal mixture of muriatic acid and water, and the result was, not only that all hexapods, but octopods, _Arachnida_, and even _Scolopendridæ_, upon immersion only emitted a few air-bubbles; while, when the other myriapods, _Polydesmus_, _Iulus_, _Glomeris_, &c. and the _Oniscidæ_, were immersed, a violent effervescence took place; proving the different nature of their substance. It is remarkable that the two great branches of the _Myriapods_, the _Scolopendridæ_ and _Iulidæ_ (_Chilopoda_ and _Chilognatha_ Latr.), should in this respect be so differently circumstanced--the latter having a calcareous integument, and the former not.--A further difference distinguishes these two tribes: _old_ specimens of the _Iulidæ_ usually lose their colour and turn white, like _Oniscidæ_; while those of the _Scolopendridæ_ retain it.

ii. The _form_ of insects is so variable, that it can be reduced to no other general rules--than that, for the most part, the length exceeds the breadth, and the breadth the depth, and that the upper surface is usually convex. But to these rules there are numerous exceptions. Thus many _Tetyræ_ F. (_Scutellera_ Latr.), a kind of bug, are as broad as they are long[1154]; in the genus _Gonyleptes_ K.[1155] amongst the _Aptera_, and _Epeira cancriformis_, a crab-shaped spider, the breadth exceeds the length; in _Cynips_, and several other _Hymenoptera_, in _Acrida_ K.[1156] (_Locusta_ F.), and other Orthopterous insects, the depth exceeds the breadth; and in that singular beetle, _Eurychora_; the cockroach (_Blatta_), &c. the upper surface is flat.

iii. The _sculpture_ of the integument of insects is often very remarkable; but as this will call for attention hereafter, I shall only here observe in general, that ornament and variety seem not to be the sole object of those elevations and depressions which form so prominent a feature of many of the animals in question; for by means of these, many important purposes, that at first sight do not strike the observer, may be served: such as giving firmness to the crust in those places where it is most wanted; diminishing its powers of resistance in others, so that it may yield somewhat to the action of the muscles; increasing or deducting from the weight of the body, so as to produce a proper equipoise during its motions, whether on the earth, in the air, or in the water. The depressions of the outer surface of the crust, in many instances, produce an elevation of it in the interior, and so afford a useful point of attachment to certain muscles. This observation seems more especially applicable to those excavations that are common to particular tribes or genera: thus the dorsal longitudinal channel to be met with on the prothorax of most of the _Carabi_ of Linné on the inside of the crust have a corresponding ridge. In _Locusta Dux_, also, (a Brazil locust,) the same part has _four_ transverse channels, corresponding with which on the inside are as many _septa_, or ridges, to which muscles are attached; and those larger impressed puncta denominated _puncta ordinaria_, which distinguish the same part in _Geotrupes_ and many of the _Scarabæidæ_, within are elevated, so as to form a kind of ginglymous articulation with the base of the anterior coxæ. The other impressed puncta so often to be seen on the different parts of various insects, which sometimes so intirely cover the surface that scarcely any interval is discoverable between them, though in many cases they appear to be mere impressions that attenuate but do not perforate the crust--yet in others, perhaps equally or more numerous, they are real _pores_, which pass through the integument. If, for instance, you take the thoracic shield of the cockchafer (_Melolontha vulgaris_), and after removing the muscle &c., hold it against the light, with the inner side towards the eye, you will see the light through every puncture: or take the elytra of _Geotrupes stercorarius_, or any common beetle in which these organs have punctate striæ, and examine them under a lens on the inside, and you will see distinctly that the punctures pass through the elytrum, and the membrane that lines it[1157]. It is not improbable that in the case last mentioned these pores may be of use, as the spiracles are usually closely covered by the elytra, for the better transmission of the air to those respiratory organs. Whether the pores in the other parts of the body are for transpiration, is more than I shall venture to affirm; but as insects sometimes perspire, at least this has been ascertained with respect to the hive-bee[1158], this must be by the means of some pores.

iv. The integument of insects is often clothed, either partially or generally, with _pubescence_, or hairs of various kinds--a circumstance which seems to have more than one object. In _Parnus_, _Heterocerus_, _Gerris_, _Argyroneta aquatica_, and some other aquatic insects, the end in view seems to be to keep the water from wetting the crust; and in this case the covering of hairs is dense, silky, and decumbent. Another object is preventing friction from being injurious: thus humble-bees, that from their mode of nidification[1159], are usually more particularly exposed to it, are well clothed with hair; and in those articulations of insects where much friction takes place, we may often observe a dense fringe or coating of the same substance. This you may see in the common stag-beetle (_Lucanus Cervus_), where the thorax receives the head; and very remarkably at the same point in the Hercules-beetle (_Dynastes Hercules_ MacLeay): but besides these uses, there is probably one more universal, which will apply as well to those thinly scattered bristles and hairs, here and there one, to be noticed in many insects: but concerning this I can only throw out a conjecture, as I do not recollect ever to have seen any experiments with regard to this use of animal hairs. But may they not act as conductors, either to introduce some invisible fluid into the body in a positive state, or to convey it out, when received by other means, in a negative state? Every one knows that the fur of a cat has electric properties, and there may be an important general use of this kind attached to the fur and hairs of animals[1160]. But, as I said, I give this as a mere conjecture; and only wish it may excite your attention to the subject, and put in exercise your natural tact for experiment.

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 the _composition_ of the integument; under which term I would include the different layers of which it consists, and its articulation.

1. With respect to the _first_ of these circumstances, the _layers_ of which the integument consists, it seems to exhibit some, although not an exact, analogy with the _skin_, rather than the _skeleton_, of the vertebrate animals[1162]. In these last, the skin is stated to consist of _four_ layers. Of these the exterior one is the _epidermis_, or scarf-skin: under this is the _rete mucosum_, or mucous tissue, which gives its colour to the skin; next follows the _papillary tissue_ formed by the extremities of the nerves, and in which the sense of _touch_ principally resides; the last and innermost layer is the _skin proper_, or leather, called _Dermis_, _Derma_, or _Corium_[1163]. _Two_ of these layers M. Cuvier assigns to insects. They have, he observes, in every state, a true _epidermis_[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 the _rete mucosum_. To this, dried and mixed with their horny substance, he attributes also the colours of the perfect insect: "for," says he, "when the _Lepidoptera_ are 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 the _Arachnida_," 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 the _Coleoptera_, and many other Orders, the colours of the skin are mixed in its horny tissue, nearly as those of the _Testacea_ are in their calcareous shells"[1166]. In the perfect insects, therefore, in most cases, we may consider the _epidermis_ and _rete mucosum_ as together forming the exterior and coloured integument of insects--that part which in the table, since it is not properly an _epidermis_, I have distinguished by the name of _Exoderma_.

The learned author just quoted has observed nothing under the skin of white-blooded animals that he regards as analogous to _nervous papillæ_[1167]. In some parts of insects, as in the lamellæ of the antennæ of the _Petalocera_, 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 of _touch_ resides, there must, even in insects, be something of a _papillary tissue_.

With regard to the innermost integument of the vertebrate animals,--the _leather_, 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, or _exoderma_, in the _Coleoptera_ and 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 of _elytra_[1169], but is not discoverable in _tegmina_. It appears to consist, in many cases, of several layers of a whitish membrane, and generally breaks into fibres. In some elytra of the larger _Dynastidæ_, 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 the _Esoderma_) 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 the _articulation_ of 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 the _nose_ in _Vespa_, &c. the head in the _Arachnida_. 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 proper _suture_; for though they exhibit the appearance both of the _harmonic_ and _squamose_ (_ecailleuse_ Cuv.) sutures[1171], yet these parts being all limited by the _esoderma_, 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 proper _Enarthrosis_, or ball and socket; though the anterior coxæ of the Capricorn-beetles (_Cerambyx_ L.) 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 the _Vertebrata_. 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.

The _number_ of 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 the _head_ and its _parts_.

I. The _Head_ of insects, as the principal seat of the organs of sensation, must be regarded in them, as well as in the vertebrate animals, as the _governing_ part of the body. It may be considered with respect to its _substance_, _figure_, _composition_, _superficies_, _proportion_, _direction_, _articulation_ with the _trunk_, _motions_--and more particularly as to its _parts_ and _appendages_.

i. With regard to its _substance_--the head may be said in general to be the _hardest_ part 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 parts _inter 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 the _figure_ or 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. The _composition_ of 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 the _Arachnida_, &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 its _superficies_, 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 many _Hymenoptera_, _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 its _proportion_, 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 the _direction_ of 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 in _Coleoptera_; but in some, as _Mylabris_, it is inflexed, forming an acute angle with it. In the Heteropterous _Hemiptera_ (_Cimex_ L. &c.) it is generally in the same line with the body, or horizontal; and in many _Diptera_ it is vertical.

vii. We now come to a circumstance which will detain us longer, namely, its _articulation with the trunk_, or rather with its anterior segment, the _prothorax_.--M. Cuvier makes _two_ principal 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. The _first_ of these kinds of articulation--that by the contact of solid parts--takes place, he says, in _four_ different 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. The _second_ mode 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 in _Curculio_, _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.--The _third_ sort 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 the _Hymenoptera_ and the majority of the _Diptera_. The disposition of the _fourth_ kind 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 of _Attelabus_ F. 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. The _second_ kind of articulation, the ligamentous, he affirms takes place only in _orthopterous_ and some _neuropterous_ insects: "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 the above. "_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:"

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----"

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 the _Orthoptera_, this part is _suspended_ by 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 the organs that are transmitted from the head through the latter part to the rest of the body. Though the head in most _Orthoptera_ is not partly received into the cavity of the prothorax, as is the case in the Order _Coleoptera_, 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 his _first_ mode 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 in _Oryctes nasicornis_[1178], and perhaps in others of the _Dynastidæ_ MacLeay, 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, the _Lamellicorns_, 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 this tribe; and in the _Capricorns_ you would find that the general conformation in this respect belonged to our learned author's _second_ mode of solid articulation, resembling that of the majority of the weevils (_Curculio_ L.), in which the head has no projecting angles or tubercles, or other elevation, but is received usually into the circular cavity of the prothorax.

His _third_ mode of this articulation, that of the _Hymenoptera_ and _Diptera_, is so peculiar, that it ought to be considered as a _primary_ kind; since in this the head moves _upon_ the prothorax as upon a pivot, and has a kind of versatile motion.

With regard to his _fourth_ mode, which from his description appears to be that of _Apoderus_ Oliv., he allows motion to the head only in _one_ direction, observing that the lower margin of the prothoracic cavity has a notch. But M. Latreille calls the articulation of the head in this genus an _Enarthrosis_[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 motion _upwards_.

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 three _primary_ sections, each admitting one or more subdivisions.

1. The _first_ consists 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 common _Harpalus_ that 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 the _Coleoptera_, the Heteropterous _Hemiptera_ (_Cimex_ L., &c.), and some of the _Neuroptera_ (_Raphidia_, _Semblis_, &c.).--It may be further divided into _two_ subsections--those, namely, whose head inosculates in the prothorax by means of a _neck_: as for instance Latreille's _Trachelides_, _Apoderus_, and the _Staphylinidæ_, amongst the beetles; the _Reduviadæ_ amongst the _Heteropterous_ insects, and _Raphidia_ in the _Neuroptera_; and those whose head inosculates in the prothorax without the intervention of a neck; as, the _Petalocera_, the aquatic beetles (_Dytiscus_, _Hydrophilus_, &c.), and most of the genus _Curculio_ L. in the first of these orders, the great body of the _Cimicidæ_ in the second, and _Semblis_, _Corydalis_, &c. in the third.

2. The _second_ section consists of those insects whose head does not inosculate in the chest, but is merely suspended to it by ligament or membrane. To this belong most of the tribes of the _Orthoptera_ Order, with the exception of the _Mantidæ_, the _Dermaptera_, the _Homopterous Hemiptera_, and such of the _Aptera_ as have the head distinct from the prothorax.--This section admits of a _triple_ subdivision: those, namely, whose head is _wholly_ covered by the shield of the prothorax, as in _Blatta_ L.; those whose head is _partly_ covered by it, as _Gryllotalpa_, and other _Gryllina_; and those whose head is quite free, not being at all impeded in its motion by the prothorax, as the _Dermaptera_, _Nirmus_, _Pediculus_, &c.

3. The _third_ section 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 the _Lepidoptera_, _Hymenoptera_, _Diptera_, the _Libellulina_, &c. in the _Neuroptera_, and the _Mantidæ_ in the _Orthoptera_. _Three_ subsections 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 any _neck_. To this belong the _Lepidoptera_, _Trichoptera_? The _second_ is of those the upper side of whose thoracic neck is _ligamentous_; and here you may range most of the _Hymenoptera_. The _third_ is of those in whom it is a _continuation_ of the ordinary _integument_. In this subsection the _Diptera_, _Libellulina_ and _Mantidæ_ will 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 the _articulation_ of the head, I must direct your attention to the _analogies_ that hold in this respect between the different tribes. Thus the _Coleoptera_ are analogous to the _Heteropterous Hemiptera_; the _Orthoptera_, with the exception of the _Mantidæ_, to the _Homopterous Hemiptera_; the _Mantidæ_ to the _Libellulina_; the _Lepidoptera_ to the _Trichoptera_; the _Hymenoptera_ to the _Diptera_, with a slight variation, and probably others might be traced.

viii. A word or two upon the _motions_ of 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 the _Harpali_, _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 the _third_ section of articulations, in which the head moves _upon_ the thorax as upon a pivot, as is the case with _Hymenoptera_ and _Diptera_, 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 the _Mantidæ_ were endued with the same faculty.

ix. The _parts_ and _appendages_ of the head are now in the last place to be considered. I shall begin with the _Mouth_, or rather the orifice in which the _trophi_ or organs of manducation are inserted. In some insects, as was before observed, they occupy all the under-side of the head, as in the _Arachnida_, _Myriapoda_, &c; but in the great majority they fill an orifice in its anterior part, which in some instances, as in _Lampyris_, the _Lepidoptera_, _Cimex_ L., _Truxalis_, appears to be nearly under the head; but in general it terminates that part, though it extends further below than above. In _Chermes_, a Homopterous genus, the promuscis is stated to be in the _Antepectus_, and consequently the _mouth_; but, as I shall endeavour to prove to you hereafter, this is a fallacy. In the males of the species of _Coccus_ there 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, the _Hemiptera_, _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 in _Apis_, _Vespa_[1182], &c., and is different in the masticators and suckers.

* * * * *

I shall next offer a few observations _seriatim_, as they stand in the Table, upon the organs of manducation; which, to avoid circumlocution, instead of _Instrumenta cibaria_, the name Fabricius gave them, I shall call _trophi_ or _feeders_. 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 their _trophi_ in general, insects of late have been divided into two great tribes[1183], _masticators_ and _suckers_; the _first_ including those that are furnished with instruments to separate and _masticate_ their food; namely, an upper- and under-lip (_labrum_ and _labium_), upper- and under-jaws (_mandibulæ_ and _maxillæ_), labial and maxillary palpi, and a tongue (_lingua_): and the _second_ those 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 it _sucks_ its juices. If, however, the mode in which insects take their food be strictly considered, it will be found that in this view they ought rather to be regarded as forming _three_ tribes; for the great majority of the _Hymenoptera_ order, 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 denominated _lappers_.

When a mouth is furnished with the _seven_ ordinary 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 a _perfect 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 an _imperfect mouth_. This last I would further distinguish, according to the nature of its _trophi_, by other and more distinctive terms, as I shall presently explain to you.

1. _Labrum_[1184].--I shall first consider the organs present in a _perfect mouth_, beginning with the _upper-lip_ (_labrum_). This part, which Fabricius sometimes confounded with the _nose_, miscalling it _clypeus_, 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 shelly _substance_; yet in some cases, as in _Copris_ and _Cetonia_, beetles that imbibe juices, it is membranous. In _form_ and 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 intire _margin_, but it is not seldom emarginate or bilobed, or even dentate. Its _surface_ is 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 its _pubescence_, 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 in _Halictus_, a little bee, in the females of which it is furnished with a slender appendage[1187].--The _direction_ of 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. The _use_ of 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 the _trophi_ properly folded; and in some cases where it is inflexed, as in the leaf-cutter bees (_Megachile_ Latr.), 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 or _maxillæ_ are 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 and _primary_ organs, 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 the _maxillæ_, as the _labium_[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 the _Hymenoptera_ he calls this part _tongue_, _under-lip_, and _proboscis_: but seems to prefer the last term[1193]. Fabricius originally regarded the whole middle piece as a _labium_[1194]; but afterwards (though his definition is not accurate, since he assigns the _palpi_ to the _ligula_, which he affirms is covered by the _labium_--circumstances by no means universal in _Coleoptera_) he considers this as consisting of _ligula_ and _labium_[1195]. Latreille at first regarded the _ligula_ of Fabricius as the _labium_, and called the _labium_ of that author the _mentum_[1196]; but afterwards he gave the name of _labium_ to the whole middle piece of the lower apparatus of the mouth--calling the upper piece, with Fabricius, the _ligula_, and retaining the denomination of _mentum_ for the lower[1197].

If the circumstances of the case are duly considered, I think you will be convinced that the term under-lip, or _labium_, 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 or _labrum_? 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 (_Scarabæus_ and _Lucanus_ L.), this is precisely the case. In the Predaceous beetles, indeed, (_Cicindela_, _Carabus_, _Dytiscus_ L. &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 miscalled _mentum_ is still the analogue of that part which helps to close the mouth in the former tribe. Should not this, therefore, which so often performs the _office_, be distinguished by the _name_, of a lip? Again, is it not rather incongruous to consider that organ which confessedly more or less performs the office of a _tongue_, as a part of the _lip_? Though it often wears that appearance, yet I believe, if the matter is 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 the _labium_; 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 the _labium_, or the middle piece of the lower oral apparatus, appears to consist of _two_ joints: this you may see in the great water-beetle (_Hydrophilus piceus_), the burying-beetles (_Necrophorus_), the Orthopterous tribes[1198], the _Hymenoptera_[1199], and others. In this case the upper or terminal piece is to be regarded as the _labium_, and the lower or basal one (which Mr. MacLeay calls the _stipes_) as the _mentum_ or 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, the _mentum_ merging in it, may be denominated the _labium_.

With respect to its _substance_, the _labium_ in most Coleopterous insects is hard and horny, in _Necrophorus_ it is membranous, and the _mentum_ harder; in _Prionus coriarius_, our largest Capricorn-beetle, both are membranous; in the bee-tribes, _Apis_ L., the _labium_ rather resembles leather, while the _mentum_ is hard. Its _surface_ is often convex, sometimes plane, and sometimes even concave; as for instance in _Melolontha 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, both _labium_ and _mentum_ are perfectly naked; in others, as in the common cockchafer, they are hairy; in _Geniates barbatus_ Kirby, another chafer in the male insect, the _labium_ is naked, while the _mentum_, which forms a piece distinct from that part, is covered with a dense rigid beard[1200]. In _shape_ the whole _labium_ varies considerably, much more than the _labrum_; 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 (_Scarabæus_ L.), in some, as in the _Rutelidæ_, the _labium_ is 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 the _labium_ of _Cremastocheilus_ Knoch, a scarce North American beetle; in another related to this, but of an African type (_Genuchus_ Kirby MS. _Cetonia cruenta_ F.), it is a trapezoid plate, which is elevated from the head, and hangs over the throat like a chin[1203]. In the _Hymenoptera_ it 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 the _labium_ exceeds its breadth; but in the Predaceous beetles the reverse of this 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].

The _mentum_ taken by itself affords no very striking characters to which I need call your attention: I shall only observe, that in _Hymenoptera_ it is generally of a triangular shape[1206]; but before I proceed to consider the labial palpi, it will be proper to notice the remarkable _labium_ of _Orthopterous_ insects, and of the _Libellulina_, 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, or _ligula_ F.; as it is indeed generally regarded by modern Entomologists[1207]. It seems, like the tongue of the _Carabi_ L., _Dytisci_, &c., to be a distinct piece, which has below it both _labium_ and _mentum_; 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 the _paraglossæ_ in bees[1209]. In a specimen of _Acrida viridissima_ (_Locusta_ F.) 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 the _Libellulina_ a similar organ is discoverable[1210], which on its upper-side terminates in the _pharynx_, like that of one of the _Harpalidæ_ before mentioned. In the _Orthoptera_, therefore, I regard the _labium_ as consisting of three articulations, the upper one divided into two, three, or more lobes[1211]; the intermediate one more directly answering to the _labium_ of other insects, and the basal one or _mentum_. This organ in the _Libellulina_ is of a different structure: it has only two articulations representing _labium_ and _mentum_; 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 the _labium_[1212].

3. _Palpi Labiales_[1213].--The last-mentioned organs, the _labial palpi_, next claim our attention; but before I advert particularly to them, it will be proper to premise a few words upon _palpi_, or feelers, in general. These are usually jointed moveable organs, of a corneous or coriaceous substance, attached by ligaments to the _labium_ and _maxillæ_, and in the _Crustacea_ even to the _mandibulæ_. 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 nervous _papillæ_ which 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 seems restrained, at least in some, to two directions, towards and from the mouth. They were called _palpi_ or 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 the _palpi_ of insects and _Crustacea_, unless, perhaps, the _cirri_ that emerge from the lips of some fishes, as the cod, red mullet, &c. which Linné defines as used in exploring (_prætentantes_). Whether the _vibrissæ_, miscalled _smellers_, 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.