Chapter XVIII: Letter XXXIV: External Anatomy of Insects (3)

The following is the structure, as to its organs, of the mouth of the myriapods, as exhibited by the centipedes (_Scolopendridæ_). The part which appears to perform the office of the upper lip (but which M. Savigny regards as the nose, calling it the _chaperon_,) is a transverse piece with a deep anterior sinus, in the centre of which is a minute tooth[1349]. This piece is separated from the forepart of the head by a suture; but it probably is not moveable: however, it covers the mouth, and may be regarded rather as analogous to the _labrum_. Below this are two mandibles, armed at their end with five sharp triangular teeth[1350], under which are the _maxillæ_, terminating in a moveable concavo-convex lobe, resembling the valve of a bivalve shell[1351]; and between them is the _labium_, of a rhomboidal shape, divisible into two lobes, attached laterally to the maxillæ: these lobes M. Savigny terms the _second maxillæ_, forming with the others, according to him, the _labium_[1352]. Affixed to the base of this labium, or covering it on the outside, are a pair of pediform palpi, which he considers as the first auxiliary _labium_, and representative of the first pair of legs of hexapods and _Iuli_[1353]. I imagine them to be also the analogues, in some degree, of the labial palpi of a perfect mouth. The last of the organs in question is a large rhomboidal plate affixed to the first apparent segment of the trunk, crowned at its vertex with two truncated denticulated teeth, and from the upper sides of which emerge a pair of moveable organs terminating in a powerful incurved claw, and which entirely covers all the other parts of the mouth[1354]. This, M. Savigny deems as a _second_ auxiliary _labium_, and the lateral organs of prehension,--which may be regarded each as a kind of maxillary hand, and as the only representatives in this tribe of the maxillary palpi, though widely different,--he looks upon as really analogous to the _second_ pair of legs in _Iulus_ and the hexapods[1355]. These two pairs of pedipalpes (to use an expressive French term) show their relation to legs by their general structure, and their analogy with palpi by their use as _oral_ organs, though belonging to the _trunk_: so that here we see the _legs_ and their appendages assume a material function in _manducation_, forming a singular contrast to what we had observed before with regard to _mandibles_ becoming instruments of _locomotion_. The mouth of the _Iulidæ_, with little variation, is upon the same plan[1356] with those here described.

The next type of form with regard to the oral organs is that of the _Arachnida_. In these, as you know, the head is confounded with the trunk; so that they are a kind of Blemmyes in the insect world. Their organs of manducation, amongst which there is no _labrum_ or upper lip, are, in the first place, a pair of mandibles planted close and parallel to each other in the anterior part of the head, which they terminate. In the spiders they consist of two tubular joints, of which the first is much the largest, more or less conical or cylindrical, and armed underneath with a double row of stout teeth; and the terminal one is more solid and harder, in the form of a very sharp crooked claw, which in inaction is folded on the first joint between the teeth. Under its extremity on the outside is a minute orifice, destined to transmit a venomous fluid, which is conducted there by an internal canal from the base of the first joint, where is the poison-bag[1357]. In the scorpion and harvest-man (_Phalangium_) the mandible consists of two joints terminated by a _chela_ or double claw, the exterior one being moveable[1358].--M. Latreille, as has been before observed, regards these not as representatives of the mandibles of hexapods, but as replacing the interior pair of antennæ, in the situation of which they are precisely placed, of the _Crustacea_[1359]: and M. Savigny is of opinion that the _Arachnida_ may in some sort be defined as _Crustacea_ without a head, and with twelve legs, of which the two first pair are converted into _mandibles_ and _maxillæ_[1360]. From the _situation_ of the organs in question, the first of these opinions seems preferable; but the conversion of the legs in other cases, at least the _coxæ_, into organs of manducation, gives some weight to the last. With regard to their _use_, it is said to be to retain the insect which the animal has seized, and to facilitate the compression which the maxillæ exercise upon it for the extraction of the nutritive matter[1361]. If this be correct, _in this respect_ the mandibles may be said to represent the _maxillæ_ of the mandibulate hexapods; and, _vice versa_, the _sciatic_ maxillæ, as they have been denominated[1362], of the _Arachnida_, their mandibles. The palpi are pediform, and the first joint of the _coxa_, or hip, acts the part of a _maxilla_:--this is composed of a single piece or plate, more or less oval or triangular, sometimes straight and sometimes inclined to the _labium_, with the interior extremity very hairy. The _labium_ consists also of a single piece, and is only an appendage of the anterior extremity of the breast. The interior of the mouth, or palate, presents a fleshy, hairy, linguiform piece, which is usually applied to the internal face of the _labium_. An opening is supposed to exist in its sides, for the transmission of the alimentary juices[1363]. If you examine the under side of the body of a scorpion, you will find that not only the palpi, but the two anterior pair of legs, by means of their _coxæ_, are concerned in manducation: so that these insects have in fact _three_ pairs of maxillæ--a circumstance that M. Savigny has observed to take place also in the harvest-men (_Phalangium_ L.)[1364]. The _palpi_ of the scorpion, which may be called its _hands_, like the anterior legs of the lobster and crab, terminate in a tremendous _chela_ or forceps, consisting of a large triangular joint, armed at the end with a double claw internally toothed; the _exterior_ one of which, contrary to what takes place in the animals just named, is moveable, and not the _interior_[1365].

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Having given you this full account of the _trophi_ of those animals that have all the organs of manducation developed, I must next advert to those in which one part receives an increment at the expense of others, and the whole oral machine is fitted for _suction_; or where some parts appear to be deficient, so that this may be called an _imperfect_ mouth. At first sight one would regard the trophi of a _bee_ as of this description; but this is not the case, since it has all the ordinary organs, though the tongue is unusually long, and looks as if it was made for suction; which, however, as you have been informed, is not the case.

There are _five_ kinds of _imperfect_ mouth to be met with in insects that take their food by suction, each of which I shall distinguish by a separate denomination. The first is that of the _Hemiptera_ Order:--this I term the _Promuscis_; the second is that of the _Diptera_, which with Linné I call _Proboscis_; the third, peculiar to the _Lepidoptera_, is with me an _Antlia_; the fourth, which I name _Rostrulum_, is confined to the _Aphaniptera_ order, or genus _Pulex_ L.; and the last is _Rostellum_, which I employ to denote the suctory organs of the louse tribe (_Pediculidæ_).

i. _Promuscis_[1366].--The organ we are first to consider has usually been denominated _Rostrum_: but since that term is likewise in general use for the snout of insects of the weevil tribes (_Curculio_ L.), I think you will concur with me in adopting the one here proposed, for the very different oral instruments of the _Hemiptera_. Illiger has employed _promuscis_ to denote those of _bees_[1367]: but since, as I have just observed, they consist of all the ordinary organs, they seem to require no separate denomination: the term, therefore, may be applied to represent a different set of _trophi_, without any risk of producing confusion. This part consists of _five_ pieces: viz. a minute, long, conical piece, commonly very slender, which covers the base of the _promuscis_, and represents the _labrum_[1368]; a jointed sheath (_vagina_), consisting of either three or four joints, the analogue of the _labium_, and four slender rigid lancets (_scalpella_), the two exterior ones, according to M. Savigny, representing the _mandibles_, and the intermediate pair the _maxillæ_[1369]. By the union of these four pieces a suctorious tube is formed, which the animal inserts into the substance, whether animal or vegetable, the juices of which form its nutriment. These pieces are dilated at their base, and serrated at their apex; and the two central ones, though at their origin they are asunder, form one tube, which has often been mistaken for a single piece. A _pharynx_ and _tongue_ have been discovered by M. Savigny in this apparatus; who thinks that in _Nepa_ there are also rudiments, but very indistinct, of _labial palpi_: so that the _maxillary palpi_ seem to be the only part absolutely wanting[1370].

The _Promuscis_ when at rest is usually laid between the legs; but when employed, in most cases its direction is outward. In the genus _Chermes_ L. (_Psylla_ Latr.) the origin of the _promuscis_ has been supposed to be in the _breast_; but if closely examined, this anomaly in nature will be found not to exist. If you take one of these insects, the first thing that strikes you upon inspecting the head, is a pair of remarkable conical processes into which the front appears to be divided. Look below these, and you will there discover the upper-lip: and from this you may follow the _promuscis_ till it gets beyond the forelegs, when it takes a direction perpendicular to the body[1371]; a circumstance which has given rise to the above false notion. Though in _Coccus_, _Chermes_, &c. this instrument is short, in some _Aphides_ it is longer in proportion than in any other insect. In _A. Quercus_ it is three times the length of the body; so that when folded, it stretches out beyond it, and looks like a long tail[1372]; and in _A. Abietis_ it even exceeds that length[1373].

ii. _Proboscis_[1374].--Linné long since, and after him Fabricius, has employed this term to designate the oral instruments, or rather their sheath, in the _Muscidæ_ and some others, calling the same organ, when without fleshy lips, _rostrum_ and _haustellum_: but as the parts of the mouth in all true _Diptera_ (for _Hippobosca_ and its affinities can scarcely be deemed as co-ordinate with the rest), are analogous to each other; although in some they are stiff and rigid, in others flexile and soft, and in _Œstrus_ (except the palpi) mere rudiments,--the same appellation ought to designate them all. I am happy to find that M. Latreille agrees with me in this opinion; and to his sensible observations on this head, if you wish for further information, I refer you[1375]. The mouth of Dipterous insects appears to vary in the number of pieces that it presents; but in all, the _theca_ or sheath is present, which represents the _labium_ (including the _mentum_) of the mandibulate Orders[1376]. It consists of _three_ joints, the last of which is formed by the liplets (_Labella_). Those in the _Muscidæ_ are large, turgid, vesiculose, and capable of dilatation; in the _Bombylidæ_ and other tribes they are small, slender, long and leathery, and sometimes recurved. The second joint or stalk, which may be said to represent the _mentum_, the liplets being properly in a restricted sense the analogue of the _labium_, its sides being turned up, forms a longitudinal cavity, which contains the _haustellum_. The upper piece of this, the _valvula_, is long, rigid, and very sharp, representing the _labrum_[1377]. Beneath this cover, in the above cavity, are the lancets; which, as far as they are at present known, vary in number and form: sometimes there are _five_ of them, sometimes _four_, sometimes _two_, and sometimes, it should seem, only _one_[1378]. In the gnat (_Culex_) they are finer than a hair, very sharp, and barbed occasionally on one side[1379]; in the horse-fly (_Tabanus_ L.) they are flat and sharp like the blade of a knife or lancet[1380]. In this tribe the upper pair, or the knives (_Cultelli_), represent the _mandibles_; the lower pair, or the lancets (_Scalpella_), usually palpigerous, the _maxillæ_; and the central one the _tongue_. In the horse-fly Reaumur has figured only _four_, exclusive of the _labrum_ and _labium_; but in a specimen I have preserved there appear to be _five_, one of which, as slender as a hair, I regard as the analogue of the tongue[1381].--When the lancets are reduced to two, they probably represent the _maxillæ_, the mandibles being absorbed in the _labrum_; and where there is only one, the maxillæ also are absorbed by the _labium_, which then bears the palpi, the lancet representing the tongue[1382]. The lancets are so constructed in many cases, as to be able by their union to form a tube proper for suction, or rather for forcing the fluid by the pressure of the lower parts to the _pharynx_[1383]. _Labial palpi_ appear not usually present in the _proboscis_; but M. Savigny thinks he has discovered vestiges of them in _Tabanus_[1384]. In this genus the maxillary ones are large, and consist of _two_ joints[1385]. The proboscis is often so folded, as to form two elbows; the base forming an angle with the stalk, and the latter with the lips, so as in shape to represent the letter Z, only that the upper angle points to the breast, and the lower one to the mouth: this is the case with the flesh-fly and many others. In other flies, as _Conops_ and _Stomoxys_, whose punctures on our legs so torment us[1386], there is only a single fold, with its angle to the breast. The _proboscis_ is received in a large oblong cavity of the underside of the anterior part of the head.

It may here be observed, that in the _promuscis_ the elongation of the organs seems to be made chiefly at the expense of _all_ the palpi, but in the _proboscis_ at that of the _labial_ only; and in some cases at that also of the _mandibles_ or _maxillæ_;--the former merging in the _labrum_ and the latter in the _labium_.

iii. _Antlia_[1387].--The _third_ kind of imperfect mouth is that of the _Lepidoptera_, which I have called _Antlia_. Fabricius denominates it _lingua_: but as this organ has no analogy with the real tongue of insects, this is confessedly improper, and it appeared necessary therefore to exchange it for another denomination: I have endeavoured to apply a term to it that indicates its use--to pump up, namely, the nectar of the flowers into the mouth of the insect. On a former occasion I described to you the structure of this instrument[1388]; but further discoveries with regard to it having since been made by MM. Savigny and Latreille, I shall here give you the result of their observations. The former of these able physiologists has detected in the mouth of the _Lepidoptera_ rudiments of almost all the parts of a perfect mouth. Of the correctness of this assertion you may satisfy yourself, if you consult his admirable elucidatory plates, and compare them with the insects. Just above the origin of the spiral tongue or pump, the head is a little prominent and rounded; and immediately below the middle of this prominence there is a very minute, membranous, triangular or semicircular piece; which from its position, as covering the base of the _antlia_, may be regarded as the rudiment of the _upper-lip_ (_labrum_)[1389]. On each side of the outer base of the _antlia_ is another small immoveable piece, resembling a flattened tubercle, the end of which is internally hairy or scaly: these pieces appear to represent the _mandibles_[1390]. Near the base of each half of the _antlia_, just below a sinus, may be distinctly seen the minute, usually biarticulate rudiment of a _maxillary palpus_[1391]; demonstrating to a certainty that these spiral organs, at least their lateral tubes or _Solenaria_, are real maxillæ[1392]. The rudiment of the _under-lip_ (_Labium_) is the almost horny triangular piece united by membrane to the two stalks of the maxillæ, and supporting at its base the recurved labial palpi; which are so well known that I need not enlarge upon them[1393]. Amongst these parts there seems at first sight no representative of the _tongue_; but M. Latreille has advanced some very ingenious, and I think satisfactory arguments[1394], which go to prove that this part, at least the tongue of _Hymenoptera_, has its analogue in the intermediate tube or _Fistula_ formed by the union of the two maxillæ, and which conveys the fluid aliment of this Order to the _pharynx_. As in _Diptera_ the _maxillæ_ sometimes merge in the _labium_, so here the _tongue_ (as it were divided longitudinally) merges in the _maxillæ_. He further observes, that in a transverse section of the maxilla of the death's-head hawk-moth (_Sphinx Atropos_), the lateral tube appeared to be divided into two by a membranous partition, and to contain in the upper cavity a small cylindrical tube, which seemed to be a _trachea_[1395]. To animals that are without lungs, and breathe by _tracheæ_, suction must be performed in a very different way from what it is by those that breathe by the _mouth_: and as in the very extended organs in question the fluid has a long space to pass before it reaches the _pharynx_, in some way or other these lateral tubes may have the power of producing a vacuum in the middle tube, and so facilitate its passage thither. We see, in the _antlia_, that the maxillæ receive their vast elongation at the expense of all the other organs, except the _labial palpi_.

iv. _Rostrulum_[1396].--An animal very annoying to us affords the type of the next kind of imperfect mouth--I mean the _flea_. Its oral apparatus, which I would name _rostrulum_, appears to consist of _seven_ pieces. First are a pair of triangular organs, the _laminæ_, which together somewhat resemble the beak of a bird, and are affixed, one on each side of the mouth, under the antennæ: these represent the _mandibles_ of a perfect mouth[1397]. Next, a pair of long sharp lancets (_Scalpella_), which emerge from the head below the laminæ: these are analogous to _maxillæ_[1398]: a pair of palpi, consisting of four joints, are attached to these near their base[1399], which of course are _maxillary palpi_. And lastly, in the midst of all is a slender setiform organ (_ligula_), which is the counterpart of the _tongue_[1400]. Rösel, and after him Latreille, seem to have overlooked this last piece, since they reckon only _six_ pieces in the flea's mouth[1401]: but the hand and eye of our friend Curtis have detected a _seventh_, as you see in his figure. From this account it appears, that the elongation of the organs of the _Aphaniptera_ Order is at the expense of the _labium_ and its _palpi_.

v. _Rostellum._--So little is known of the composition of the next kind of imperfect mouth, that I need not enlarge upon it. It is peculiar to the louse tribe (_Pediculidæ_), and it consists of the tubulet (_Tubulus_), and siphuncle (_Siphunculus_). The former is slenderer in the middle than at the base and apex, the latter being turgid, rather spherical, and armed with claws which probably lay hold of the skin while the animal is engaged in suction. When not used, the whole machine is withdrawn within the head; the siphuncle, which is the suctorious part, being first retracted within the tubulet, in the same way as a snail retracts its _tentacula_[1402]. This apparatus seems formed at the expense of all the other organs.

There are some other kinds of imperfect mouth, which, though they seem not to merit each a distinct denomination, should not be passed altogether without notice. The first I shall mention is that of the family of _Pupipara_ Latr. (_Hippobosca_ L.). It consists of a pair of hairy coriaceous valves, which include a very slender rigid tube or siphuncle, the instrument of suction, which Latreille describes as formed by the union of two setiform pieces[1403]. In _Melophagus_, the sheep-louse, the union of the valves of the sheath is so short, that they appear like a tube; but if cut off they will separate, and show the siphuncle, as fine as a hair, between them. This organ is of a type so dissimilar, as was before observed, to that of the _Diptera_ in general, and approaches so near to that of the dog-tick (_Ixodes_), that they may be deemed rather apterous insects with two wings, than to belong to that Order; and the circumstance that some of the family are apterous confirms this idea. In fact they are a _transition_ family that connects the two Orders, but are nearest to the _Aptera_. In _Nycteribia_ the oral organs differ from those of the other _Pupipara_ in having _palpi_. This also is the case with those of the genus _Ixodes_, the palpi of which are placed upon the same base with the instrument of suction, than which they are longer: they appear to consist of _two_ joints, the last very long and flat. The instrument of suction itself is formed by three hard rigid laminæ; two shorter parallel ones above, that cover the third, which is longer and broader, and armed on each side with several teeth like a saw, having their points towards the base[1404]. Many of the other _Acari_ L. have mandibles, and several have not: but their oral organs have not yet been sufficiently examined; and from the extreme minuteness of most of them, this is no easy task; nor to ascertain in what points they differ or agree.

If you consider the general plan of the organs of manducation in the vertebrate animals, how few are the variations that it admits! An upper and a lower jaw planted with teeth, or a beak consisting of an upper or a lower mandible with a central tongue, form its principal features. But in the little world of insects, how wonderful and infinite is the diversity which, as you see, in this respect they exhibit! Consider the number of the organs, the varying forms of each in the different tribes, adjusted for nice variations in their uses:--how gradual, too, the transition from one to another! how one set of instruments is adapted to prepare the food for deglutition by mastication; another merely to lacerate it, so that its juices can be expressed; a third to lap a fluid aliment; a fourth to imbibe it by suction--and you will see and acknowledge in all the hand of an almighty and all-bountiful CREATOR, and glorify his wisdom, power, and goodness, so conspicuously manifested in the structure of the meanest of his creatures. You will see also, that all things are created after a pre-conceived plan; in which there is a regular and measured transition from one form to another, not only with respect to beings themselves, but also to their organs--no new organ being produced without a gradual approach to it; so that scarcely any change takes place that is violent and unexpected, and for which the way is not prepared by intermediate gradations. And when you further consider, that every being, with its every organ, is exactly fitted for its functions; and that every being has an office assigned, upon the due execution of which the welfare, in certain respects, of this whole system depends, you will clearly perceive that this whole plan, intire in all its parts, must have been coeval with the Creation; and that all the species,--subject to those variations only that climate and different food produce,--have remained essentially the same, or they would not have answered the end for which they were made, from that time to this.

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Having given you this particular account of the _trophi_ or organs of the mouth of insects, I must now make some observations upon the _other_ parts of the head. I have divided it, as you see in the Table, into _face_ and _subface_; the former including its _upper_ and the latter its _lower_ surface. Strictly speaking, some parts of the face, as the temples and cheeks, are common to both surfaces; but I do not therefore reckon them as belonging to the subface, which, exclusive of the mouth and its organs, consists only of the _throat_, and where there is a neck, the _gula_.

i. _Nasus_[1405].--I shall consider the parts of the face in the order in which they stand in the Table, beginning with the _nasus_ or nose. Fabricius has denominated this part the _clypeus_, in which he has been followed by most modern Entomologists. You may therefore think, perhaps, that I have here unnecessarily altered a term so generally adopted, and expect that I assign some sufficient reasons for such a change. I have before hinted that there is good ground for thinking that the sense of _smell_ in insects resides somewhere in the vicinity of this part; and when I come to treat of their senses, I shall produce at large those arguments that have induced me to adopt this opinion: and if I can make out this satisfactorily, you will readily allow the propriety of the denomination. I shall here only state those _secondary_ reasons for the term, which, in my idea, prove that it is much more to the purpose than _clypeus_. This last word was originally applied by Linné in a metaphorical sense to the ample covering of the head of the _Scarabæidæ_, and the thoracic shield of _Silpha_, _Cassida_, _Lampyris_, and _Blatta_: in all which cases there was a propriety in the figurative use of it, because of the resemblance of the parts so illustrated to a _shield_. But when _Fabricius_ (though he sometimes employs the term, as Linné did, merely for illustration,) admitted it into his orismological table, as a term to represent universally the anterior part of the face of insects to which the labrum is attached (though in some cases he designates the _labrum_ itself by this name), it became extremely inappropriate; since in every case, except that of the _Scarabæidæ_, the part has no pretension to be called a _shield_;--so that the term is rather calculated to mislead than illustrate. This impropriety seems at length to have struck M. Latreille, since in a late essay[1406] he has changed the name of this part to _Epistomis_, a term signifying _the part above the mouth_. But there are reasons, exclusive of those hereafter to be produced concerning the sense of smell, which seem to me to prove that _nasus_ is a preferable term; not to mention its claim of priority, as having been used to signify this part a century ago[1407]. When we come to consider the terms for the other parts of the head, as _lips_, _jaws_, _tongue_, _eyes_, _temples_, _cheeks_, _forehead_, &c. the concinnity, if I may so speak, and harmony of our technical language, seem to require that the part analogous in point of situation to the _nose_ of vertebrate animals should bear the same name. And any person who had never examined an insect before, if asked to point out the _nose_ of the animal, would immediately cast his eye upon this part: so that one of the principal uses of imposing names upon parts--that they might be more readily known--would be attained. If it is objected, that calling a part a _nose_ that has not the sense of smell, supposing it to be so, might lead to mistakes--I would answer, that this objection is not regarded as valid in other cases: for instance, the _maxillæ_ are not generally used as _jaws_, and yet no one objects to the term; because, from their situation, they evidently have an analogy to the organs whose name they bear. But enough on this subject--we will now consider the part itself.

To enable you to distinguish the nose of insects when it is not separated from the rest of the face by an impressed line, you must observe that it is the terminal middle part that sometimes overhangs the upper-lip, and at others is nearly in the same line with it; that on each side of it are the cheeks, which run from the anterior half of the eyes to the base of the mandibles. Just below the antennæ is sometimes another part distinct from the nose, which I shall soon have to mention; so that the nose must not be regarded as reaching always nearly to the base or insertion of the antennæ, since it sometimes occupies only half the space between them and the upper-lip, which space is marked out by an impressed line. But you will not always be left at such uncertainty when you want to ascertain the limits of the nose; for it is in many cases a distinct piece, separated by an elevated or impressed line from the rest of the face. This separation is either partial or universal. Take any species of the genera _Copris_, _Onitis_, or _Ateuchus_, and you will see the nose marked out in the centre of the anterior part of the face by two _elevated_ lines, forming nearly a triangle and bounded by the horn[1408]. Or take a common wasp or hornet, and you will find a similar space, though approaching to a quadrangular figure, marked out by _impressed_ lines[1409]. In _Rhagio_ and _Sciara_, two Dipterous genera, this impression is so deep as to look like a suture. Between these lines, in those cases, is included what I call the _nose_. As to _substance_, in general it does not differ from the rest of the head; but in the _Cleridæ_ it is almost membranous. You must observe, that in all these, what at first sight appears to be the termination of the front, is not the nose, but the narrow depressed piece that intervenes between it and the lip. With regard to its _clothing_, it is most commonly naked, but in some genera it is covered with hair; in _Crabro_ F. often with golden or silver pile, which imparts a singular brilliance to the mouth of the insects of that genus: M. Latreille supposes that the brilliant colours of the golden-wasp (_Chrysis_ L.) may dazzle their enemies, and so _promote_ their escape[1410]; the brilliance of the mouth of the _Crabro_ may on the contrary at first dazzle their prey for a moment, so as to _prevent_ their escape. The _form_ of the nose, where distinct from the rest of the face, admits of several variations: thus in the _Staphylinidæ_ and _Cleridæ_ it is transverse and linear; in _Copris_ it is triangular, with the vertex of the triangle truncated; in _Vespa Crabro_ it is subquadrate and sinuated. In many Heteromerous beetles[1411] it is rounded posteriorly: in _Pelecotoma_, a new genus in this tribe, related to _Asida_, there is a deep anterior sinus; in _Blaps_ the anterior margin is concave; in _Cetonia_[1412] _Brownii_, and _atropunctata_ (forming a distinct subgenus), it is bifid: it varies in the _Scarabæidæ_, in some being bidentate, in others quadridentate, and in others again sexdentate, including the cheeks: in _Mylabris_, a kind of blister-beetle, it is transverse and nearly oval; in _Lamia_, a capricorn-beetle, it represents a parallelogram; and in most _Orthoptera_ it is subtriangular: in _Tettigonia_ F. it is prominent, transversely furrowed, and divided by a longitudinal channel: in _Otiocerus_ K. it presents the longitudinal section of a cone[1413]: in the _Diptera_ Order, with the exception of the _Tipulidæ_ and some others, in which it unites with the cheeks, &c. to form a rostrum, the nose in general, as to _form_, answers to its name, resembling that of many of the _Mammalia_: in some of the _Asilidæ_ it is very tumid at the end, and terminates in a sinus, to permit the passage of the proboscis to and fro: in many of the _Syrphidæ_, &c. it is first flat and depressed, and then is suddenly elevated, so as to give the animal's head the air of that of a monkey: in some tribes, as _Rhingia_, _Nemotelus_, _Eristalis_, &c., in conjunction with the cheeks it forms a conical rostrum: in _Tabanus bovinus_, and other horse-flies, it terminates in three angles or teeth. Many more forms might be mentioned, but these will suffice to give you a general idea of them. In _size_ and _proportions_ the nose also varies. It is frequently, as in _Tettigonia_, the most conspicuous part of the face, both for size and characters; but in the _Staphylinidæ_ it is very small, and often scarcely discernible, being overshadowed by its ample front: and it may be observed in general, that when the antennæ approximate the mouth, as in this genus and many others, the _front_ becomes _ample_, and the _nose_ is reduced to its _minimum_: but when they are distant from the mouth, the reverse takes place; and the nose is at its _maximum_ and the front at its _minimum_. _Mutilla_, _Myrmecodes_, _Scolia_, &c. in the _Hymenoptera_, are an example of the former; and the _Pompilidæ_, _Sphecidæ_, _Vespidæ_, &c. of the latter. In _Myopa buccata_, &c. its length exceeds its width; but more commonly the reverse takes place. The _circumscription_ of the _nose_ also deserves attention. It is usually terminated behind by the front (_frons_), or, where it exists, by the _postnasus_, in the sides by the cheeks, and anteriorly by the _labrum_. But this is not invariably the case; for in the _Cimicidæ_, in which the cheeks form the bed of the _Promuscis_, the front embraces it on each side by means of two lateral processes, that sometimes meet or lap over each other anteriorly, which gives the nose the appearance of being insulated; but it really dips below these lobes to join the _labrum_. This structure you may see in _Edessa_ F., and many other bugs. This part sometimes has its _arms_. Thus in _Copris_, and many _Dynastidæ_, the horns of the head seem, in part at least, to belong to this portion of it; in _Tipula oleracea_ (the crane-fly), &c. it terminates before in a horizontal mucro. In _Osmia cornuta_, a kind of wild-bee, each side of the nose is armed with a vertical horn. The _margin_ of the nose in most Lamellicorn insects, though mostly level, curves upwards.

I am next to mention a part of the nose which merits a distinct name and notice, which I conceive in some sort to be analogous to the _nostrils_ of quadrupeds, and which I have therefore named the _Rhinarium_ or nostril-piece. I had originally distinguished it by the plural term _nares_, nostrils; but as it is usually a single piece, I thought it best to denote it by one in the singular. When I treat of the senses of insects, I shall give you my reasons, as I have before said, for considering this part as the organ of scent, or connected with it, which you will then be able to appreciate. I shall only here observe, that the piece in question is in the usual situation of the nostrils--between the nose and the lip. In a large number of insects this part may be regarded as nearly obsolete; or at least it is merely represented by the very narrow membranous line that intervenes between the nose and the lip and connects them; which, as in the case of the head of _Harpali_ before noticed, may be capable of tension and relaxation, and so present a greater surface to the action of the atmosphere. But I offer this as mere conjecture. In the lady-bird (_Coccinella_) this line is a little wider, and becomes a distinct _Rhinarium_; as it does also in _Geotrupes_. With respect to its _insertion_, the _rhinarium_ is a piece that either entirely separates the nose from the lip, or only partially: the former is the most common structure. It is particularly remarkable in a New Holland genus of chafers (_Anoplognathus_ Leach). In _A. viridiæncus_ it is very ample, and forms the under side of the recurved nose, so that a large space intervenes between the margin of the latter and the base of the _labrum_. In _Macropus_ Thunb., of the Capricorn tribe (_Cerambyx_ L.), the nostril-piece, which forms a distinct segment, is narrower than the nose, and the upper-lip than the nostril-piece, forming as it were a triple gradation from the front to the mouth. Again, in others the part in question is received into a sinus of the nose. This is the case with the dragon-flies (_Libellulina_), in which this sinus is very wide; in the burying-beetle (_Necrophorus_)[1414], in some species of which it is deep but narrow; and in a species of _Tenebrio_ from New Holland, which perhaps would make a subgenus. If you examine with a common glass any of the larger rove-beetles (_Staphylinidæ_), you will find that the nose itself seems lost in the nostril-piece, both together forming a very narrow line across the head above the _labrum_, without any apparent distinction between them; but if you have recourse to a higher magnifier, you will find this divided into an upper and lower part, the former of the hard substance of the rest of the head, and the latter membranous. I once was of opinion that the prominent transversely furrowed part, so conspicuous in the face of _Tettigonia_ F.[1415], was the _front_: but upon considering the situation of this, chiefly below the eyes and antennæ, and comparing it with the analogous piece in _Fulgora laternaria_ and other insects of the Homopterous section of the _Hemiptera_, I incline to think that it represents the _nose_, and that the longitudinal ridge below it is the nostril-piece[1416]. In the Heteropterous section it is merely the vertical termination of their narrow nose. In other insects again, this part approaches in some measure to the common idea of nostrils; there being _two_, either one on each side the nose, or two approximated ones. If you catch the first humble-bee that you see busy upon a flower, you will discover a minute membranous protuberance under each angle of the nose. Something similar may be observed in some species of _Asilus_ L. In the _Orthoptera_, especially in _Blatta_, _Phasma_, and some _Locustæ_, two roundish or square pieces, close to each other on the lower part of the nose, represent the nostrils[1417].--With regard to _substance_, in the chafer-tribes, at least those that feed on leaves or living vegetable matter, as the _Melolonthidæ_, _Anoplognathidæ_, and in many other insects, the _rhinarium_ is of the same substance with the rest of the head; but in _Macropus_ Thunb., _Staphylinus_, _Necrophorus_, &c., it consists of membrane.

ii. _Postnasus_[1418].--This is a part that appears to have been confounded by Entomologists with the front of insects; in general, indeed, it may be regarded as included in the nose, and does not require separate notice: but there are many cases in which it is distinctly marked out and set by itself, and in which it forms a useful diagnostic of genera or subgenera. There is a very splendid and beautiful Chinese beetle, to be seen in most collections of foreign insects (_Sagra purpurea_), in which this part forms a striking feature, and helps to distinguish the genus from its near neighbour _Donacia_. If you examine its face, you will discover a triangular piece, below the antennæ and above the _nasus_, separated from the latter and from the front by a deeply-impressed line: this is the _postnasus_ or _after-nose_. Again: if you examine any specimens of a Hymenopterous genus called by Fabricius _Prosopis_ (_Hylæus_ Latr.), remarkable for its scent of baum, you will find a similar triangle marked out in a similar situation[1419]. In many Coleopterous insects, besides _Sagra_, you will discover traces of the part we are considering: as in _Anthia_, _Dytiscus_, and several others of the Predaceous beetles. In _Cistela_ it is larger than the nose itself; but it is more conspicuous in the _Orthoptera_, particularly in _Locusta_ (_Gryllus_ F.), in which it is the space below the antennæ, distinguished by two or four rather diverging ridges[1420]. In the _Libellulina_, _Myrmeleonina_, &c. it is a distinct transverse piece. In _Dasyga_ Latr., a kind of bee, it is armed with a transverse ridge or horn--But enough has been said to render you acquainted with it; I shall therefore proceed to the next piece.

iii. _Frons_[1421].--The _Front_ of insects may be denominated the _middle_ part of the face between the eyes, bounded anteriorly by the nose, or after-nose, where it exists, and the cheeks; laterally by the eyes; and posteriorly by the vertex. Speaking properly, it is the region of the _antennæ_; though when these organs are placed before the eyes, under the margin of the nose, as in many Lamellicorn and Heteromerous beetles, they seem to be rather _nasal_ than _frontal_. This part is often elevated, as in the elastic beetles (_Elater_), whose faculty of jumping, by means of a pectoral spring, has been related to you[1422]. In _Anthia_, a Predaceous beetle, it has often three longitudinal ridges. In many of the Capricorn beetles (_Cerambyx_ L.), it is nearly in the shape of a Calvary cross, with the arms forming an obtuse angle, and then terminating at the sinus of the eyes in an elevation for the site of the antennæ. In the ants also (_Formicidæ_), the front is often elevated between those organs. In _Ponera_, one tribe of them, this elevation is bilobed, and receives between its lobes the vertex of the _postnasus_. In the hornet (_Vespa Crabro_) the elevation is a triangle, with its vertex towards the mouth. In _Sagra_ it is marked out into three triangles, the _postnasus_ making a fourth, with the vertexes meeting in the centre. In the _Dynastidæ_ and _Scarabæidæ_ the horns are often _frontal_ appendages, as is that of _Empusa_ Latr., a leaf-insect, and probably those of _Sphinx Iatrophæ_ F., which affords a singular instance of a horned Lepidopterous one. Sometimes it is an ample space, reducing the nose to a very narrow line, as in the _Staphylinidæ_, or sending forth a lobe on each side, as before mentioned, which embraces the nose. In a species of bug from Brazil, related to _Aradus_ F., these lobes are dilated, foliaceous, and meet before the nose, so as to form a remarkable extended frontlet to the head. In others this part is extremely minute: thus in many _male_ flies and other insects, as the _Libellulina_, where the eyes touch each other, the front is cut off from the vertex and reduced to a small angle. In the female flies the communication with the vertex is kept open, and the front consequently longer. In the horse-flies (_Tabanidæ_), in _Hæmatopota_, and _Heptatoma_, the frontal space is wider than in the rest of that tribe. Many of these are distinguished by a levigated area behind the antennæ in the part we are treating of. In the _Libellulina_, and in the drone-bee, whose eyes are confluent, the stemmata are in the front. In many _Orthoptera_ also, as _Locusta_ Leach, one of them is below the antennæ; and in the lanthorn-fly tribe (_Fulgoridæ_), both these organs, which are situate between them and the eyes, as they do also in _Truxalis_, appear to be in it[1423]. In this tribe the rostrum is an elongation of the part in question; and perhaps you would think at first that what I have considered as the _nose_ in _Tettigonia_ F. was also a tendency to this kind of rostrum; but if you examine the great lanthorn-fly (_Fulgora laternaria_), you will find besides, at the lower base of the lanthorn, a triangular piece analogous to the nose of _Tettigonia_, and below it another representing its nostril-piece:--the horizontal part of the nose in that genus may perhaps be regarded as part of the front. In _Truxalis_ F. the face consists of a supine and prone surface, and the latter is composed of the front, after-nose, nose, and organs of the mouth. I may notice here a most remarkable and singular tribe of bugs, of which two species have been figured by Stoll[1424]: in these the head, or rather those parts of it that we have now been describing, the nose, namely, the after-nose, and front, are absolutely divided longitudinally in two, each half having an eye and antenna planted in it; or perhaps, as it is stated to be divided in one instance to the commencement of the _promuscis_, the nose is left intire, and dips down, as in cases before alluded to: so that in this the nose appears to leave the lobes of the front, which in others embrace its sides.

iv. _Vertex_[1425].--We now come to the _vertex_, or crown of the head; which is situated behind the front, and, except where the communication is intercepted by confluent eyes, adjoins it. It is laterally bounded by the hind part of the eyes and the temples; and _posteriorly_, where that part exists, by the _occiput_. The vertex may be denominated the ordinary region of the _stemmata_: for though in several cases, as we have just seen, one or more of them are planted in the front; yet this in the great majority, especially in the _Hymenoptera_, is their natural station. In _Blatta_ and some other _Orthoptera_ the posterior angle of the head is the vertex. In many dung-chafers of Latreille's genus _Onthophagus_, which are said to have _occipital_ horns, as _O. nutans_, _nuchicornis_, _Xiphias_, &c., the horn really arms the part I regard as the vertex. In _Locusta_ Leach, this part is very ample, and in _Truxalis_ very long; but more generally it is small, and not requiring particular notice.

v. _Occiput_[1426].--The _occiput_, or _hind-head_, is that part of the face that either forms an angle with the vertex posteriorly, or slopes downwards from it. It has for its lateral boundaries the temples, and behind it is either terminated by the orifice of the head, or in many cases by the neck. In those beetles that have no neck, as the Lamellicorn and Capricorn, the hind-head is merely a declivity from the vertex, usually concealed by the shield of the thorax, very lubricous, to facilitate its motion in the cavity of that part, and at its posterior margin distinguished by one or two notches, which I shall notice hereafter, for the attachment of the levator muscles: but in those beetles or other insects that have a neck, or a versatile head, the occiput forms an angle with the vertex, often rounded, and sometimes acute. This structure may be seen in Latreille's _Trachelides_, and several other beetles. In the _Hymenoptera_, _Diptera_, and others with a versatile head, the part now under consideration curves inwards from the vertical line, so as with the temples and under parts of the head to form a concavity adapted to its movement upon the trunk.

vi. _Genæ_[1427].--The _cheeks_ of insects (_Genæ_) usually surround the anterior part of the eyes, and lie between them and the mandibles or their representatives. Where they approach the latter, as in the Predaceous beetles (_Cicindela_, _Carabus_ L. &c.), they are very short, and of course longer where the eyes are further removed from the mouth; as in the Rhyncophorous beetles (_Curculio_ L.), where they form the sides of the rostrum, and often contain a channel which receives the first joint of the antennæ, when they are unemployed. In the _Scarabæidæ_ and many other Lamellicorn beetles, their separation on each side from the nose is marked by a ridge[1428]; and in the wasps (_Vespa_) by an impressed line or channel. In an African tribe at present arranged with _Cetonia_ F., to which _C. bicornis_ Latr.[1429] and another, which he has named, I believe, _C. vitticollis_, belong, the cheeks are porrected on each side of the mouth into a horizontal horn. These horns have at first the aspect of a pair of open mandibles. In the magnificent _Goliathi_ Lam., the horns of the male are rather a process of the _cheek_ than of the _nose_. In _Alurnus_, _Hispa_, and other beetles, these parts, by their elevation and conjunction with the lower side of the head, form a kind of fence which surrounds and protects the oral organs; in many _Cimicidæ_, by a similar elevation of the cheeks, the bed of the _promuscis_ is formed. In the Homopterous _Hemiptera_ they run parallel nearly with the _rhinarium_ or nostril-piece. In the _Hymenoptera_ they are almost always ample, but they are confined to the lower side of the eye. In _Sirex grandis_, and others of that genus, the cheek at the base of the mandible is dilated so as to form a rounded tooth below it. In the Capricorn-beetles it is considerable, and sometimes terminates, at the base of the mandible, in two or three notches. In _Scaurus_ and _Eurychora_, darkling-beetles, the cheek below projects into a lobe that covers the base of the _maxilla_. But the animal distinguished by the most remarkable cheeks is a species of _Phryganea_ L. (_Phryganea personata_ Spence); for from this part projects a spoon-shaped process, which curves upwards, and uniting with that of the other cheek, forms an ample mask before the face, the anterior and upper margin of which, in the insect's natural state, are closely united; and the posterior part being applied to the anterior part of the eye, causes the face to appear much swoln. It looks as if it was a single piece; but upon pressing the thorax it opens, both above and in front, into two parts, each convex without and hollow within, and each having attached to its inside a yellow tuft of hair resembling a feather. The use of this machinery at present remains a mystery[1430].

vii. _Tempora_[1431].--The temples (_Tempora_) are merely a continuation of the cheeks to the posterior limit of the head, forming its sides and posterior angles, and including the hinder part of the eyes, the vertex, and the occiput. They seldom exhibit any tangible character, except in certain ants (_Atta_ Latr.), in which their angle terminates in one or two strong spines, giving the animal a most ferocious aspect; and in that remarkable genus _Corydalis_ they are armed below with a tooth or point, which was not overlooked by De Geer[1432].

viii. _Oculi_[1433].--I must now call your attention to organs of more importance and interest, and which indeed include a world of wonders: I mean the eyes (_Oculi_) of insects. These differ widely from those of vertebrate animals, being incapable of motion. They may be regarded as of _three_ descriptions--_simple_, _conglomerate_, and _compound_.

1. _Simple Eyes_[1434]. We will consider them as to their _number_, _structure_, _shape_, _colour_, _magnitude_, _situation_, and _arrangement_.

As to their _number_, they vary from two to sixteen. In the flea, the louse, the harvest-man (_Phalangium_), there are only _a pair_; in the bird-louse of the goose (_Nirmus Anseris_), and probably in others of the same genus, there are _four_[1435]; in some spiders (_Scytodes_, _Dysdera_, and _Segestria_ Latr.[1436]), and some scorpions[1437], there are _six_. In the majority of spiders and _Scolopendra morsitans_, _Scorpio maurus_, &c. there are _eight_; and in _Podura_ and _Sminthurus_ Latr. there are sixteen[1438].

As to their _structure_, nothing seems to have been ascertained; probably their organization does not materially differ from that of one of the lenses of a compound eye; which I shall soon explain to you.

Their _colour_ in the many is black and shining, but in the bird-louse of the goose they are quite white and transparent. In spiders they are often of a sapphirine colour, and clear as crystal. In _Scolopendra morsitans_ and many spiders, scorpions, and _phalangia_[1439], they appear to consist of iris and pupil, which gives them a fierce glare, the centre of the eye being dark and the circumference paler. In the celebrated _Tarantula_ (_Lycosa Tarantula_), the pupil is transparent, and red as a ruby; and the iris more opaque, paler, and nearly the colour of amber.

Where there are more than two, they vary in _magnitude_. In the enormous bird-spider (_Mygale avicularia_) the four external eyes are larger than the four internal[1440]; but in the Tarantula and _Sphasus_, the _two_ or _four_ internal are the largest. In _Clubiona_ and _Drassus_ they are all nearly of the same size[1441]; and in the _Micrommata_ family they are very small[1442].

They vary also in _shape_. In _Scolopendra morsitans_ the three anterior ones are round, and the posterior one transverse, and somewhat triangular. In _Mygale calpeiana_, a spider, the two smallest are round and the rest oval[1443]. In the trapdoor or mason spider (_Mygale cæmentaria_), the four small internal ones are round, and the large external ones oval[1444]; and those that are circumscribed posteriorly with an impressed semicircle, are shaped like the moon when gibbous[1445].