Were I to ask you what your idea is with regard to the use of the organs we are considering, you would perhaps reply without hesitation, "Of what possible use can thejawsof insects be but tomasticatetheir food?" But in this you would in many instances be much mistaken; as you will own directly if you only look at the mandibles of the stag-beetle—these protended and formidable weapons, as well as those of several other beetles, cannot be thus employed. "Of what other use, then, can they be?" you will say. In the particular instance here named, their use, independent of mastication, has not been satisfactorily ascertained; but in many other cases it has. Recollect, for instance, what I told you in a former letter, of those larvæ that use their unguiform mandibles as instruments ofmotion[1256]. Again: amongst the Hymenopterous tribes, whose industry and varied economy have so often amused and interested you,many have no other tools to aid them in their various labours and mechanical arts: to some they supply the place of trowels, spades, and pick-axes; to others that of saws, scissors, and knives—with many other uses that might be named. In fact, with the insects of this intire Order mastication seems merely asecondary, if it is at any time their use. Still comprehending in one view all the mandibulate Orders, though some use their mandibles especially for purposes connected with their economy, yet their most general andprimaryuse is the division, laceration, and mastication of their food; and this more exclusively than can be affirmed of the under-jaws (maxillæ). This will appear evident to you, when you consider that insects in their larva state, in which universally their primary business isfeeding, with very few exceptions use the organs in question for the purpose of mastication, even in tribes, as theLepidoptera, that have only rudiments of them in their perfect state—while themaxillæordinarily are altogether unapt for such use. The exceptions I have just alluded to are chiefly confined to the instance of suctorious mandibles; or those which, being furnished at the end with an orifice, the animal inserting them into its prey, imbibes their juices through it. This is the case with the larvæ of someDytisci,Hemerobius, andMyrmeleon[1257]; and spiders have a similar opening in the claw of their mandibles, which is supposed to instil venom into their prey[1258].
Under this head I must not pass without notice an appendage of the mandibles, to be found in some of therove-beetles (Staphylinidæ), as inOcypus,Staphylinus, andCreophilusKirby. In the first of these it is a curved, narrow, white, subdiaphanous, submembranous, or rather cartilaginous piece, proceeding from the upper side of the base of the mandible[1259]; in the second it is broader, straighter, and fringed internally and at the end with hairs; and in this at first it wears the appearance of being attached laterally to the mandible under the tooth[1260], but if closely examined, you will find that it is separate: inCreophilus maxillosusit is broader. This is the part I have namedprostheca. It is perhaps useful in preventing the food from working out upwards during mastication.
5.Maxillæ[1261]. The antagonist organs to the mandible in the lower side of the head, are the under-jaws, ormaxillæ—so denominated by the illustrious Entomologist of Kiel. Linné appears to have overlooked them, except in the case of his genusApis, in which he regards them, and properly, as the sheath of the tongue. De Geer looked upon them in general as part of the apparatus of the under-lip orlabium; and such in fact they are, as will appear when we consider them more particularly. Fabricius has founded his system for the most part upon these organs, the principal diagnostic of ten out of his thirteen Classes (properly Orders) being taken from them; and in the modern, which may be termed theeclectic, system, although the Orders are not founded upon them, yet the characters of genera, and sometimes of large tribes, are derived from them: and as they appear lessliable to variation than almost any other organ, as Mr. W. S. MacLeay has judiciously observed, there seems good reason for employing them—it is therefore of importance that you should be well acquainted with them.
Theirsituationis usually below each mandible, on each side of thelabium;towards which they are often somewhat inclined, so that their tips meet when closed. In some cases, as in the Predaceous beetles (CarabusL. &c.), they exactly correspond with the mandibles; but in others their direction with respect to the head is more longitudinal, as in theHymenoptera, &c. Insubstancethey may be generally stated to be less hard than those organs; yet in some instances, as in theLibellulina,Anoplognathidæ, &c. they vie with them, and in theScarabæidæandCetoniadæexceed them, in hardness. In the bees, and many otherHymenoptera, they are soft and leathery. Theirarticulationis usually by means of the hinge on which they sit: it appears entirely ligamentous, and they are probably attached to thelabiumat the base, ormentum—at least this is evidently the case with theHymenoptera, in which the opening of themaxillæpushes forth thelabiumand its apparatus. In that remarkable genus related to the glow-worms, now calledPhengodes(Lampyris plumosaF.), and in the case-worm flies (TrichopteraK.), themaxillæappear to be connate with thelabium, or at least at their base.—As to theircomposition, these organs consist of several pieces or portions. At their base they articulate with a piece more or less triangular, which I call the hinge (Cardo)[1262]. This on its inner side is often elongated towards the interiorof the base of thelabium, to which it is, as I have just observed, probably attached. This elongate process of the hinge inApis,Bombus, &c. appears a separate articulation; and the two together form an angle upon which thementumsits[1263], and by this themaxillaacts upon the labial apparatus.
The next piece is thestipesor stalk of themaxilla. This is the part that articulates with the hinge, and may be regarded in some cases, as in theOrthoptera&c., as the whole of themaxillabelow the feeler; and in others, as in theGeotrupidæ,Staphylinidæ&c., as only the back of it, the inside forming the lower lobe. This piece is often harder and more corneous than the terminal part, is linear, often longitudinally angular, and in the bee-tribes (ApisL.) is remarkable on its inner side for a series of bristles parallel to each other like the teeth of a comb[1264]. InPogonophorusLatr., a kind of dor or clock-beetle, it is armed on the back with four jointed spines, the intermediate one being forked[1265]. M. Latreille has thus described the stipes of themaxillæofColeoptera: "Next comes the stalk," says he, "which consists of three parts: one occupies the back and bears the feeler; the second forms the middle of the anterior face, and its figure is triangular; the third fills the posterior space comprised between the two preceding; and is that which is of most consequence in the use of themaxilla; the anterior feeler, where there are two, thegalea, and the other appendages that are of service in deglutition, are part of that piece[1266]."
Thethirdand terminal portion of themaxillais formed by the lobe, or lobes (Lobi). This may be called the most important part of the organ, since it is that which often acts upon the food, when preparing for deglutition. When armed with teeth or spines at the end, its substance is as hard as that of the mandibles; but when not so circumstanced, it is usually softer, resembling leather, or even membrane[1267]; and sometimes the middle part is coriaceous, and the margin membranous. This part is either simple, consisting only ofonelobe, as you will find to be the case with theHymenoptera,Dynastidæ,Nemognatha, and several other beetles; or it is compound, consisting oftwolobes. In the former case, the lobe is sometimes very long, as in the bee tribes, and the singular genus of beetles mentioned above[1268],Nemognatha; and at others very short, as inHister, &c. The bilobedmaxillæpresent several different types of form. Nearest to those with one lobe are those whose lower lobe is attached longitudinally to the inner side of the stalk of the organ, above which it scarcely rises. Of this description is themaxillain the common dung-beetle (Geotrupes stercorarius), and rove-beetle (Staphylinus olens).[1269]Another kind of formation is where the lower lobe is only a little shorter than the upper: this occurs in a kind of chafer (Macraspis tetradactylaMacLeay).[1270]A third is where the upper lobe covers the lower as a shield; as you will find in theOrthopteraorder, and theLibellulina, and almost inMeloe[1271]. A fourth form is where the upper lobe somewhat resembles the galeatemaxillajust named; but consists of two joints. This exists inStaphylinidæ, &c.[1272]The last kind I shall notice is when the upper lobe not only consists of two joints, but is cylindrical, and assumes the aspect of a feeler orpalpus[1273]. This is the common character of almost all the Predaceous beetles (EntomophagiLatr.). This lobe, which has been regarded as an additional feeler, is strictly analogous to the upper lobe in other insects, and therefore should rather be denominated a palpiform lobe than apalpus. Where there are two lobes, the upper one is most commonly the longest; but in many species of the tribe last mentioned the lower one equals or exceeds it in length[1274].
The lobes vary in form, clothing, and appendages. The upper palpiform lobe in those beetles just mentioned, in general varies scarcely at all inform; but the genusCychrus(which is remarkable for a retrocession from the general type of form of theCarabiL. making an approach towards that of thoseHeteromerawhich, from their black body and revolting aspect, Latreille has namedMelosomes,) affords an exception, the upper joint being rather flat, linear-lanceolate, incurved, and covering the lower lobe[1275], which it somewhat resembles. The lowerlobe also in this tribe varies as little as the upper, being shaped like the last joint of that lobe inCychrusjust described, except that inCicindelait is narrowest in the middle[1276]. In other tribes the upper valve is sometimes linear and rounded at the apex, and the lower truncated, as inStaphylinus olens[1277]; sometimes the upper one is truncated or obtuse, and the lower acute, as inTrogositaandParnus[1278]. InPtinus, another tribe of beetles, before noticed as injurious to our museums[1279], the reverse of this takes place, the upper-lobe, which is the smallest and shortest, being acute, and the lower truncated[1280]. InBlapsboth are acute[1281]. InRhipiphorusandScolytusthe lobes are nearly obsolete. The lower lobe is bifid inLanguria, a North American genus of beetles, so as to give the maxilla the appearance of three lobes[1282]; and inErotylus, a South American one, the upper is triangular[1283]: it is often oblong, quadrangular, linear, &c. in others.—In those that have only one lobe the shape also varies. InGyrinus, the beetle that whirls round and round on the surface of every pool, which, though it belongs to the Predaceous tribe, has only one lobe, the lobe represents a mandible in shape of the laniary kind, beingtrigonal and acute[1284]; and in theAnoplognathidæ, a New Holland tribe of chafers, in which it is, as it were, broken, the lobe forming an angle with the stalk, it is concavo-convex and obtuse, and somewhat figures a molary tooth[1285]. In the first tribe into which the bees (ApisL.) have been divided (MelittaKirby), the lobe is often linear or strap-shaped, and bifid at the apex; and in the second (ApisK.) lanceolate and intire[1286]. InCerocomait is long and narrow[1287]. More variations in form might be named, but these are sufficient to give you a general idea of them in this respect. With regard to theirclothing, I have not much to observe—in examining the Predaceous beetles you will observe, that the interior margin of the lower incurved lobe is fringed with stiff bristles or slender spines, and in many other beetles either one or both lobes have a thick coating or brush of stiffish hairs[1288]; but in several cases only the apex of the lobe is hairy. In theOrthopteraorder, and many of theMelolonthidæor chafers, the wholemaxillais without hairs, or nearly so.
Theappendagesof themaxillæare next to be noticed. These are principally their claws, or laniary teeth; for they are seldom armed with incisive or molary teeth. The whole tribe of Predaceous beetles, with few exceptions, have the inner lobe of theirmaxillaarmed with a terminal claw, which in theCicindelidæarticulates with the lobe, and is moveable, but in the rest of the tribe isfixed[1289]. InPhoberusMacLeay the lower lobe has two spines[1290]. InLocustathis lobe has three or four spines or laniary teeth, and inÆshnathere are six, which, like the claw ofCicindela, are moveable[1291]. In others both lobes terminate in a single spine or claw: this is the case withPaxillusMacLeay[1292]. InPassalus, nearly related to the last genus, the upper lobe is armed with a single spine, and the lower one with two[1293]. Thosemaxillæthat terminate in a single lobe are also often distinguished by the spines or teeth with which it is armed; thus in a nondescript chafer belonging to theDynastidæ(ArchonK. MS.) it terminates intwoshort teeth; in that remarkable Petalocerous genusHexodonOliv. inthreetruncatedincisiveones[1294]; inDynastes Herculesin threeacutespines[1295].Foursimilar ones arm the apex of themaxillain that tribe ofRutelidæwhich have striated elytra; andfivethat are stout and triquetrous those ofMelolontha StigmaF. Many others havesixspines, sometimes arranged in a triple series[1296]. Besides teeth or spines, in some cases the lobes ofmaxillæterminate in several long and slenderlaciniæor lappets fringed with hairs. At least those of aLeptura(L. quadrifasciataL.) described by De Geer, appear to be thus circumstanced. He conjectures that this beetle uses itsmaxillæto collect the honey from the flowers[1297].
As the principal use of the mandibles is cutting and masticating, so that of the organs we are considering seems to be primarily that ofholdingthe food and preventing it from falling while the former are employed upon it. I say this is theirprimaryuse; for I would by no means deny that they assist occasionally in comminuting or lacerating it. In fact, were there no organs appropriated to this use, and if both mandibles andmaxillæwere employed at the same time in comminuting the food, it seems to me that it must fall from the mouth. In a large proportion of insects the lobes of themaxillæare not at all calculated for laceration or comminution; and in those tribes—as theMelolonthidæ,Rutelidæ,Dynastidæ—in which they seem most fitted for that purpose, the mandibles haveincisiveteeth at their apex, and at their base a powerfulmolaor grinder: circumstances which prove, that even in this case the business of mastication principally devolves upon them.
6.Palpi Maxillares[1298]. There is one circumstance that particularly distinguishes themaxillæfrom the mandibles—they arepalpigerous, as well as the under-lip. The feelers, or palpi, emerge usually from a sinus observable on the back of themaxillæwhere the upper lobe and stalk meet. Their articulation does not materially differ from that of the labial palpi. Eachmaxillahas properly onlyonefeeler; but, as was lately observed[1299], in certain tribes the upper lobe is jointed and palpiform, which has occasioned it to be considered as a feeler, and these tribes have been regarded as having six feelers. The most general rule with regard to thelengthof thepalpiis, that the maxillary shall be longer than the labial; but the reverse often takes place. In manybeesthe maxillary consist only of asinglejoint, and are veryshort; while the labial consist offour, and are verylong[1300]: and in some insects (as inPogonophorusLatr.) the four palpi are of equal length[1301]. The antennæ are most commonly longer than the palpi; but in several aquatic beetles, asElophorus,Hydrophilus, &c., whose antennæ in the water are not in use, the organs we are considering are the longest.—As to thenumberof their articulations, it varies from one to six; which number they are not known to exceed. In each of the Orders a kind of law seems to have been observed as to the number of joints both in the maxillary and labial palpi, but which admits of several exceptions. Thus in theColeoptera, thenaturalnumber may be set atfourjoints for themaxillary, andthreefor thelabialpalpi: yet sometimes, as inStenus,Notoxus, &c., theformerhave onlythreejoints, and thelatter, as inStenusandTillus, onlytwo. In theOrthopterathe law enjoinsfivefor themaxillary, andthreefor thelabial; and to this I have hitherto observed no exception. In theHymenoptera, the rule issixandfour, but with considerable exceptions, especially as to themaxillarypalpi, which vary fromsixjoints to asingleone: thus in the hive-bee and the humble-bee, the labials, including the two flat joints or elevators, have four joints, while the maxillaries are not jointed at all[1302]. InChrysis, in which the latter consist offive, the former are reduced tothree. TheLibellulinamay almost be regardedas having no maxillary palpi, since they exhibit no organ that is distinctly palpiform. It seems to me that the upper lobe of their maxilla, which articulates with the stalk in the same manner as a feeler, may be regarded as an instance in which that lobe and the feeler coalesce into one; and the mucro that proceeds from the lobe has the aspect of an emerging feeler, and corresponds somewhat with the labial one above noticed[1303]. In the remainder of theNeuropteraand theTrichoptera, the prevailing number isfiveandthree. In the latter there are exceptions, which will furnish good characters for genera. In theLepidopterawe findtwo, and sometimesthree, the maxillary being very minute[1304]. TheDipteraOrder presents two tribes in this respect quite distinct from each other. The most natural number of joints in the maxillary palpi of theTipulidæ,Culicidæ, &c. isfourorfive: the last joint, however, inTipula,Ctenocera, &c. like that of the antennæ inTabanusL., appears to consist of a number of very minute joints[1305]; but in theAsilidæandMuscidæ, &c., the numbertwoseems to be most prevalent[1306]. Thelabialpalpi in this order are obsolete.—As toshape, the maxillary palpi, as well as the labial, are usually filiform; but in the weevil tribes (CurculioL.) they are most commonly very short and conical[1307]; in the chafers (ScarabæusL.) they usually are thickest at the apex[1308]; inMegachileandEuglossa, wild bees, they are setaceous, growing graduallymore slender from the base to the summit[1309]: a tribe of small water-beetles (Haliplus), the saw-flies (TenthredoL.), and several otherHymenoptera, have them thickest in the middle[1310]. Their most important part, however, and that which varies most in form, is theterminaljoint:—of this I have already related some singular instances[1311], and shall now describe a few more. This joint is sometimes acute, at others blunt, at others truncated: in figure it is ovate, oblong, obtriangular, hatchet-shaped, lunate, transverse, conical, mammillate, subulate, branched, chelate, laciniate, lamellate, &c. &c.[1312]: terms which I shall more fully explain to you hereafter, and which I only mention here to show the numerous variationsas to figure, of which this joint exhibits examples. The palpi in general at their vertex are often rather concave; and this concavity is formed by a thin papillose membrane, which it is supposed the animal has the power of pushing out a little, so as to apply it to surfaces. Theprimaryuse of the palpi of insects will be considered when I treat of their senses; but they probably answer more purposes than one. For instance, when I was once examining, under a lens, the proceedings of a species ofMordella, which was busily employed in the blossom of some umbelliferous plant, it appeared to me to open the anthers with its maxillary palpi, and they often held the anther between them: when not so employed, they were kept in intense vibration, more than even its antennæ; and at the same time, as far as I could judge, anElatermade the same use of them.
7.Lingua[1313].—This name was applied by Linné to the part in insects representing thetonguein vertebrate animals; and as it performs most of the common offices of a tongue, and thepharynxis situated with respect to it, as we shall presently see, nearly as it is in those animals, there seems no more reason for giving it a new name, than there is for giving a new name to the head or legs of insects, because in some respects they differ from those of the higher animals. I shall not therefore call itLigula, with Fabricius and Latreille, norLabium, with Cuvier and others, but adhere to the original term, which every one understands.
Thetonguelies between the twolips—thelabrumandlabium. On its upper side, at the base, it meets the palateor roof of the mouth, below which it is attached, it may be presumed, by its roots to the crust of the head, on each side thepharynxor swallow; and on its lower side, in many cases, it is attached to thelabium, and that very closely, so as to appear to be merely a part of it, and to form its extremity: but in theOrthopteraandLibellulina, it is more free, and in form somewhat resembling the tongue of the quadrupeds[1314].—Insubstancethe tongue varies. In general it seems something between membrane and cartilage; but in the Predaceous beetles, in which it is not covered by thelabium, it approaches nearer to the substance of the general integument, and inAnthiaF. it is quite hard and horny:—that just mentioned of theOrthopteraandLibellulinais more fleshy[1315]. With regard to itsstation, in many cases, as in the instance just named, in the Lamellicorn tribe (ScarabæusL.) and others, it is, when unemployed, concealed within the mouth; the lips, mandibles, and maxillæ all closing over it. The tongue of someHymenopteraalso is retractile within the mouth. "Whenantsare disposed to drink," says M. P. Huber, "there comes out from between their lower jaws, which are much shorter than the upper, a minute, conical, fleshy, yellowish process, which performs the office of a tongue, being pushed out and drawn in alternately: it appears to proceed from the lower-lip.—This lip has the power of moving itself forwards in conjunction with the lower jaws: and when the insect wishes to lap, all this apparatus moves forward; so that the tongue, which is very short, does not require to lengthen itself much to reachthe liquid[1316]." M. Lamarck thinks that thelabiumof insects has averticalmotion (de haut en bas ou de bas en haut)[1317]. This it certainly has in some degree; but it has also, as in the above case, a more powerfulhorizontalone, which is produced, inHymenopteraat least, by the opening of the maxillæ—as I have already observed[1318].
I have little to say with respect to thestructureof the tongue: it generally seems to be without articulations; but in many bees it articulates with thelabiumwhere it enters it, so as when unemployed to form a fold with it. In the hive-bee it terminates in a kind of knob or button, which has been falsely supposed to be perforated for imbibing the honey by suction. The upper part of this tongue is cartilaginous, and remarkable for a number of transverse rings: below the middle, it consists of a membrane, longitudinally folded in inaction, but capable of being inflated to a considerable size: this membranous bag receives the honey which the tongue, as it were, laps from the flowers, and conveys it to thepharynx[1319]. InStenusthis organ is retractile, and consists of two joints[1320].
Theshapeof the tongue of insects probably varies as much as any other part; but as it is apt to shrink when dried[1321], and is not easy to come at, we know but little of its various configurations:—in the bees it is very long, in most other insects very short. Though frequently simple and undivided, in many cases it presents a different conformation. Thus in the saw-flies (TenthredoL.)it terminates in three equal lobes[1322]; inStomisandGeotrupesin three unequal ones, the intermediate being very short[1323]; inCarabus, in three short teeth[1324]; inPogonophorusit represents a trident[1325]; in the wasp it is bifid, each lobe being tipped with a callosity[1326]; inMelolontha Stigmait is bipartite[1327]; inElaphrus, the analogue of the tiger-beetles, it terminates in a single tooth or point; in the aquatic beetles,DytiscusL., it is quadrangular and without teeth[1328]; in someIchneumonidæit is concavo-convex, and forms a demitube; and in others it is nearly cylindrical[1329].
In many insects it has nohairs, but in the Predaceous beetles it generally terminates in a couple of bristles[1330]. In the hive- humble- and other bees, it is extremely hairy[1331]; a circumstance which probably enables it more effectually to despoil the flowers of their nectar. InGeotrupes stercorarius, the common dungchafer, andMelolontha Stigmalately mentioned, the lobes of the tongue are fringed with incurved hairs[1332]; and inÆshnait is hairy on the upper side, each hair or bristle crowning a minute tubercle. In many cases the tongue is attended, and sometimes sheathed at the base, by two usually membranous appendages:—these the learned Illiger has denominatedparaglossæ; and I shall adopt his term. You will find them frequently attached to the tongue of the Predaceous beetles[1333], and to that of manyHymenoptera. In the hive-bee and humble-bee they are short, and take their origin within thelabialfeelers[1334]: inEuglossa, another bee, they are long, involute at the tips, and, what is not usual with them, very hairy[1335]: in the wasp, like the lobes of the tongue, they are tipped with a callosity.
Under this head I may observe to you, that the insects whose oral organs we are considering besides atongueappear likewise to be furnished with apalate(Palatum). This, though a part of the roof of the mouth, is not precisely in the situation of the palate of vertebrate animals, since it seems rather the internal lining of thelabrum. If you take the common dragon-fly (Æshna viatica), you will find that the under side of this part and of therhinariumis lined with a quadrangular fleshy cushion, beset, like the upper surface of the tongue, with minute black tubercles, crowned with a bristle. This cushion is divided transversely into two parts by a depression; the anterior or outer piece being attached to thelabrum, and the other piece to therhinarium. The former has a central longitudinal cavity, black at the bottom, on the sides of which the tubercles are flat and without a bristle. From its base on each side a spiniform process emerges, forming a right angle with it. These processes seem the antagonists of those mentioned above[1336], that emerge from thelabium. The posterior orinner piece has on each side a roundish space, attached to the under surface of the two sides of therhinarium, beset also with bristle-bearing tubercles. You will find something similar lining thelabrumandnasusof someColeoptera,—sayGeotrupes,Necrophorus, andDytiscus. The first piece I regard as the analogue of the palate, and the second as connected with the sense ofsmelling. InNecrophorusthe circular pieces are covered with a finely striated membrane, and inDytiscuseach has a little nipple.
8.Pharynx[1337].—On the upper side of the tongue, usually at its base or root, is thepharynx, or aperture by which the food passes from the mouth to theœsophagus. This orifice, which is situated with respect to the tongue of theOrthopteraandLibellulinanearly as in those insects (at least as far as I have been able to examine them), whose tongue is called aligulaorlabium,—of course exists in all the mandibulate Orders whose mouth we are now considering. In theHymenopterait is covered by a valve, theEpipharynxof Savigny; and it appeared to me to be so likewise in one of theHarpalidæthat I examined. The formation seems different inGeotrupes, as far as I can get an idea of it; but it is so difficult to examine the interior of the mouth without laceration of some of the parts, that I can only tell you what the appearances were in one instance, upon removing thelabrumfrom themandibles; and in another, separating the whole apparatus of thelabium, including themaxillæ, from themandiblesandlabrum. In the former case, the mandibles coincidedat the base, the two molary plates (molæ), which in this genus are narrow, transverse and not furrowed, are so applied as evidently to have an action upon each other, as the mandible opens and shuts, proper for trituration. Within these is the base of the tongue, under the form of a ventricose sack. The upper part of this last organ, which forms the internal covering of the labium, appears to consist of three (in the recent insectfleshy) lobes, the middle one being bent downwards internally, so as to form a kind of sloping cover to an orifice in the part I call the base. After two or three days, the tongue shrinks and dries to a hard substance;—between the mandibles and the base of the tongue I could not discover thepharynx. The above apparent opening covered by the tongue was the only one I could perceive. In the latter case, the form and structure of the base of the tongue is more visible: it is an oblong ventricose tubular sack, projecting above anteriorly into an acute angle formed by a fine white membrane, most beautifully and delicately striated with oblique striæ, to be seen only under a powerful lens: on the anterior side of this sack are two parallel cartilaginous ridges close to each other, fringed with short hairs, which take their origin from the angle. I could not be certain whether the orifice covered by the intermediate lobe was only apparent, or real; but I did not succeed in my endeavour to find any otherpharynx, though from the molary structure of the base of the mandibles one may conjecture that there must be one situated at the base of this sack to receive the food they render after trituration. The excrement of this animal is not fluid. In theLibellulinathepharynxseems closed by two valves meeting. This part inHymenoptera,and probably in other Orders, has the aspect of being cartilaginous and fitted to sustain the action of the substances that have to pass through it[1338].
TheEpipharynxis a valve, called by M. Latreillesublabrum(sous labre[1339]), attached by its base to the upper margin of thepharynx, or that next thelabrum. In the bees it is said by Reaumur to be of a fleshy substance, and capable of changing its figure. He seems to think it the realtongueof the bee[1340]; but as it does not appear to have any of the uses of a tongue, and merely closes the orifice of the mouth, it surely does not merit that name. M. Savigny calls it a membranous appendage which exactly closes thepharynx[1341]. De Geer has examined theepipharynxof the wasp, which he describes as of a scaly substance, and regards merely as the cover of the part just named[1342].
With regard to theHypopharynx, which Latreille considers as a support and appendage of theepipharynx, I have little to add to the definition I have given of it above. In theLibellulinathe base of the tongue terminates towards thepharynxin a fleshy cushion, armed at each angle next to that part with a short hard horn or tooth of a black colour. This cushion, I suppose, may be analogous to thehypopharynxof M. Savigny[1343]. On the opposite side the pharynx is closed by another fleshy cushion (epipharynx?), which appears to line the nose, behind those two mammillæ before described[1344], which form the internal covering of therhinarium.
Before I call your attention to what I would denominate animperfect mouth, in which some one or more of the seven organs above enumerated exist under another form, or only as rudiments,—I must say something upon the mouth of theMyriapodsandArachnida, in which there seem to beredundantorgans of manducation.—M. Latreille, in the Essay lately quoted, in which, though some of his notions seem fanciful, he has shown a vast depth and range of thought and research, has asserted,—from the admirable and curious observations of M. Savigny, and those which since their publication he has made himself,—that the masticating organs ofannuloseanimals (called by himcondylopes) are a kind oflegs[1345]. And M. Savigny, whose indefatigable labours and unparalleled acuteness have opened the door to a new and vast field in what may be denominated analogical anatomy,—has observed, that with certainApiropods[1346]the organs that serve for manducation do not differ essentially from those which, with the otherApiropodsand theHexapods, serve forlocomotion[1347]: and the unguiform mandibles of the larvæ of certainDiptera, you have before been told, are used not only in manducation, but also as legs[1348]. These remarks will satisfactorily prove to you, that organs which at first sight possess no visible affinity or analogy—as for instance, jaws and legs—may, if traced through a long series of beings, exhibit a very great one;—and will lessen your surprisewhen you find, that in certain tribes such commutations of organs and their use take place.
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 thechaperon,) 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 thelabrum. Below this are two mandibles, armed at their end with five sharp triangular teeth[1350], under which are themaxillæ, terminating in a moveable concavo-convex lobe, resembling the valve of a bivalve shell[1351]; and between them is thelabium, of a rhomboidal shape, divisible into two lobes, attached laterally to the maxillæ: these lobes M. Savigny terms thesecond maxillæ, forming with the others, according to him, thelabium[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 auxiliarylabium, and representative of the first pair of legs of hexapods andIuli[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 fromthe 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 asecondauxiliarylabium, 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 thesecondpair of legs inIulusand 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 asoralorgans, though belonging to thetrunk: so that here we see thelegsand their appendages assume a material function inmanducation, forming a singular contrast to what we had observed before with regard tomandiblesbecoming instruments oflocomotion. The mouth of theIulidæ, 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 theArachnida. 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 nolabrumor 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 armedunderneath 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 achelaor 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 theCrustacea[1359]: and M. Savigny is of opinion that theArachnidamay in some sort be defined asCrustaceawithout a head, and with twelve legs, of which the two first pair are converted intomandiblesandmaxillæ[1360]. From thesituationof the organs in question, the first of these opinions seems preferable; but the conversion of the legs in other cases, at least thecoxæ, into organs of manducation, gives some weight to the last. With regard to theiruse, 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 respectthe mandibles may be said to represent themaxillæof the mandibulate hexapods; and,vice versa, thesciaticmaxillæ, as they havebeen denominated[1362], of theArachnida, their mandibles. The palpi are pediform, and the first joint of thecoxa, or hip, acts the part of amaxilla:—this is composed of a single piece or plate, more or less oval or triangular, sometimes straight and sometimes inclined to thelabium, with the interior extremity very hairy. Thelabiumconsists 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 thelabium. 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 theircoxæ, are concerned in manducation: so that these insects have in factthreepairs of maxillæ—a circumstance that M. Savigny has observed to take place also in the harvest-men (PhalangiumL.)[1364]. Thepalpiof the scorpion, which may be called itshands, like the anterior legs of the lobster and crab, terminate in a tremendouschelaor forceps, consisting of a large triangular joint, armed at the end with a double claw internally toothed; theexteriorone of which, contrary to what takes place in the animals just named, is moveable, and not theinterior[1365].
Having given you this full account of thetrophiof those animals that have all the organs of manducation developed, I must next advert to those in which one partreceives an increment at the expense of others, and the whole oral machine is fitted forsuction; or where some parts appear to be deficient, so that this may be called animperfectmouth. At first sight one would regard the trophi of abeeas 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 arefivekinds ofimperfectmouth 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 theHemipteraOrder:—this I term thePromuscis; the second is that of theDiptera, which with Linné I callProboscis; the third, peculiar to theLepidoptera, is with me anAntlia; the fourth, which I nameRostrulum, is confined to theAphanipteraorder, or genusPulexL.; and the last isRostellum, 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 denominatedRostrum: but since that term is likewise in general use for the snout of insects of the weevil tribes (CurculioL.), I think you will concur with me in adopting the one here proposed, for the very different oral instruments of theHemiptera. Illiger has employedpromuscisto denote those ofbees[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 oftrophi, without any risk of producing confusion. This part consists offivepieces: viz. a minute, long, conical piece, commonly very slender, which covers the base of thepromuscis, and represents thelabrum[1368]; a jointed sheath (vagina), consisting of either three or four joints, the analogue of thelabium, and four slender rigid lancets (scalpella), the two exterior ones, according to M. Savigny, representing themandibles, and the intermediate pair themaxillæ[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. Apharynxandtonguehave been discovered by M. Savigny in this apparatus; who thinks that inNepathere are also rudiments, but very indistinct, oflabial palpi: so that themaxillary palpiseem to be the only part absolutely wanting[1370].
ThePromusciswhen at rest is usually laid between the legs; but when employed, in most cases its direction is outward. In the genusChermesL. (PsyllaLatr.) the origin of thepromuscishas been supposed to be in thebreast; 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 whichthe front appears to be divided. Look below these, and you will there discover the upper-lip: and from this you may follow thepromuscistill 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 inCoccus,Chermes, &c. this instrument is short, in someAphidesit is longer in proportion than in any other insect. InA. Quercusit 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 inA. Abietisit 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 theMuscidæand some others, calling the same organ, when without fleshy lips,rostrumandhaustellum: but as the parts of the mouth in all trueDiptera(forHippoboscaand 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 inŒstrus(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, thethecaor sheath is present, which represents thelabium(including thementum) of the mandibulate Orders[1376]. It consists ofthreejoints, the last of which is formed by the liplets (Labella). Those in theMuscidæare large, turgid, vesiculose, and capable of dilatation; in theBombylidæand other tribes they are small, slender, long and leathery, and sometimes recurved. The second joint or stalk, which may be said to represent thementum, the liplets being properly in a restricted sense the analogue of thelabium, its sides being turned up, forms a longitudinal cavity, which contains thehaustellum. The upper piece of this, thevalvula, is long, rigid, and very sharp, representing thelabrum[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 arefiveof them, sometimesfour, sometimestwo, and sometimes, it should seem, onlyone[1378]. In the gnat (Culex) they are finer than a hair, very sharp, and barbed occasionally on one side[1379]; in the horse-fly (TabanusL.) 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 themandibles; the lower pair, or the lancets (Scalpella), usually palpigerous, themaxillæ; and the central one thetongue. In the horse-fly Reaumur has figured onlyfour, exclusive of thelabrumandlabium; but in a specimen I have preservedthere appear to befive, 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 themaxillæ, the mandibles being absorbed in thelabrum; and where there is only one, the maxillæ also are absorbed by thelabium, 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 thepharynx[1383].Labial palpiappear not usually present in theproboscis; but M. Savigny thinks he has discovered vestiges of them inTabanus[1384]. In this genus the maxillary ones are large, and consist oftwojoints[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, asConopsandStomoxys, whose punctures on our legs so torment us[1386], there is only a single fold, with its angle to the breast. Theproboscisis received in a large oblong cavity of the underside of the anterior part of the head.
It may here be observed, that in thepromuscisthe elongation of the organs seems to be made chiefly at the expense ofallthe palpi, but in theproboscisat that of thelabialonly; and in some cases at that also of themandiblesormaxillæ;—the former merging in thelabrumand the latter in thelabium.
iii.Antlia[1387].—Thethirdkind of imperfect mouth is that of theLepidoptera, which I have calledAntlia. Fabricius denominates itlingua: 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 theLepidopterarudiments 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 theantlia, may be regarded as the rudimentof theupper-lip(labrum)[1389]. On each side of the outer base of theantliais another small immoveable piece, resembling a flattened tubercle, the end of which is internally hairy or scaly: these pieces appear to represent themandibles[1390]. Near the base of each half of theantlia, just below a sinus, may be distinctly seen the minute, usually biarticulate rudiment of amaxillary palpus[1391]; demonstrating to a certainty that these spiral organs, at least their lateral tubes orSolenaria, are real maxillæ[1392]. The rudiment of theunder-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 thetongue; 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 ofHymenoptera, has its analogue in the intermediate tube orFistulaformed by the union of the two maxillæ, and which conveys the fluid aliment of this Order to thepharynx. As inDipterathemaxillæsometimes merge in thelabium, so here thetongue(as it were divided longitudinally) merges in themaxillæ. He further observes, that in a transverse section of the maxilla of the death's-head hawk-moth (Sphinx Atropos), the lateral tube appearedto be divided into two by a membranous partition, and to contain in the upper cavity a small cylindrical tube, which seemed to be atrachea[1395]. To animals that are without lungs, and breathe bytracheæ, suction must be performed in a very different way from what it is by those that breathe by themouth: and as in the very extended organs in question the fluid has a long space to pass before it reaches thepharynx, 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 theantlia, that the maxillæ receive their vast elongation at the expense of all the other organs, except thelabial palpi.