These little organs inosculate each in an appropriate socket of the end, or in many cases of the middle of thetibia; and that part of their head or base that is received by it, is often constricted for the purpose: from hence it follows that they are capable of some degree of motion, but in some insects, as those on the four posterior legs ofScarabæus sacerand its more immediate affinities, and those at the end of the cubit ofGryllotalpa, they are immoveable, and appear almost processes of the joint to which they belong. They are commonly sharp, of a subtriquetrous figure, with the lower side flat: where there are two, the outer one is usually the longest; and in general the spurs on the hind legs are longer than those on the four anterior: but there are exceptions—thus, inAcanthopusLatr. theintermediatespurs are the longest; and inCicindelatheanteriorare longer than the former; inBlaps mortisagathose on all the legs are nearly equal in length. They vary sometimes inshape—those on the middle of the cubit of manyLepidoptera,which may be regarded as a kind of thumb[2053], are of a lanceolate shape; inMeloethe external posterior one is flat and obtuse; inŒnasLatr. it is obconical, concave at the extremity, and apparently furnished with a sucker; inAteuchus smaragdulustheanterior, and inCopris Carolinatheposterioris forked and emarginate; inSirexthe former is hooked and winged; inLamprimait is triangular and dilated; inAphodius analisit is dolabriform; inDynastes retususandJuvencusthe spurs are bent like a bow. In manyHymenoptera, as theSphecidæ, they are pectinated[2054], with a series of minute parallel spines—a structure which assists the animal in burrowing[2055]; inAcanthopusLatr. they are armed with little teeth or spines[2056]; in the hive bee the spur of the cubit is furnished with a membranous appendage which I have called the velum[2057]; and in a subgenus related toSaropodaLatr. (CtenoplectraK. MS.), the interior spur of the posterior leg is crescent-shaped, fixed transversely, and fitted on the inner side with a membrane, the edge of which is finally pectinated.
e.TarsusorManus[2058]. This is thelastportion of the leg, usually supposed to be analogous to the hand or foot of vertebrate animals; but, according to the hypothesis so often alluded to, rather the representative of their jointed finger or toe. In treating of this part I shall consider itsarticulationwith thetibia, and of its jointsinter se; thenumberof those joints; theirproportionandshape; theirpartsandappendages.
I seem to have observedthreekinds of tarsalarticulation. Thefirstis a species ofenarthrosisor ball and socket, the joints terminating in a globular head, perforated indeed for the transmission of muscles, &c., and which is received by a corresponding cavity of thetibiaor preceding joint, as may be seen in many weevils (CurculioL.[2059]). This admits of some rotatory motion.—Thesecondis amixedarticulation betweenenarthrosisandginglymus, when at the base of the ball a deep transverse channel receives a corresponding ridge of thetibiæor preceding joint: this may be found inRutelaand probably many other Lamellicorn beetles; and something very similar in the Predaceous ones.—Thethirdkind is where there is little or no inosculation, and the joints are scarcely more thansuspended: this takes place in theOrthoptera,Neuroptera, &c.; but inBlattaand the hind legs ofMantisthere is some approach to the foregoing kinds.
We are now to consider thenumberof joints of thetarsus, which varies considerably in the different Orders, and in one has been assumed as a clue for a subdivision of it into sections[2060], which, though not perfectly natural, is very convenient, and has been adopted by most modern Entomologists. In treating of this head, I shall use those denominations that have been employed by M. Latreille and others to express the variations of the number of the tarsal joints in theColeoptera, but shall apply them to insects in general. Insects in this view, therefore, may be calledpentamerous;heteromerous;tetramerous;trimerous;dimerous; ormonomerous.
Pentamerousinsects are those which havefivejoints inalltheirtarsi. This is the most universal, and may be called thenaturalnumber of these joints. More than half theColeopterabelong to this section; in theOrthoptera—theBlattidæ,Mantidæ, andPhasmidæ; all theLepidopteraexcept those butterflies calledtetrapi(Vanessa, &c.); all theTrichoptera,Hymenoptera, andDiptera; in theNeuroptera—Ascalaphus,Myrmeleon,Hemerobius,Corydalis, &c.; and in theAptera—Pulex[2061].
Heteromerousinsects are those in which the number of these joints varies in the different pairs of legs[2062]. These variations, like the spurs, may be expressed by three figures, the first representing theanterior tarsus, the second theintermediate, and the third theposterior. I begin with 5:5:4. This number represents those beetles that have beenexclusivelyregarded asheteromerousby modern Entomologists—of this description is the LinneanTenebrio,Meloe, &c., now subdivided into numerous genera; they havefivejoints in the two anterior pair, andfourin the posterior. The tarsal joints of the aquatic genusHydroporus(a singular anomaly in the Order to which they belong) are expressed by 4:4:5,thus reversing the number in the preceding tribe: other Heteromerous genera are to be found amongst theHemiptera. Thus, inRanatrathe numbers are 2:1:1; inSigaraandNauceris1:2:2; in a new subgenus betweenBelostomaandNaucoris(XiphostomaK. MS.), brought by Dr. Bigsby from Canada, 3:2:2; in theLepidopterathe butterflies calledtetrapi(Vanessa, &c.) may be expressed by 1:5:5. Amongst theApteraandArachnidathere are three remarkable genera, which if their pedipalps are included may be deemed Heteromerous. I meanPhrynus,Thelyphena, andGaleodes;—in the former the numbers will be *:4:4:4, the asterisk denoting more than ten; in the second, 8:4:4:4.; and inGaleodes(in which the first pair of pedipalps are not chelate, the mandibles performing their office) the numbers are 1:1:3:3:3.[2063]
Tetramerousinsects are those in which all thetarsiconsist offourjoints; these in theColeopteraare next in number to thepentamerous—indeed a very large proportion of them strictly speaking are really of the latter description, since in Linné's four great genera,Curculio,Cerambyx,Chrysomela, andCassidaand some others, theclaw-joint(ungula) consists oftwoarticulations, one very short, forming merely the ball at its base[2064], which inosculates in the socket of the preceding joint, and the other constituting the remainder: if you carefully separate these two pieces, you will find that the last inosculatesin the summit of the ball, and is moved by appropriate muscles[2065]. This structure probably permits the readier elevation and depression of this joint. In theOrthopterathe tetramerous genera are those which Linné calledTettigoniaamongst hisGrylli(LocustaF.);Acheta monstrosaalso, and in theNeuroptera,Raphidiabelong to this section.
Trimerousinsects are those whosetarsiconsist of onlythreejoints. Amongst beetles the Lady-birds (CoccinellaL.) are remarkable for this structure, but in themthe claw-joint is alsobiarticulate, so that strictly speaking they aretetramerous; in theOrthopterousOrder the migratory locusts (LocustaLeach) belong to this section, as likewiseGryllusLatr. andGryllotalpaLatr.: in the first of these genera is an appearance of there being more joints in thetarsus, because there is more than one cushion below the first[2066]. To this section also belong the great majority of theHemiptera, excluding only those tribes that connect the two sections of the Order constituting the two Linnean generaNepaandNotonecta; theLibellulinalikewise belong here, as do also theScorpionidæandScolopendridæ.
Dimerousinsects are those that havetwojoints in all theirtarsi. Such are thePselaphidæin the Coleoptera Order[2067]; in theHemiptera—BelostomaandNotonecta; in the hexapodAptera—Pediculus; in the octopod—theAcariof Linné; in the myriapod—Iulus; and in theArachnida—theAraneidæ.
Monomerousinsects are those which have only a single tarsal joint. Only one Coleopterous and also one Hemipterous genus is so distinguished: the first isDermestes ArmadillusDe Geer[2068], and the second the common water-scorpion,NepaLatr. Among theApterawe findNirmus,Podura,Sminthurus, &c., that belong to this section.
To the above sections another may be added for those insects whosetarsihave more thanfivejoints, which may be denominatedPolymerous. Here belong the generaGonyleptesK.,PhalangiumandScutigeraLatr. In the first the number of joints varies fromsixtoeleven, and in the two last they far exceed that number, amounting in some species ofPhalangiumto more thanfifty, and becoming convolute like the antennæ of Ichneumons[2069].
I am next to notice theproportionsandshapeof thetarsusand its joints. The most general law is, that it shall be shorter and more slender than thetibia; but it admits of several exceptions—thus, inMegasomaK.[2070], inallthe legs; inAgrostiphilaMcL. MS.[2071]in theintermediate, and inAmphicoma lineatain theposteriorpair thetarsiare the longest; inTrichius Deltathese last are longer than the thigh andtibiatogether. In some insects thetarsiare disproportionally short, as inCassida, thePselaphidæ,LocustaLeach, &c. Though generally more slender than thetibia, in several instancesthey are as thick or thicker, or more dilated, as in most of the tetramerous beetles, which being climbers require a dilatedtarsus. Again, comparing the three pairs of this joint with each other, the most general rule is, that theanteriorshould be theshortest, and theposteriorthelongest: but in some, as the Capricorn beetles, &c., they are nearly equal in length; in others, asLytta marginata, theanteriorpair, and inRhipiphorustheintermediate, are the longest; inTrichius Deltathese last are the shortest. With respect tothickness, the anteriortarsi, except in many males[2072], are not very strikingly different from the rest.
With regard to the proportion of the joints of thetarsusto each other,—according to the most general law, the first is the longest, the last next in length, then the second and third, and the fourth is the shortest. InGonyleptesK. and otherPhalangidæthe first is almost thrice the length of all the rest taken together; but there are numerous exceptions to the rule. In the femaleCarabithe first joint is not longer than the last, and in the males not so long; and inHydrophilus, &c., it is the shortest of all. Again, the second joint is longer than the three following ones inDasytes ater[2073]; and than the last inCicindela sylvatica: thethirdjoint is shorter than the fourth inLampyris ignita: it is longer than the first inDonacia, manyMelolonthidæ, &c. Once more, thefourthjoint, usually the shortest of all, is longer than the second and third inAnthia, &c. Lastly, the claw-joint, usually the second in length, in theEproboscideaLatr. (HippoboscaL.) is very long and large, while the four first joints are soextremely short as to be scarcely distinguishable from each other: it is the shortest of all inColymbetes, &c.; it is of the length of thethirdinCicindela sylvatica, of thefourthinC. sexguttata. Though commonly the slenderest joint of all, particularly so inRaphidia, in many Heteromerous and Lamellicorn beetles it is the largest, conspicuously so inMellinus tricinctus. Sometimes, as inBuprestis chrysis, &c., all the tarsal joints are nearly equal in length and thickness.
We are next to say something upon theshapeof thetarsiand their joints. In general we may first observe that their upper surface is commonly more or less convex, and the lower flat or concave: in insects that are swiftrunners, as the terrestrial Predaceous beetles, they are usually slender and filiform[2074]; in those that swim, asDytiscus, the two posterior pair taper nearly to a point from the base to the apex[2075]; in some that climb, asBuprestis, they are rather flat and linear; and in others (the Weevils,CurculioL.) they grow gradually wider towards the claw-joint[2076]; sometimes, as inMordellaLatr., the four anteriortarsiare of this shape, and the posterior pair setaceous. InGyrinusthe four posterior are flat and triangular; and in that extraordinary insectGryllus monstrosusthetarsiare foliaceous and lobed[2077]. In many males and some others theanteriorpair orhandsare of a different shape from the twoposterior: thus, in severalCarabithey are lanceolate; inStaphylinus,Creophilus, &c. in both sexes they are often nearly circular, like those of maleDytisci[2078]. With regard to theshapeofindividual joints it may be said in general that they are rather triangular, with an anterior sinus for the reception of the succeeding joint: the first joint usually departs most from this form; in the bees it is commonly much larger than the rest, especially in the last pair of legs, and nearly forming a parallelogram[2079]; inEuglossait is trapezoidal; in the majority nearly linear or filiform. With regard to theirtermination—inBrachycerusand some ants (Ponera,Myrmica, &c., Latr.) thethree firstjoints; inDascillus,Lycus reticulatusand affinities, thethirdandfourth; and in the great majority of the Tetramerous insects thepenultimatejoint is bilobed; although in most Predaceous beetles this joint is entire or simply emarginate, yet inColliurisit terminates in a single oblique lobe; and inLebia,Drypta, &c., it is nearly bipartite. I must now advert to theUngulaor claw-joint: it is usually clavate or thickest at the end and curved; but in theAsilidæit is shaped like a vase or cup; inPhanæus, in the four posteriortarsi, in which the claws are obsolete, it is thickest at the base and sharpest at the extremity[2080]; it usually forms an angle with the rest of thetarsus, rising upwards, which enables the insect to move more easily without hindrance from the claws, and also more readily to lay hold of any object it meets with; but in the Lamellicorn beetles and many other insects it is in the same line with it. As in the beetles last mentioned this joint is often inserted in the extremity of the preceding one; but inŒdemerait articulates with the middle of its upper surface; and inLycusand a numerous host ofTetramerousbeetles it springs from its base, just behind where it diverges into two lobes.
I shall next call your attention to the different kinds ofappendageswith which thetarsiare furnished. They are seldom armed, like thetibiæ, with teeth, or spines, or horns; but something of the kind occasionally distinguishes them. InPhileurus,Oryctes, and several otherDynastidæ, the first joint is armed at the apex externally with a considerable mucro; in the fore-leg ofDasytes atera similar process is prolonged into a crooked horn[2081]. But the most important appendages of thetarsiare theclawswhich almost universally arm their extremity, and which appear clearly analogous to those ofbirds,quadrupeds, &c., though probably differing as to their substance[2082]. Some few, however, are without them; this, as I lately observed, is the case withPhanæuswith respect to the fourposteriorlegs; theanteriorones ofVanessaamongst theLepidoptera, and all those ofStylopsand manyAcariL., are also without them: this is likewise the case with the first pair of legs, or the second of the pedipalps ofGaleodes. In this genus these organs consist of two joints[2083]. With respect tonumberthey vary in different tribes, but not so much as thecalcaria: these variations may likewise be represented by three numbers. The most natural istwoin all thetarsi, exhibited by the Predaceous beetles and the great majority; 2.2.1. are to be found inHoplia,Anisonyx, &c.[2084]; 1.2.2. inBelostoma; three in all the legs in theAraneidæ[2085]; inMeloe[2086],Elater, &c., each claw is double or consisting of two, which makesfourin each leg; and in manyHippoboscidæthere aresix[2087]; inNepaand the Myriapods there is onlyone. In most insects, perhaps, the claws are simple or undivided[2088]; but inGaleruca,Melolontha subspinosa[2089], &c., they are bifid at the apex; as is the exterior claw of the four posterior legs inChasmodiaandMacraspis[2090]McL., and ofallinMelolontha horticola; inSerica brunneaMcL. the claws are all cleft at the extremity, but the internal tooth is broad, flat, and obtuse[2091]; inMelolontha vulgarisandPelidnota punctataMcL.[2092], the claws are armed with an internal tooth near the base[2093]. In theAraneidæ, which have three claws, the two external ones are furnished with several parallel teeth, which the animal uses to keep separate the threads of its web, and probably for other purposes[2094]; and some Predaceous beetles, asLebiaandCymindis, have both their claws similarly furnished[2095]. These organs vary in their relativeproportions: thus, inAnoplognathusthe inner claw is much smaller than the other[2096]; and inElater sulcatus,fuscipes, &c., it is represented by a mere bristle; inHoplia, in the anteriortarsusit is not half the length of the outer one[2097]; inAreodaandPelidnotaMcL. this last is the smallest. They vary also in length—inRynchænus,Ascalaphus, &c., they are very short; in the Lamellicorns,Galeodes, &c., very long; and inMyrmeleonlonger than the claw-joint. With regard to theircurvaturethey generally form the segment of a circle; in manyAsilidæthey are crooked like the claws of the eagle[2098], and the posterior one of theHopliæis bent like a hook[2099]; they most commonly diverge from each other; but in theRutelidæ,Anoplognathidæ, &c., they are perfectly parallel, and in the former often inflexed[2100]. With regard tootherappendages of the part we are treating of, if you examine the stag-beetle and many other Lamellicorns, you will find between the claws a minute but conspicuous joint terminated by two bristles which seem to mimic theungulaand its claws; these parts are what are denominated in the table thepalmula,plantula, andpseudonychia: in the stag-beetle these are long[2101]; in theMelolonthidæshort[2102]; and in manyCetoniadæthey resemble an intermediate claw.
The most remarkable of theappendagesof thetarsiare to be looked for on their under side or sole (solea), and are the means by which numbers of insects can overcome atmospheric pressure and walk against gravity. Many of these have been fully described in a former letter[2103]; but much that relates to them was there omitted, which I shall now detail to you.Fourkinds ofpulvilli, as I would call these appendages, are found in the sole of insects, upon each of which I shall make a few remarks.
Thefirstis acushionor brush composed of very thickly set hairs or short bristles: examples of this you will find in the majority ofTetramerousandTrimerousbeetles. InChrysomela,Timarcha, &c., there isoneof these cushionson each of thethreefirst joints; inPrionus,Liparus, &c., there is a pair; and inCoccinellaon thetwofirst; in others (Balaninus Nucum, &c.) a pair only on the penultimate joint; inCalandra Palmarum,Rhina barbirostris, &c., that joint has an intire cushion; inEurynotus muricatusK.[2104]the three first joints of the four anterior tarsi are similarly circumstanced, but the cushions resemble sponge[2105].
Thesecondkind of cushion is a vesicularmembranecapable of being inflated. This distinguishes the tarsi ofThrips[2106], and manyAcariL.[2107]; likewise those ofXenos[2108]; and also of manyOrthopterafully described on a former occasion[2109], though the fact of their capacity of inflation has not been ascertained, belong to this section.
Thethirdkind of covering of the sole is when the three or four first joints of thetarsuseach terminate inoneortwomembranous lobes or appendages: of the first description isPrioceraK., in which the lobes are involute[2110]; and of the secondRhipiceraLatr.[2111], in which there is a pair on each joint, in the Brazil species set with very fine hairs.
Thefourthand last kind are what may with the utmost propriety be denominatedsuckers, since their use as such is clearly ascertained. These are not only to be found in a large proportion of theDiptera, in some of which there aretwoof them, as in theAsilidæ[2112]; and in othersthree, as theTabanidæ[2113]; but also in many of the subsequent Orders: thus, in the HeteropterousHemiptera, inScutelleraandPentatoma, but not theReduviadæ, and in the Neuropterous genusNymphesLeach there is a minute one under each claw. It is discoverable between the claws in manyHymenoptera, asApis[2114],Vespa, &c. But the genus that exhibits to the curious Entomologist the most singular and elaborate apparatus of this kind isDytiscusLatr.; and the examination of the under side of thehandof anymaleof this genus will almost compel the most inattentive observer to glorify the wisdom and skill of theAllfatherso conspicuously manifested in the structure of these complex organs. For this part in these, instead of two or three pedunculate cups as in the insects just mentioned, is composed of a vast number, some large and some small. If you take a male specimen of the commonD. marginalis, you will find that the three first joints of the hand are very much dilated, so as to form a plate or shield nearly circular, fringed all round with stiffish hairs; if you next examine the under side of this plate with a good magnifier, you will discover at the base, where it is united to the cubit, two circular cups, the external one more than three times the size of the other, with an umbilicated centre[2115]; besides these two larger cups the rest of the shield is covered by a vast number of minute ones of a similar construction[2116]: the larger cups are nearly sessile, but the smaller are elevated upon a tubular footstalk[2117]; the three first joints of the intermediatetarsiare also dilated, but not into anorbicular shield, and thickly set with minute pedunculated suckers[2118]. The structure varies however in different species. Thus inD. limbatusthe shield is triangular with the smaller suckers at the base, and two rows of larger oblong ones, concave but not umbilicated, at the apex; in another Brazilian undescribed species (D. obovatusK. MS.) the shield is oblong and quite covered with suckers like those last mentioned; inD. sulcatus(AciliusLeach) almost the whole plate is occupied by a very large sucker, above which, at some distance in the inner side, are two smaller ones, while the extremity of the shield is covered by minute ones elevated on long footstalks: the central umbilicated elevation of the large one, which nearly fills its cavity, is in this species beautifully radiated. The male ofColymbetes transversalishas also an orbicular shield, but the suckers are much less strongly marked. The use of this organ has been before sufficiently explained[2119].
A few words will be necessary upon thefoldingof the legs inrepose. When insectswalk, the thigh is usually in anascendingposition, risingabovethe horizontal line, thetibiaforming with it rather an obtuse angle, and thetarsusnearly a right one with thetibia; but in the Myriapods, as far as I can unravel their swift many-footed motions, these angles in walking do not take place; in repose however, in many insects, thecoxaforms an angle with the thighbelowthe horizontal line and with thetibiaabove it, and thetibiaandtarsuscontinue in the same line, and point downwards nearly vertically; in others, as in theTetramerousbeetles, the last-mentioned jointsform an angle with each other and turn upwards, thetibiahaving an external oblique cavity to permit this; but the insects most remarkable for packing close their legs are those carnivorous generaDermestes,Anthrenus,Byrrhus, &c. In the last-mentioned genus there are cavities in the under side of the trunk, in each division of the breast, and at the base of the abdomen, to receive the legs when folded; thecoxæhave also a cavity to receive thebaseof the thigh. In theanteriorlegs this last part has a longitudinal one on itsupperside, and in the fourposterioron theunder, which receives thetibiæ, which at the inner edge are straight, and at the outer curvilinear, and thetarsiare turned up and received by the concave part, on theanteriorside of thefirstpair and theposteriorside of the twolastof thetibiæ, so as to lie between it and the body: when the legs are close packed, the animal looks almost as if it had none. I have observed that whenDytiscirepose on the water, the posterior legs are turned up and laid over the elytra, and curved towards the head.
vi.Pectines.I must next say a few words upon a remarkable organ, which seems in some degree supplementary to thelegs, by which theCreatorhas distinguished the genusScorpio, called from its parallel teeth, set in a back, theirpectenor comb[2120]. This back consists of two or more articulations, is attached by its anterior extremity to the sides of the posterior piece of themesostethium, and is marked by a longitudinal furrow or channel. The teeth, which vary in number in the different species, andin the same species at different periods of its growth, are usually ovato-lanceolate, or obtusangular, furnished on their exterior edge with what appears to be a longitudinal sucker, and supported between their bases, or at the base, both within and without, by triangular, conical, or subglobose props. With regard to theuseof these organs, it has not been clearly ascertained. Amouroux states that he has seen the animals use them as feet, and he conjectures that by them they may fix themselves and turn upon them as on a pivot, when they have to make a retrograde movement[2121]. M. Latreille, from their having branchial pouches immediately under them, seems to think that they are connected with respiration[2122]. This may be true; but from the suckers just described, I am inclined to think with Amouroux, that they are useful to the animal in itsmotions, and that like the suckers of the Gecko, flies, &c., they enable it to support itself against gravity and to climb perpendicular surfaces.
Whether the five obtriangular plates, elevated on a pedicle, which are found arranged in a series on the under side of each of the jointedcoxæof the posterior legs inGaleodes, are at all analogous to the pectens of scorpions, has not been ascertained[2123]. M. Leon Dufour watched them very attentively in one species (G.intrepidus), but he could observe no motion in them[2124].
Theabdomenof insects, which we are next to consider, is thethirdgreat section of the body, and is the seat of the organs of generation, as well as of a principal part of those connected with respiration. My remarks upon it will be under the following heads: Itssubstance;articulation with the trunk;composition;shape and proportions; itsappendages; and itsclothing.
i.Substance.Under this head I may observe in general, that where the abdomen is protected byhardelytra or tegmina, as in mostColeoptera, and many HeteropterousHemiptera, theupperside is generally of a softer and more flexible substance than theunder, which from its exposure requires a greater degree of hardness and firmness to prevent its being injured. In some,—as theDynastidæand those beetles whose elytra are connate, or as it were soldered together, the former is scarcely more thanmembrane. In others of the above tribes, nearly thewholeof the back of whose abdomen, as inStaphylinus; or only its analextremity, as inMelolontha; or itssides, as inLygæus, &c., is not covered by the elytraor tegmina, that part, as was requisite for its protection, is harder than the covered portion.
ii.Articulation with the trunk.Two distinct modes of this articulation take place:—in the first the abdomen is united to the trunk by thewholediameter of its base, without any appearance of incision; in the other only asmall partof that diameter, with a very visible incision. All the Orders, except the majority of theHymenopteraandDiptera, and theAraneidæ, belong to thefirstof these sections; for in all these the aperture by which the abdomen is suspended to the trunk, occupies the whole of the base; I saysuspended, because, though in many cases it inosculates in the posterior cavity of the latter part, it does not in all, and the margins of the orifice are united by ligament to those of that cavity. Indeed, in theColeopteraand others that have a somewhat prominent metaphragm[2125], the trunk may with more propriety be said to inosculate in the abdomen. With regard to thesecondsection,—those in which the orifice is of less diameter than the base, occupying only a portion of it,—it may be further subdivided into those whose abdomen issessile, and those in which it is united to the trunk by the intervention of a long or shortpedicleor footstalk: to the first of these subdivisions belong all thoseDipterathat have an incision between the trunk and abdomen—for many tribes of this Order, as theTipulidæ,Asilidæ, &c., belong rather to thefirstsection—and theAraneidæ; the abdomen, however, inallis merely suspended, without any inosculation. To thesecondsubdivision belong all theHymenoptera, except theTenthredinidæandSiricidæ, the abdomen of which is united to the trunk by the whole diameter of its base; these may be further subdivided into those that have a veryshortpedicle and those that have alongone; but as the mode of articulation in both these is the same, there will be no necessity to consider them separately. M. Cuvier has included theDipteraandAraneidæin the same tribe with suchHymenopteraas have a petiolate abdomen[2126]; but as the manner in which the latter articulates with the trunk is widely different from that of theDiptera&c., I thought it best to consider them as distinct; especially as in theDipterathere is no tendency to a pedicle, while only the above two tribes ofHymenopteraare wholly without it. This learned author thus describes the articulation where the abdomen is connected by a pedicle. "They have," says he, "a real solid articulation, a kind of hinge in which the first segment is emarginate above, and receives a saliant portion of the trunk upon which it moves; this articulation is rendered solid by elastic and powerful ligaments; muscles which have their attachment in the interior of the trunk are inserted in this first segment, and determine the extent of its movement[2126]." But this passage by no means conveys an adequate idea of the singular mechanism by which theDivine Artificerhas enabled these little creatures to impart the necessary movements to an organ so bulky compared with its very diminutive point of attachment. As no author that has fallen in my way has examined the articulation of the abdomen with the trunk in theseHymenopterawith the attention which it merits[2127],I shall enlarge a little upon it. You would be surprised, and not without reason incredulous, were I seriously to assert that these insects lift their weighty posteriors by means of aropeandpulley; yet something like this really does take place, though not with all in a manner equally striking. The point of articulation in the insects in question, except inEvania, is at the base of the metathorax just above the posterior pair of legs: here you see a small orifice, either insulated or connected by a narrow opening with the larger one, when the abdomen is removed, which in many instances, as in the common wasp, is surmounted by another still smaller, through which, if you examine it attentively, you will find there is transmitted a flat and sometimes broadish ligament or rathertendon, in which the levator muscles of the abdomen, attached by their other end to the metaphragm[2128], terminate: another minute orifice above the base of the pedicle affords a point of attachment to the tendon, so as to give it prize upon the abdomen. Here the upper orifice in the trunk is thepulley(trochlea)[2129], the tendon is therope(funiculus)[2129], and the abdomen is the weight to be lifted. When the muscles contract, the tendon, running over the edge of the aperture, is pulled in, and the part just named is elevated; and when they are relaxed the tendon is let out, and it falls. Some little variation in the structure takes place in different tribes: thus, in theFormicidæ,Scoliadæ, &c., instead of a separate orifice, the part I call the pulley is merely an upper sinus of the large orifice that receives the pedicle of the abdomen. The shape of these orifices, both of the trunk and abdomen, varies in different genera: thus, in the bee it is triangular, with the vertex reversed; and in the wasp the upper one is circular, and the lower one transversely oblong; but in all, the apertures of the trunk correspond with those of the abdomen. InEvania, in which the minute abdomen is inserted in the upper side of the metathorax, there is scarcely any trace of this structure. With regard to the articulation of the pedicle itself with the lower orifice of the trunk, it appears simply suspended, with little or no inosculation. I may observe under this head, that though the abdomen in almost all insects is wholly clear of the cavity of the trunk, yet in somePhalangidæ(GonyleptesK.) it appears almost retracted within it[2130].
iii.Composition.I shall next consider thesegmentsinto which the abdomen is usually divided, their number, and other circumstances connected with them. In theHippoboscidæ,Acaridæ,Phalangidæ, andAraneidæ, the part we are considering is not divided into segments, though in some instances, as inGonyleptesand the cancriformEpeiræ[2131], they are represented byfolds; but in the great majority of insects it consists of several dorsal and ventral pieces or segments, forming by their union theannulior rings into which it appears divided. Thenumberof these abdominal segments varies in differentinsects; I have noticed more thantwentysuch variations, and probably there are many more. Before I give you them in detail, I must first observe that the dorsal and ventral segments, though sometimes they correspond in number, yet very often do not, the dorsal most commonly exceeding the ventral by a segment; in a few cases however the reverse takes place. In the sexes also there is frequently a difference in the number of segments, as has been before observed[2132]. I shall express the variations in question by two figures, thefirstrepresenting the number ofdorsalsegments, and thesecondtheventral—they usually only express theapparentsegments: perhaps a very general examination and dissection might bring many of them nearer to a common type.
I shall next explain the articulation of the segments with each other, both that of the rings formed by the union of the dorsal and ventral pieces, and that of those pieces themselves. In general it may be stated with respect to the former, that each ring is suspended by ligament to that which precedes it; but this takes place inthreeways—in some the margins of the suspended ringstoucheach other only, with little or noinosculation; in others thedorsalsegments only touch, and the base of eachventralis covered more or less by the apex of the preceding one; and in others again the base of the whole ring, both above and below, is so covered, or inosculates. The first kind here mentioned you will find exemplified inMelolontha,Geotrupes,Musca, &c.; the second inScorpio; and the third inStaphylinus, theHymenoptera, and many others. In theColeoptera, says M. Cuvier, speaking of the movements of the abdomen, the rings only touch each other at the margin, and themovement is very limited; whilst in theHymenopterathey are so many little hoops, which inosculate in each other as the tubes of a telescope, one third only of their extent often appearing uncovered[2136]. We see the reason of this structure when we consider the calls they have for greater powers of movement in this part in laying their eggs, and annoying their enemies and assailants; and also in theStaphylinidæto enable them to turn up their abdomen like a scorpion, both as a posture of attack, and to fold their wings: in all cases, however, as far as my observation goes, these animals, when they want to lengthen this part, can disengage the rings from almost all inosculation, so that no impediment remains to any movement.
The articulation of the dorsal and ventral segments with each other is next to be considered. InIulusand someCentrotithe ring appears to be formed of a single piece, with scarcely any trace of the existence of any such division; it is however almost universal, and is ofthreedescriptions; in the first the dorsal segments are united to the ventralatthe lateral margin or edge of the abdomen; in the second it isabovethis margin, and in the thirdbelowit. You will find that inFulgoraand many other HomopterousHemipterathese segments unite at the margin, as they do likewise inCimex lectulariusbelonging to the other Hemipterous section; but in the rest of the Heteropterous tribes, the ventral segments turn upwards, and their union with the dorsal is in the back of the abdomen; in these theHemelytraand wings only cover the dorsal segments, leaving the edge, formedof the ends of the ventral, uncovered. The Lamellicorn beetles also, and many otherColeoptera, exhibit the same structure. To the last description, in which the dorsal segments turn down to meet the ventral, belong theLepidoptera,LocustaLeach; likewiseSirex,Chrysis, and many otherHymenoptera. The articulation between these segments is by means of an elastic membranous ligament, which usually is not externally visible; but in many instances, in which the connecting ligament is of a firmer substance, as inScorpio,Thelyphonus, andPhrynus, it is very conspicuous, and in the latter genus exhibits many longitudinal folds, as it does likewise inGryllotalpa, which must permit a vast extension of the abdomen. In this membrane, in some cases, as inDynastesMcL.,Melolontha, &c., the two or three first spiracles are fixed[2137]. In theHymenopteraand many other insects the dorsal segments do not unite by theirmarginwith the ventral, but theendof each dorsal laps over that of the corresponding ventral.
Dorsal segments[2138]. I shall next notice the segmentsseriatim, in the order of their occurrence, beginning with thedorsalones. The most remarkable circumstance with respect to these that occurs to my recollection takes place in the Cancroid spiders (Epeira cancriformis,aculeata, &c.), in which the back of the abdomen is formed by a plate, in some extended in a transverse direction (E. cancriformis), in others in a longitudinal one (E. aculeata), of a much harder substance than the under side and quite flat, set with strong sharp spines, in the former species apparently moveable, and terminating behindin a piece resembling in some measure the scutellum of theStratyomidæand similarly armed with a pair of spines[2139]: inE. aculeatathe sides of the abdomen, under the plate, have a number of longitudinal folds like those ofPhrynus. InCryptocerus, a genus of ants peculiar to South America, thefirstsegment, not reckoning the pedicle, forms almost the whole back of the abdomen, and the three last are so minute as scarcely to be distinguishable. Nothing very remarkable is exhibited by the other segments, except that inTrichiusthepenultimateis the largest; in someStaphylinidæ(S. splendens) andBrachini(B. melanocephalus) it is emarginate, and in the former tribe also often terminating in a white membrane. Thedorsalsegment most worthy of notice is the last, which is called thepodex; for though in general it is a minute piece, often retracted within the abdomen and invisible, as in manyDiptera, yet sometimes it is the most conspicuous of the dorsal segments. It is most commonly triangular, and usually deflexed and forming an angle with a horizontal line; but inClytra,Chlamys, andOryctes, it is inflexed; in many Lamellicorns it is nearly vertical. InTettigoniaF., many other HomopterousHemiptera, and someHymenoptera(Cimbex), its sides turn down and become ventral; on its lower side it has in these a longitudinal cavity which receives the ovipositor in repose[2140]. In many other insects it unites with the last ventral segment, thehypopygium, to form a tube for that organ, as you will find inCallidium violaceum[2141], manyMuscidæ, andThelyphonus. As to itsterminationthe podex is sometimes bifid,Blatta; bipartite,Ranatra;mucronate,Sirex; acuminate,Melolontha vulgaris,Trichius hemipterus. Generally this part is flat; but the disk is elevated or gibbous inOryctesand some other Lamellicorns. In the majority of theColeopteraOrder it is quite covered by the wings and elytra; but in many of the last-mentioned tribe, and sometimes the penultimate segment also, it is not covered by them[2142]. In some insects the piece we are considering appears to consist oftwosegments; in the male ofLocusta morbillosathe whole podex is rhomboidal, but it is formed by two triangular pieces which articulate with each other; this structure permits the more easy elevation of the terminal one for the extrusion of the feces.
Ventral Segments[2143]. We are now to turn our attention to theventralsegments of the abdomen. The first of them is what is called theepigastrium[2144]in the table. This part, according to M. Chabrier, is of considerable importance to the animal in flight, as, by its pressure against the trunk, not only regulating the movements of the abdomen, but as, in his opinion, contributing to push forward the trunk[2145]in the descent of the animal. It is remarkable only in theColeopteraand HeteropterousHemiptera, to which my observations upon it will be confined. It may be stated as usually consisting of two articulations, that nearest the trunk being narrow, and in the Predaceous beetles[2146], as also inScutellera,Pentatoma,&c., interrupted in the middle[2147]. In manyLamellicornsthis joint is concealed under the posteriorcoxæ, and with the anterior part of the second forms a hollow cavity for their reception; this last joint is what is properly theEpigastrium, the former, especially when distinct, being called in the table theHypochondria. InSagraandBrentustheepigastriumis particularly conspicuous for its size, in the former occupying half, and in the latter nearly two-thirds of the under side of the abdomen; but in general it is distinguished from the remaining segments only by the central mucro or point that terminates it towards the trunk[2148], and which is received by a sinus of themetasternum; this point is generally minute and triangular, but inSagrait is large and rounded at the extremity, and inCalandrait terminates nearly in a transverse line somewhat waving. It is most remarkable, however, in some species of the Heteropterous genusEdessaF.; for inE. nigripesand affinities it is a sharpsterniformconical horn, which passing between the four posterior legs covers the end of thepromuscis. In fact, this part appears a kind of abdominalsternum. In theCetoniadæ, &c., theHypochondriaunite before this mucro, and form a ridge which articulates with it, and dips towards the abdominal cavity; inScolytustheepigastriumis much elevated from the rest of the ventral segments, so that the under side of the abdomen appears as if it were suddenly cut off, whence Herbst's awkward though not inexpressive name,Ekkoptogaster; this part in this genus has something of a posterior mucro.
The intermediate ventral segments exhibiting no veryremarkable peculiarities, I shall pass them without further notice, and call your attention to the last, which is opposed to thepodex, and which I have named thehypopygium[2149]. Though usually asinglesmall piece, inEdessaand manyPentatomæit consists ofseveralplates; and inTrichiusit is very large: it is mostlyintire, but in the maleDytisciit iscleft; inLamia ocellatatrilobed; inEdessatripartite; inCentrotus Taurusit is boat-shaped and hollowed out to receive the stalk of the ovipositor. It is also generally in the same line with the body, but inXenosit is turned up and bent inwards[2150].
iv.Shape.With regard toshape, in some Orders the abdomen varies considerably; but the most general form is one that approaches to trigonal, so that a transverse section will be a triangle, with the vertex more or less obtuse, and the base more or less convex; some tendency to this form will often be found even in those insects whose abdomen appears almost as flat as a leaf, as in manyAradi. In the hive-bee the transverse section is almost an equilateral triangle; inBelostoma grandisthe disk of the under side of the part in question is longitudinally elevated into a trigonal ridge, the section of which is an equilateral triangle, the sides being quite flat. In general, in the vertical section of an abdomen, the vertex of the triangle pointsdownwards, but inLibellulaF. it pointsupwards. InBlattathis section is nearly lanceolate; inStaphylinus olensit is a segment of a circle with the convex side downwards; inÆshnaF. with that side upwards; and inAgrionthe section is circular. InCopris,Ateuchus, &c., the abdomen is very short andthick; inStaphylinusslender and long; inAradus,Nepa, &c., depressed and flat; compressed inOphionandEvania; conical inCœlyoxis; rhomboidal in manyMantes; boat-shaped in manyLygæi; fusiform in variousPapilionidæ; lanceolate in someIchneumonidæ, falcate in others; nearly round inDiapria pupurascens; ovate inLyrops; elliptical inAndrena; oblong in manyXylocopæ; heart-shaped in the nakedEuglossæ; triangular inDytiscus; gibbous inFlata; and vaulted inChrysis. At its base it is truncated inSirex; retuse in mostbees; forming the segment of a circle inAndrena; in general sessile, but in the majority ofHymenoptera, as has been already observed, terminating in a pedicle. The pedicle is very short in theAndrenidæandApidæ; long in theSphecidæ; thick in theFormicidæ; slender inEvania; fusiform inPelecinus; clavate inAmmophila; campanulate in manyVespidæ; nodose inMyrmica[2151]; squamigerous inFormica[2152]: it sometimes also consists oftwojoints, as inAmmophilaand manyVespidæ. As tomargin, some have none, asCentrotus; in others, asDytiscus, it is very narrow; in others again, wide and flat, as in theNepidæ; inStaphylinus, &c., it is distinguishable only on the upper side of the abdomen; inLocustaLeach only on the under side, though mostly intire; it is serrated inBlatta, sinuated inAcanthia paradoxa, and crenated inCerceris.
v.Proportions.These vary greatly in the different tribes; in some the abdomen is long and slender, as inLocusta, andStaphylinus; disproportionably so in a remarkable degree in someAgrionidæfrom South America,asA. lineare, &c.[2153]; in others it is extremely short and thick, as inCopris, &c.; a mere appendage inEvania; it is shorter than the elytra inTrox; of the same length in most beetles; longer inMelolontha,Hister, &c.; disproportionably so inStaphylinus: though usually of the same width with the trunk, in manyMantidæit is much wider[2154]; and more slender in theLibellulina,Myrmeleon, &c.
vi.Arms and Appendages[2155]. These are various; and may be considered under the following heads:processes;organsofrespiration,motionandprehension;weapons; and otheranal appendagesthe use of which is unknown.
1.Processes.Under this term I include all prominences of whatever kind, whethertubercles,teeth,spines, orhorns, that arm any part of the abdomen. Many of these aresexualcharacters, and have been sufficiently described in a former letter[2156]; I need not therefore detain you long on this head. Of the first kind is a remarkable elevation that distinguishes the second ventral segment ofScolytus Destructor(Ips ScolytusMarsh.) or of a species allied to it[2157]; inS. pygmæus(I. multistriatusMarsh.) the same segment is armed by a flat horizontaltoothor horn; in anAradusfrom Brazil, before alluded to[2158](A. laminatusK. MS.), the margin of the abdomen is surrounded by eight flat subquadrangular laminæ; in another species figured by Stoll[2159], it is cut out into bays by a number ofdenticulated teeth; and inAcanthia paradoxaby long spinose lobes[2160]. InEdessaF., another genus of bugs, the abdomen usually terminates in four strong sharp dentiform spines, the intermediate ones being the shortest, and in some themarginis also armed with spines[2161]; occasionally the anal spines are very long[2162]. In addition to the ventral horns before mentioned that distinguish the sexes of some insects[2163]; the males of the genusConops, a two-winged fly, have, on the antepenultimate ventral segment, a singular process, varying in length and shape in the different species, standing nearly at right angles with the belly, convex towards the trunk, and concave towards the anus. De Geer supposes that with the anal extremity this forms a forceps with which this fly seizes the other sex[2164].
2.Organs of respiration[2165]. I shall defer my account of thespiracles, and other external respiratory organs, till I come to treat of the system ofrespirationin insects, when every thing connected with that subject will be most properly discussed; but there are certain appearances in some insects, which at first sight seem to partake of the same character, but which being really independent of that vital function, may here have their place. If you examine the abdomen of the mole-cricket (Gryllotalpa vulgaris), you will easily discover the true spiracles in the folds of thepulmonarium, which separates the back of that part from the belly; if you next inspect the five intermediate segments of the latter, you will discover on each nearer the base a pair of oblique little channels,which precisely resemble closed spiracles. These may be denominated false or blind spiracles. Again, if you examine the pupa of anyScutelleraorPentatoma, in which tribe the true spiracles areventral, you will discover, placed in a square on the two or three intermediatedorsalsegments, four or six elevated points resembling spiracles, but not perforated, connected often by corrugations in the skin or crust[2166]; in the larvæ also of someReduviithe first minute dorsal segment, at each lateral extremity, has a similar elevation with a central umbilicus precisely resembling a spiracle, but still not perforated: another instance of false spiracles in this section of theHemiptera, is furnished byAradus laminatusbefore mentioned, in the perfect insect; between the spiracle and the margin of each ventral segment is a white round callus, with a dark point resembling a perforation on its exterior side, and terminating internally in a channel covered by membrane leading to the disk of the segment, so that the whole in shape resembles a tobacco-pipe[2167]. A number of similar callosities with a central impression, but without any channel, variously disposed, are also to be found in another bug,Rhinuchus compressipesK.[2168]In the Homopterous section of this Order, a series of impressed points, which may be easily mistaken for spiracles, are to be discovered on both sides of the abdomen, at the margin inCentrotus, in which the real spiracles are quite concealed.