iv.Rostrulum[1396].—An animal very annoying to us affords the type of the next kind of imperfect mouth—I mean theflea. Its oral apparatus, which I would namerostrulum, appears to consist ofsevenpieces. First are a pair of triangular organs, thelaminæ, which together somewhat resemble the beak of a bird, and are affixed, one on each side of the mouth, under the antennæ: these represent themandiblesof a perfect mouth[1397]. Next, a pair of long sharp lancets (Scalpella), which emerge from the head below the laminæ: these are analogous tomaxillæ[1398]: a pair of palpi, consisting of four joints, are attached to these near their base[1399], which of course aremaxillary palpi. And lastly, in the midst of all is a slender setiform organ (ligula), which is the counterpart of thetongue[1400]. Rösel, and after him Latreille, seem tohave overlooked this last piece, since they reckon onlysixpieces in the flea's mouth[1401]: but the hand and eye of our friend Curtis have detected aseventh, as you see in his figure. From this account it appears, that the elongation of the organs of theAphanipteraOrder is at the expense of thelabiumand itspalpi.
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 When not used, the whole machine is withdrawn within the head; the siphuncle, which is the suctorious part, being first retracted within the tubulet, in the same way as a snail retracts itstentacula[1402].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 ofPupiparaLatr. (HippoboscaL.). 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 setiformpieces[1403]. InMelophagus, 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 theDipterain 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 atransitionfamily that connects the two Orders, but are nearest to theAptera. InNycteribiathe oral organs differ from those of the otherPupiparain havingpalpi. This also is the case with those of the genusIxodes, the palpi of which are placed upon the same base with the instrument of suction, than which they are longer: they appear to consist oftwojoints, 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 otherAcariL. 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 thevariations 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-bountifulCreator, 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 climateand 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.
Having given you this particular account of thetrophior organs of the mouth of insects, I must now make some observations upon theotherparts of the head. I have divided it, as you see in the Table, intofaceandsubface; the former including itsupperand the latter itslowersurface. 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 thethroat, and where there is a neck, thegula.
i.Nasus[1405].—I shall consider the parts of the face in the order in which they stand in the Table, beginning with thenasusor nose. Fabricius has denominated this part theclypeus, 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 ofsmellin 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 thosesecondaryreasons for theterm, which, in my idea, prove that it is much more to the purpose thanclypeus. This last word was originally applied by Linné in a metaphorical sense to the ample covering of the head of theScarabæidæ, and the thoracic shield ofSilpha,Cassida,Lampyris, andBlatta: in all which cases there was a propriety in the figurative use of it, because of the resemblance of the parts so illustrated to ashield. But whenFabricius(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 thelabrumitself by this name), it became extremely inappropriate; since in every case, except that of theScarabæidæ, the part has no pretension to be called ashield;—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 toEpistomis, a term signifyingthe 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 thatnasusis 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, aslips,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 thenoseof vertebrateanimals should bear the same name. And any person who had never examined an insect before, if asked to point out thenoseof 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 anosethat 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, themaxillæare not generally used asjaws, 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 ofthe generaCopris,Onitis, orAteuchus, and you will see the nose marked out in the centre of the anterior part of the face by twoelevatedlines, 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 byimpressedlines[1409]. InRhagioandSciara, 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 thenose. As tosubstance, in general it does not differ from the rest of the head; but in theCleridæit 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 itsclothing, it is most commonly naked, but in some genera it is covered with hair; inCrabroF. 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 (ChrysisL.) may dazzle their enemies, and sopromotetheir escape[1410]; the brilliance of the mouth of theCrabromay on the contrary at first dazzle their prey for a moment, so as topreventtheir escape. Theformof the nose, where distinct from the rest of the face, admits of several variations: thus in theStaphylinidæandCleridæit is transverse and linear; inCoprisit is triangular, with the vertex of the triangle truncated; inVespa Crabroit is subquadrate and sinuated. In many Heteromerousbeetles[1411]it is rounded posteriorly: inPelecotoma, a new genus in this tribe, related toAsida, there is a deep anterior sinus; inBlapsthe anterior margin is concave; inCetonia[1412]Brownii, andatropunctata(forming a distinct subgenus), it is bifid: it varies in theScarabæidæ, in some being bidentate, in others quadridentate, and in others again sexdentate, including the cheeks: inMylabris, a kind of blister-beetle, it is transverse and nearly oval; inLamia, a capricorn-beetle, it represents a parallelogram; and in mostOrthopterait is subtriangular: inTettigoniaF. it is prominent, transversely furrowed, and divided by a longitudinal channel: inOtiocerusK. it presents the longitudinal section of a cone[1413]: in theDipteraOrder, with the exception of theTipulidæand some others, in which it unites with the cheeks, &c. to form a rostrum, the nose in general, as toform, answers to its name, resembling that of many of theMammalia: in some of theAsilidæit is very tumid at the end, and terminates in a sinus, to permit the passage of the proboscis to and fro: in many of theSyrphidæ, &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, asRhingia,Nemotelus,Eristalis, &c., in conjunction with the cheeks it forms a conical rostrum: inTabanus 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. Insizeandproportionsthe nose also varies.It is frequently, as inTettigonia, the most conspicuous part of the face, both for size and characters; but in theStaphylinidæit 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, thefrontbecomesample, and thenoseis reduced to itsminimum: but when they are distant from the mouth, the reverse takes place; and the nose is at itsmaximumand the front at itsminimum.Mutilla,Myrmecodes,Scolia, &c. in theHymenoptera, are an example of the former; and thePompilidæ,Sphecidæ,Vespidæ, &c. of the latter. InMyopa buccata, &c. its length exceeds its width; but more commonly the reverse takes place. Thecircumscriptionof thenosealso deserves attention. It is usually terminated behind by the front (frons), or, where it exists, by thepostnasus, in the sides by the cheeks, and anteriorly by thelabrum. But this is not invariably the case; for in theCimicidæ, in which the cheeks form the bed of thePromuscis, 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 thelabrum. This structure you may see inEdessaF., and many other bugs. This part sometimes has itsarms. Thus inCopris, and manyDynastidæ, the horns of the head seem, in part at least, to belong to this portion of it; inTipula oleracea(the crane-fly), &c. it terminates before in a horizontal mucro. InOsmia cornuta, a kind of wild-bee, each side of the nose is armed with a vertical horn. Themarginof 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 thenostrilsof quadrupeds, and which I have therefore named theRhinariumor nostril-piece. I had originally distinguished it by the plural termnares, 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 ofHarpalibefore 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 distinctRhinarium; as it does also inGeotrupes. With respect to itsinsertion, therhinariumis 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 (AnoplognathusLeach). InA. viridiæncusit 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 thelabrum. InMacropusThunb., of the Capricorn tribe (CerambyxL.), the nostril-piece, which forms adistinct 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 ofTenebriofrom 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 thelabrum, 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 ofTettigoniaF.[1415], was thefront: but upon considering the situation of this, chiefly below the eyes and antennæ, and comparing it with the analogous piece inFulgora laternariaand other insects of the Homopterous section of theHemiptera, I incline to think that it represents thenose, 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 beingtwo, 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 willdiscover a minute membranous protuberance under each angle of the nose. Something similar may be observed in some species ofAsilusL. In theOrthoptera, especially inBlatta,Phasma, and someLocustæ, two roundish or square pieces, close to each other on the lower part of the nose, represent the nostrils[1417].—With regard tosubstance, in the chafer-tribes, at least those that feed on leaves or living vegetable matter, as theMelolonthidæ,Anoplognathidæ, and in many other insects, therhinariumis of the same substance with the rest of the head; but inMacropusThunb.,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 neighbourDonacia. If you examine its face, you will discover a triangular piece, below the antennæ and above thenasus, separated from the latter and from the front by a deeply-impressed line: this is thepostnasusorafter-nose. Again: if you examine any specimens of a Hymenopterous genus called by FabriciusProsopis(HylæusLatr.), remarkable for its scent of baum, you will find a similar triangle marked out in asimilar situation[1419]. In many Coleopterous insects, besidesSagra, you will discover traces of the part we are considering: as inAnthia,Dytiscus, and several others of the Predaceous beetles. InCistelait is larger than the nose itself; but it is more conspicuous in theOrthoptera, particularly inLocusta(GryllusF.), in which it is the space below the antennæ, distinguished by two or four rather diverging ridges[1420]. In theLibellulina,Myrmeleonina, &c. it is a distinct transverse piece. InDasygaLatr., 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].—TheFrontof insects may be denominated themiddlepart 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 theantennæ; 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 rathernasalthanfrontal. 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]. InAnthia, a Predaceous beetle, it has often three longitudinal ridges. In many of the Capricorn beetles (CerambyxL.), 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. InPonera, one tribe of them, this elevation is bilobed, and receives between its lobes the vertex of thepostnasus. In the hornet (Vespa Crabro) the elevation is a triangle, with its vertex towards the mouth. InSagrait is marked out into three triangles, thepostnasusmaking a fourth, with the vertexes meeting in the centre. In theDynastidæandScarabæidæthe horns are oftenfrontalappendages, as is that ofEmpusaLatr., a leaf-insect, and probably those ofSphinx IatrophæF., 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 theStaphylinidæ, or sending forth a lobe on each side, as before mentioned, which embraces the nose. In a species of bug from Brazil, related toAradusF., 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 manymaleflies and other insects, as theLibellulina, 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), inHæmatopota, andHeptatoma, 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 theLibellulina, and in the drone-bee, whose eyes are confluent, the stemmata are in the front. In manyOrthopteraalso, asLocustaLeach, one of them is below the antennæ; and in the lanthorn-fly tribe (Fulgoridæ), both these organs, which are situate between them andthe eyes, as they do also inTruxalis, 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 thenoseinTettigoniaF. 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 ofTettigonia, 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. InTruxalisF. 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 thepromuscis, 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 thevertex, 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; andposteriorly,where that part exists, by theocciput. The vertex may be denominated the ordinary region of thestemmata: 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 theHymenoptera, is their natural station. InBlattaand some otherOrthopterathe posterior angle of the head is the vertex. In many dung-chafers of Latreille's genusOnthophagus, which are said to haveoccipitalhorns, asO. nutans,nuchicornis,Xiphias, &c., the horn really arms the part I regard as the vertex. InLocustaLeach, this part is very ample, and inTruxalisvery long; but more generally it is small, and not requiring particular notice.
v.Occiput[1426].—Theocciput, orhind-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'sTrachelides, and several other beetles. In theHymenoptera,Diptera, and others with aversatile 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].—Thecheeksof 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,CarabusL. &c.), they are very short, and of course longer where the eyes are further removed from the mouth; as in the Rhyncophorous beetles (CurculioL.), 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 theScarabæidæand 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 withCetoniaF., to whichC. bicornisLatr.[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 magnificentGoliathiLam., the horns of the male are rather a process of thecheekthan of thenose. InAlurnus,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 manyCimicidæ, by a similar elevation of the cheeks, the bed of thepromuscisis formed.In the HomopterousHemipterathey run parallel nearly with therhinariumor nostril-piece. In theHymenopterathey are almost always ample, but they are confined to the lower side of the eye. InSirex 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. InScaurusandEurychora, darkling-beetles, the cheek below projects into a lobe that covers the base of themaxilla. But the animal distinguished by the most remarkable cheeks is a species ofPhryganeaL. (Phryganea personataSpence); 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 thehead, 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 (AttaLatr.), in which their angle terminates in one or two strong spines, giving the animal a most ferocious aspect; and in that remarkable genusCorydalisthey 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 ofthreedescriptions—simple,conglomerate, andcompound.
1.Simple Eyes[1434]. We will consider them as to theirnumber,structure,shape,colour,magnitude,situation, andarrangement.
As to theirnumber, they vary from two to sixteen. In the flea, the louse, the harvest-man (Phalangium), there are onlya pair; in the bird-louse of the goose (Nirmus Anseris), and probably in others of the same genus, there arefour[1435]; in some spiders (Scytodes,Dysdera, andSegestriaLatr.[1436]), and some scorpions[1437], there aresix. In the majority of spiders andScolopendra morsitans,Scorpio maurus, &c. there areeight; and inPoduraandSminthurusLatr. there are sixteen[1438].
As to theirstructure, 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.
Theircolourin 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. InScolopendra morsitansand many spiders, scorpions, andphalangia[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 celebratedTarantula(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 inmagnitude. In the enormous bird-spider (Mygale avicularia) the four external eyes are larger than the four internal[1440]; but in the Tarantula andSphasus, thetwoorfourinternal are the largest. InClubionaandDrassusthey are all nearly of the same size[1441]; and in theMicrommatafamily they are very small[1442].
They vary also inshape. InScolopendra morsitansthe three anterior ones are round, and the posterior one transverse, and somewhat triangular. InMygale calpeiana, a spider, the two smallest are round and the restoval[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].
Thesituationandarrangementof simple eyes are also various. In many they are imbedded, as usual, in the head; but in the little scarlet mite, formerly noticed[1446], (Trombidium holosericeum), they stand upon a small foot-stalk[1447]: the hairiness of this animal might otherwise have impeded its sight. In spiders they are planted on the back of the part that represents the head, sometimes four on a central elevation or tubercle, and the remaining four below it—as inLycosa; sometimes the whole eight are on a tubercle, as inMygale; and sometimes, as in the common garden-spider (Epeira Diadema), upon three tubercles, four on the central one and two on each of the lateral ones. Other variations in this respect might be named in this tribe. In the scorpions a pair are placed one on each side, on a dorsal tubercle, and the other four or six on two lateral ones of the anterior part of the head[1448]. In thePhalangidæthefrontaleyes of the scorpion cease, and only a pair ofdorsalones are inserted vertically in the sides of a horn or tubercle, either bifid or simple, often itself standing upon an elevation which emerges from the back of the animal[1449]. If their eyes were not in a vertical and elevated position, the sight ofthese insects would be very limited; but by means of the structure just stated, they get a considerable range of surrounding objects, as well as of those above them. With regard to thearrangementof the eyes we are considering, it varies much. Sometimes they are placed nearly in the segment of a circle, as in those spiders that havesixeyes only, before noticed[1450]; sometimes in two straight lines[1451]; at others in two segments of a circle[1452]; at others, inthreelines[1453], and at others infour[1454]. Again, in some instances they form a cross, or two triangles[1455]; in others, two squares[1456]; in others, a smaller square included in a large one[1457]; in others, a posterior square and two anterior triangles[1458]; sometimes a square and two lines. Though generally separate from each other, in several cases two of the eyes touch[1459]; and in one instance three coalesce into a triangle[1460]. But it would be endless to mention all the variations, as to arrangement, in the eyes of spiders.
2.Conglomerate Eyes[1461]differ in nothing from simple eyes, except that instead of being dispersed they are collected into a body, so as at first sight to exhibit the appearance of a compound eye:—they are, however, not hexagonal, and are generally convex. They occur inLepisma, theIulidæ, and several of theScolopendridæ. InScolopendra forficatathe eye consists of about twenty contiguous, circular, pellucid lenses, arranged in five lines, with another larger behind them, as a sentinel or scout, placed at some little distance from the main body. In the common millepede (Iulus terrestris) there are twenty-eight of these eyes, placed in seven rows, and forming a triangle, thustriangular eye shape—the posterior row containing seven lenses, the next six, and so on, gradually losing one, till the last terminates in unity. Each of these lenses is umbilicated, or marked with a central depression. InCraspedosomaLeach, you will find a similar formation. InGlomeris zonata, a kind of wood-louse that rolls itself into a ball, the lenses are arranged in a line curved at the lower end, with a single one by itself at the posterior end on the outside; they are oblong and set transversely, and their white hue and transparency give them the appearance of so many minute gems, especially as contrasted with the black colour of the animal[1462]. Between these eyes and the antennæ is another transverse linear white body, but opaque, seemingly set in a socket, and surrounded by a white elevated line, like the bezel of a ring. Whether it is an eye, or what organ, I cannot conjecture[1463]. Its aspect is that of a spiracle.
3.Compound Eyes[1464].—These are the most common kind of eye in hexapod insects, when arrived at their perfect state; in their larva state, as we have seen, their eyes being usually simple[1465]; except, indeed, those whose metamorphosis issemicomplete, which have compound eyesin every state.—In considering compound eyes, I shall advert to theirstructure,number,situation,figure,clothing,colour, andsize.
As to theirstructure,—when seen under the microscope they appear to consist usually of an infinite number of convexhexagonalpieces. If you examine with a good glass the eye of any fly, you will find it traversed by numberless parallel lines, with others equally numerous cutting them at right angles, so as apparently to form myriads of little squares, with each a lens of the above figure set in it. The same structure, though often not so easily seen, obtains in the eyes ofColeopteraand other insects. When the eye is separated and made clean, these hexagons are as clear as crystal. Reaumur fitted one eye to a lens, and could see through it well, but objects were greatly multiplied[1466]. In Coleopterous insects they are of a hard and horny substance; but inDiptera, &c. more soft and membranous. The number of lenses in an eye varies in different insects. Hooke computed those in the eye of a horse-fly to amount to nearly 7,000[1467]; Leeuwenhoeck found more than 12,000 in that of a dragon-fly[1468]; and 17,325 have been counted in that of a butterfly[1469]. But of all insects they seem to be most numerous in the beetles of Mr. W. S. MacLeay's genusDynastes. In the eyes of these the lenses are so small as not to be easily discoverable even under a pocket microscope, except the eye has turned white[1470]: it is not, therefore, wonderful, that Fabriciusshould call these eyessimple[1471]. In some insects, however, as in theStrepsipteraKirby, the lenses are not numerous: inXenosthey do not exceed fifty, and are distinctly visible to the naked eye[1472]. These lenses vary in magnitude, not only in different, but sometimes in the same eyes. This is the case in those of male horse-flies and flies, those of the upper part of the eye being much larger than those of the lower[1473]. The partitions that separate the lenses, or rather bezels, in which they are set, are very visible in the eyes just mentioned, and those ofXenos; but in many insects they are only discernible at the intersecting lines of separation between the lenses. In hairy eyes, such as those of the hive-bee, the hairs emerge from thesesepta. Every single lens of a compound eye may be considered as acornea, or acrystalline humour, it being convex without and concave within, but thicker in the middle than at the margin: it is the only transparent part to be found in these most remarkable eyes. Immediately under thecorneais an opaque varnish, varying according to the species, which produces sometimes in one and the same eye spots or bands of different colours. These spots and bands form a distinguishing ornament of many of theTabaniand other flies. And to this varnish the lace-winged flies (Hemerobius, &c.) are indebted for the beautiful metallic hues that often adorn them. When insects are dead, this varnish frequently loses its colour, and the eye turns white: hence many species are described as havingwhiteeyes which when alive hadblackones. The consistenceof this covering is the same with that of the varnish of thechoroidin the eyes of vertebrate animals; but it entirely covers the underside of the lens, without leaving any passage for the light. Below this varnish there are numbers of short white hexagonal prisms[1474], every one of which enters the concavity of one of the lenses of the cornea, and is only separated from it by the varnish just described: this may be considered as theretinaof the lens to which it is attached; but at present it has not been clearly explained how the light can act upon aretinaof this description through an opaque varnish. Below this multitude of threads (for such the bodies appear), perpendicular to thecornea, is amembranewhich serves them all for a base, and which consequently is nearly parallel with that part. It is very thin, of a black colour, not produced by a varnish; and in it may be seen very fine whitetracheæ, which send forth branches still finer, that penetrate between the prisms of thecornea: this membrane may be called thechoroid. Behind this is a thin expansion of the optic nerve, which is a true nervous membrane, precisely similar to theretinaof red-blooded animals. It appears that the white pyramidal threads which form theretinaof each lens are sent forth by this generalretina, and pierce the choroid by a number of almost imperceptible holes[1475]. From this description it appears that the eyes of insects have nothing corresponding with theuveaorhumoursof those of vertebrate animals, but are of a type peculiar to themselves.
Having explained to you the wonderful and complexstructure with which it has pleased theCreatorto distinguish the organs of vision of these minute beings, proving, what I have so often asserted, that when animals seem approaching to nonentity, where one would expect them to be mostsimple, we find them in many cases mostcomplex, I shall now call your attention to the next thing I am to consider—thenumberof the eyes in question. Most insects have onlytwo; but there are several exceptions to this rule. Those that have occasion to see both above and below the head, the eyes of all being immovable, must have them so placed as to enable them to do this. This end is accomplished in many beetles, for instanceScarabæusL.,HelæusLatr., &c., by having these organs fixed in thesideof the head, so that part looks upward and part downward; but in othersfourare given for this purpose. If you examine the common whirlwig (Gyrinus Natator) that I have so often mentioned[1476], which has occasion, at the same time, to observe objects in the air and in the water, you will find it is gifted with this number of eyes.Lamia Tornator(Cerambyx tetrophthalmusForst.) and some others, of which I make a genus, under the appellation ofTetrops, are also so distinguished. In these insects, one eye is above and the other below the base of the antennæ; in fact, in these thecanthus, instead of dividing the eye partially, as in the other Capricorn-beetles, runs quite through it at considerable width[1477]. InRyssonotusMacLeay(Lucanus nebulosusK.) the eye appears also to be divided in two by thecanthus. In theNeuropteraOrder there is more than one instance of the same kind. InAscalaphusthere are two considerable eyes on each side of the head, which, though clearly distinct, meet like those of many male flies and the drone. The male, likewise, of more than one species ofEphemera, besides the common lateral eyes and thestemmataon the back of the head, have a pair of compound eyes on the top of a short columnar process[1478]. In theHemipteraOrder, also, an instance occurs of four eyes in the genusAleyrodes[1479]. Amongst the vertebrate animals, there is an example of eyes with two pupils inAnableps, a genus of fishes[1480], but no vertebrate animal hasfourof these organs. That many insects should have more thantwoeyes, will not seem to you so extraordinary as that any should be found that, like the Cyclops of old, have onlyone. There is, however, an insect, before celebrated for its agility[1481](Machilis polypodaLatr.), which has a single eye in its forehead; or we may say, its eyes are confluent, without any line of distinction between them except a small notch behind. Now that I am treating of thenumberof eyes, I must not forget to observe to you, that in some insects no eyes at all have been discovered. InPolydesmus complanatus, on each side of the head there is an eye-shaped portion separated by a suture, in which under a powerful lens I cannot satisfy myself that I can discern any thing like the facets that usually distinguish compound eyes. InGeophilus electricus, another myriapod, theycertainly do not exist[1482]. Whence we may conclude, as was before observed[1483], that the faculty of emitting light is rather given it as a means of defence than to guide it in its path.
Thesituationof compound eyes differs in different tribes. In some, as in theStaphylinidæ, they are planted laterally in theanteriorpart of the head; in others, theCarabi&c., in themiddle; in others again,LocustaLeach &c., in theposteriorpart. In some, their station is more in the upper surface, either before or behind; so that a very narrow space separates them, or perhaps none at all. Instances of this position of the eyes occur in a minute weevil (RamphusClairv.[1484]), and manyDiptera, &c. Of those that form an union on the top of the head, some are placed obliquely, so as to leave a diverging space below them, as in manyLibellulina[1485], the drone[1486], &c. Others, asAtractocerus, in which the eyes occupy nearly the whole head, and unite anteriorly, have this diverging spaceabovetheir conflux. InRhina barbirostrisLatr., another kind of weevil, they are confluentbelowthe head, at the base of the rostrum, and a very narrow interval separates them above. In a large number of theHeteromerousbeetles, they are settransversely, in theCapricornoneslongitudinally. Their surface, when they are lateral, has usually two aspects, oneproneto see below, the othersupineto see above. In general the eyes are situated behind the antennæ, so that their position, whether it shall be anterior or posterior, depends upon that of those organs. Often, indeed, as inthe last-named beetles, part of the eye is behind and part before the antennæ; but except where there arefoureyes, as inTetrops, they are never placed before or below them.
Though the eyes of insects are generallysessile, yet to give them a wider range they are sometimes, but it rarely occurs, placed, like those of manyCrustacea, on afootstalk, but not amoveableone. An instance of this in certain maleEphemeræhas already been mentioned. In theHemipteraDe Geer has figured two species of bugs (Cimicidæ) that are so circumstanced[1487]; as are also all the knownStrepsipteraK., though in these the footstalk is very short[1488]: but the most remarkable example of columnar eyes is afforded by that curiousDipterousgenusDiopsis, in which both eyes and antennæ stand upon a pair of branches, vastly longer than the head, which diverge at a very obtuse angle from its posterior part[1489].
In theirfigureeyes vary much. Sometimes they are so prominent as to be nearlyspherical: this is the case with some aquatic bugs, asRanatra,Hydrometra, and several maleEphemeræ[1490]. Very often they arehemispherical, as in the tiger-beetles (CicindelaL.), and the clocks or dors (CarabusL.); but in a large number of insects they are flat, and do not rise above the surface of the head.—With regard to theiroutline, they are often perfectlyround, as in many weevils;oval, as in various bees;ovate, as in other bees (AndrenaF.);triangular, as in the water-boatman (Notonecta). They are also oftenoblong, and occasionally narrow andlinear; as in that singular beetleHelæus. In many of theMuscidæthey form nearly asemicircle, or rather, perhaps, the quadrant of a sphere. The eyes of the Capricorn-beetles (CerambyxL.) have a sinus on their inner side, as it were, taken out of them; so that they more than half surround the antennæ, before which is the longest portion of them. An approach to this shape is more or less observed in the darkling-beetles (TenebrioL.); but in these the sinus is not so deep. I may under this head observe, that in thoseMantidæthat represent dry leaves, and some others, these organs usually terminate in a spine[1491].
Though not distinguished by the beauty and animation that give such interest to the eye of vertebrate animals, and exhibiting no trace of iris or pupil, yet from the variety of theircoloursthe compound eyes of insects, though most commonly black or brown, are often very striking. Look at those of one of the lace-winged flies that commit such havoc amongst theAphides[1492], and it will dazzle you with the splendour of the purest gold, sometimes softened with a lovely green. The lenses of those ofXenosblaze like diamonds set in jet[1493]. You have often noticed the fiery eyes of many horse-flies (TabanusL.) with vivid bands of purple and green[1494]. Others are spotted[1495]; and Schellenberg has figured one (Thereva hemiptera)[1496], that exhibits the figure of a flowerpainted in red on a black ground. These colours and markings are all most vivid and brilliant in thelivinginsect, and often impart that fire and animation to the eyes for which those of the higher animals are remarkable. Take one of the large dragon-flies that you see hawking about the hedges in search of prey, examine its eyes under a lens, and you will be astonished at the brilliance and crystalline transparency which its large eyes exhibit, and by the remarkable vision of larger hexagons which appear in motion under thecornea, being reflected by theretina—all which give it the appearance of a living eye. This moving reflexion of the hexagonal lenses in living insects was noticed long since in some bees (NomadaF.,CœlioxysLatr.)[1497]
Compound eyes differ greatly in theirsize. In some insects, asAtractocerus, the drone-bee, many maleMuscidæ, &c., they occupy nearly the whole of the head; while in others, as numerousStaphylinidæ,LocustaLeach, &c., they are so small as to be scarcely larger than some simple eyes of spiders: and they exhibit every intermediate difference of magnitude in different tribes, genera, and species.
Under this head I must say something of theCanthusof the eye; by which I mean an elevated process of the cheek, which in almost all the genera of the Lamellicorn beetles enters the eye more or less, dividing the upper portion from the lower. Though usually only aprocessof the cheek, yet in theScarabæidæthe whole of that part forms thecanthus[1498]. It onlyentersthe eye in theRutelidæ,Cetonidæ, &c.; it extends throughhalfof it inCopris; it goesbeyondthe half inAteuchus; and inRyssonotusMacLeay (Lucanus nebulosusK.) it quite divides the eye into two[1499], as I before observed. InLucanus,Passalus&c. it projects before the eye into an angle; inLucanus femoralisnearly into a spine; but inLamprimaandŒsalusit does not exist. The part, also, that enters the eye in the Capricorn-beetles may be regarded as a kind ofcanthus, though it is merely a dilatation of thefront.
4.Stemmata[1500].—Having given so full an account of the kinds and structure of the ordinary eyes of insects, you may perhaps expect that I should now dismiss the subject: you would, however, have great cause to blame me, did I not make you acquainted with a kind of auxiliary eyes with which a large portion of them are gifted; I mean those pellucid spots often to be found on the posterior part of the front of these animals, or upon the vertex, frequently arranged in a triangle. These, Linné, from his regarding them as a kind of coronet, calledStemmata. They have been of late denominatedOcelli; but as this latter term is also in general use for the eyelets on the wings ofLepidoptera, I have adhered to that of the illustrious Swede. Neither he nor Fabricius has expressed any opinion as to theuseof these organs; but Swammerdam and Reaumur were aware that they were realeyes. The former found that there are nerves that diverge to them though not easily traced, and that theyhave acornea, and what he takes for theuvea[1501]; and the latter has supposed that the compound eyes and these simple ones have, the one the power of magnifying objects much, and the other but little, so that the former are for surveying those that are distant, and the latter those that are near[1502]. The same author relates some experiments that he tried with the common hive bee, by which he ascertained that thestemmata, as well as the compound eyes, were organs of vision. He first smeared thelatterover with paint, and the animals, instead of making for their hive, rose in the air till he lost sight of them. He next did the same with theformer, and placing the bees whosestemmatahe had painted within a few paces of their hive, they flew about on all sides among the neighbouring plants, but never far: he did not observe that these ever rose in the air like the others[1503]. From this experiment it seems as if the compound eyes were forhorizontalsight, and thestemmataforvertical.
The definition of them by Linné and Fabricius as smooth, shining, elevated or hemispheric puncta, conveys a very inadequate idea of them; for, except in a very few instances, they are perfectly clear and transparent, and their appearance is precisely the same as that of the simple eyes ofArachnida&c., under which head they might very well have been arranged; but as the last areprimaryeyes, and the stemmatasecondary, it seemed to me best that they should stand by themselves. The structure of both is probably the same, and their internal organization that of one of the lenses of a compound eye, and both are set in a socket of the head.
Though a large number of insects have them, they are by no means universal, since some Orders, as theStrepsiptera,Dermaptera, andAptera, are altogether without them. TheColeoptera, also, have been supposed to afford no instance of species furnished with them; but in the last number of Germar and Zincken Sommer'sMagasin, it is affirmed that they are discoverable in Gravenhorst's genusOmalium, but not in the kindred generaMicropeplusandAnthophagus[1504]. Upon examining the former genus, I find, that althoughOmalium planumand affinities,O. striatulum, and some others, appear not to have them, yet with the aid of a good magnifier they may be discovered in most species of that genus; as likewise inEvœsthetusGrav. I find them also very conspicuous inA. Caraboidesand otherAnthophagi, but some species appear to want them. In these insects they are two in number, situated in the vertex a little behind the eyes but within them, and either at each end of a transverse furrow, or at the posterior termination of two longitudinal ones. Nor are they found in all the genera of the other Orders. In theOrthoptera, theBlattidæ, unless a white smooth spot on the inner and upper side of the eyes may be regarded as representing them, have them not; but in all the other genera of that Order they are to be found[1505]. In theHemipteraall theCicadiadæare gifted with them; as are likewiseTetyra,Pentatoma, with many otherCimicidæ, and theReduviadævery remarkably; but many others in both sections of this order, asThrips,Coccus,Aphis,Capsus,Miris,Naucoris,Nepa, andNotonecta, &c. are deprived of them[1506]. Of theNeuropteratheLibellulinaaddstemmatato their large eyes, in the anterior angle of which they are stationed[1507]; but many other genera of that Order are without them; asMyrmeleon,Ascalaphus,Hemerobius, &c. TheTrichopteraandLepidopterauniversally have them; though in the latter, except inCastniaand theSphingidæ, they are not easily seen. In theHymenopterathey are usually very conspicuous, but inLarraandLyrops, two genera of this order, the posterior pair are scarcely discernible; and in the neuter ants they are quite obsolete. In theDiptera, though many genera are furnished with them, yet many also want them; amongst the rest Latreille'sTipulariæ, and all the horse-flies (TabanusL.). ThePupiparæ(HippoboscaL.) usually have none; but inOrnithomyia avicularia, one of that tribe, though extremely minute they are visible, arranged in a triangle, in the polished space of their vertex.