As to the Number of the stemmata, three appears to be most universal. Reaumur mentions an instance in which he counted four in a fly with two threads at its tail; but great doubt rests upon this statement[1508]. Some Orthopterous genera, as Gryllotalpa, and many Hemipterous, as Tetyra, Pentatoma, Reduvius[1509], Cercopis, Fulgora[1510], &c., have no more than two; and in Larra and its affinities, as just observed, the posterior ones are obsolete, so as to leave only one discernible.
Where there are three of these organs, they are usually arranged in an obverse triangle in the space behind the antennæ, at a greater or less distance from them. In those male flies (Muscidæ) whose eyes are confluent, the stemmata are in a little area behind their conflux; but, as before observed, in the drone-bee and the Libellulina they are before it. This triangle is in some cases nearly equilateral, as in Perla related to the may-flies, and many Hymenoptera; in others it is acutangular, as in Locusta &c., in which the stemma forming the vertex of the triangle is before the antenna[1511]: in others, again, it is obtusangular, as you will see in Pepsis and various Hymenoptera. In the humble-bees (Bombus), a line drawn through them would form a slight curve. Their situation also varies. In insects that have only two, they are sometimes placed a little behind the eyes, or in the back part of the space between them: this is the case with most of the bugs (Cimex L.) that have them.—They are often distant, as in Tetyra F., Edessa F.; and sometimes approximated, as in Reduvius F.[1512] In many of the Homopterous Hemiptera, as Cercopis, Ledra, &c. they are planted in the upper part of the head[1513], but in Iassus their situation is on the under part; and in a North American subgenus, as yet without a name, they are exactly between the two, being placed in the frontal angle. In Fulgora their station is between the eyes and antennæ[1514]. They are most commonly sessile, and as it were set in the head; but in some, as Fulgora candelaria, they stand on a footstalk. The stemmata are set in the side of a frontal tubercle in that four-winged fly of threatening aspect, Corydalis, which in its perfect state has mandibles, but longer and more tremendous, like those that distinguish the larva only of the kindred genus Hemerobius[1515]. These organs differ little in shape, being usually perfectly round and somewhat convex; but occasionally they vary in this respect. In Fulgora serrata they are oblong, with a longitudinal depression; in F. Diadema they are also umbilicated, but the umbilicus is circular; in Corydalis they are oval; in other insects they are ovate; in some semicircular, and in a few triangular. They vary much in size: in some of these animals being so minute as to be scarcely visible, while in others, as Corydalis, Dorylus, Vespa pallida F., Reduvius, &c.[1515], they are as large as some compound eyes. They differ also in colour, though often black: in Fulgora laternaria they are of a beautiful yellow; in F. candelaria they are white; in many Hymenoptera they are crystalline, in others red: the fierce look of Reduvius personatus is rendered more hateful by its stemmata having a pale iris round a dark pupil[1516].
Let us here stop and adore the goodness of a beneficent Creator, who, though he has deprived these little beings of the moveable eyes with which he has gifted the higher animals, has made it up to them by the variety and complex structure of their organs of vision, where we have only two points of sight, giving them more than as many myriads.
5. Antennæ.—But of all the organs of insects, none appear to be of more importance to them than their Antennæ, and none certainly are more wonderful and more various in their structure, and probably uses. Upon this last particular I shall enlarge hereafter. Their structure, as far as it differs in the sexes, I fully discussed in a former letter[1517]; and the most remarkable kinds of them will be included in a set of definitions which I shall draw up for you before our correspondence on this part of my subject closes: I shall therefore now confine myself to the following particulars—namely, their number, insertion, substance, situation, proportion, general form and structure, clothing, expansion, motions, and station of repose.
As to their Number, in the majority of crustaceous animals the antennæ amount to four, but no insect has more than two. A genus recently established (Otiocerus Kirby[1518]) seems to afford an exception to this rule, since the species composing it at first sight appear to have four, and in some instances even six antennæ; but as only two of them terminate in a bristle, the other, though proceeding from the same bed of membrane, may perhaps be regarded as merely appendages. Germar, who has described a species of this genus[1519] under the name of Cobax Wintheri, considers these appendages as analogous to palpi: but as they do not proceed from the oral organs, but from the bed of the antennæ at the base of the nose[1520], they ought certainly to be regarded rather as accessories to the latter, than as representing the former. In the Aptera order the mites (Acacus L.) appear to be without these organs. In the pupiparous tribe Hippobosca they seem about to disappear; and in the Arachnida &c., as has been more than once observed[1521], the mandibulæ have been thought to represent, not indeed the antennæ of insects, but the inner pair of those of the Crustacea.
In considering the insertion of antennæ, by which I mean their articulation with the head, we must advert first to the orifice (Torulus) that receives them[1522]. This is a perforation of the crust of the head; commonly, though not invariably, circular: in Coleopterous insects often with concave lubricous sides, forming an acetabulum, with processes usual in ginglymous articulations, larger than the bulb or root of the antennæ; and which is commonly covered, except the central space occupied by the bulb, with a tense membrane. Though not in general remarkable, in some cases it merits attention. In the genus Rhipicera Latr., the elegant antennæ of whose males I have described in a former letter[1523], particularly the Brazilian species, it is a long process on each side of the nose, and might be mistaken for the first joint: in another Coleopterous genus, Priocera K.[1524], it has somewhat of the shape of a trumpet: in Cupes a tubercle rises just above the base of the antenna: a circular process forms the torulus in Fulgora and others. It is also often placed in a cavity of the front, as in several wild-bees, Melitta K., and in Locusta Leach on the sides of an elevation of that part[1525]. In a large majority of insects the bulb (Bulbus) or ball which is received by the bed, wears the appearance, especially in the Hymenoptera, of a distinct joint; but if you carefully examine it, you will clearly see that it is merely the base of the scape swelled out into a spherical or other kindred form[1526]; and often marked, as in the Cicindelidæ, with impressed points: as it is the piece by which the antenna moves in its socket, this form of a rotula was doubtless given for its more ready motion in all directions. This structure is principally conspicuous in the Coleoptera and Hymenoptera Orders: in the others the base is not so distinguished from the rest of the scape. If you carefully extract the antennæ of a beetle, say a Copris or Lamia, and examine its base or bottom, you will find that it is open for the transmission of muscles and nerves; that in its upper margin it has a deep notch or sinus, on each side of which is a smaller notch; and that all round the margin, which is very lubricous, a membranous ligament is attached, by which it was affixed in the torulus. Its articulation, therefore, seems of a mixed kind, like that of most other organs and parts of insects, partaking of the ligamentous, ginglymous, and ball and socket. In the Orthoptera, Hemiptera, &c. the articulation seems more purely ligamentous.
With regard to their substance—these organs are regulated, in some degree, by the nature of the integument of the animal of which they are appendages; in the softer insects being of a softer substance than they are in hard ones. The vertex of the joints, where they receive the succeeding one, appears in many cases to be softer than the rest of it, and especially towards the apex, often papillose. The antennæ are generally opaque; but in Nebria complanata, a beetle common on the sea-coast in Wales and Lincolnshire, they are semitransparent.
The situation of antennæ must next be considered. In this respect it seems necessary that they should be so situated as to be under the direction of the eyes: for if you examine ten thousand insects (except, as was before observed[1527], where there are four eyes), you will not find one in which these organs are situated either above or immediately behind them; their station being always either somewhere in the space between the eyes or that below them. In Ptinus F. they are placed near the vertex; but in Gibbium, which is so nearly related to that destructive genus[1528], they are beneath them. In many Melittæ K. they are in the middle of the space between the eyes; and in many other Hymenoptera and Coleoptera (Staphylinus &c.), in the anterior part of it. In many Lamellicorn genera (except in some Acridæ, as A. viridissima) as Melolontha, Cetonia, Lucanus, &c. they may be regarded as planted in the lower surface of the cheek before the eyes; but in Copris &c., in which they are inserted further under the shield of the head, they are properly in the prone surface of the front. In the Capricorn-beetles (Cerambyx L.) and Cnodalon F. they may be termed inocular, or placed in a sinus of the eye; in the former tribe in its interior, and in the latter its anterior side. In the Rhynchophorous or rostrum-bearing beetles (Curculio L.) they vary in their situation. Thus in Macrocephalus Oliv. they are inserted at its apex; in Anthribus in its middle, and in Calandra at its base[1529]. In the water-scorpions (Nepa, Belostoma, &c.) they may be called extraocular, being placed under the head in its prone part, outside the eyes[1530]. In Nirmus Fringillæ, a kind of bird-louse, they appear to be oral, being situated, according to De Geer, under the head near the mouth, at a great distance from the eyes[1531].
In their proportions, both as to length and thickness, antennæ vary extremely. Thus sometimes they are very short—much shorter than the head; as in the aquatic beetles Gyrinus, Parnus, and the water-scorpion; and some land-beetles, as Anthrenus, &c. At other times they far exceed the length of the insect: the males of many Capricorn-beetles are so distinguished. In that of Lamia ædilis they are more than four times as long as the body; and every intermediate length between these two may be found amongst them. They vary also greatly in thickness: in Paussus, whose antennæ emit light in the night[1532], and Cerapterus, they are nearly as thick,—at least their knob, which forms the chief part of them,—as the body of the insect[1533]; while in Mantis, Acrida K. and Psocus, they are as slender as a hair. The antennæ in many of the Prioni, especially in P. imbricornis, are thick from base to tip; while in other Capricorn-beetles they are quite the reverse.
It will not be necessary to enlarge here upon the general form of these organs: I shall therefore only notice the two principal divisions of them in this respect.—Antennæ, regard being had to one of their uses, may be divided into two sections, distinguished by forms extremely different: those, namely, that are employed by insects as tactors to explore their way, and those that cannot be so employed. The great majority are of the former kind; but those that may be denominated setigerous,—as the antennæ of the Libellulina, Ephemerina, of the Homopterous Hemiptera, and of many Diptera, the last joint of which terminates in a bristle, or is furnished with a lateral one, and of some gnats that have short feathered antennæ,—appear not fitted to be used as tactors to explore by touch, and form the latter description. This difference in these organs, as I shall have occasion to prove more at large hereafter, furnishes a strong presumption that their primary function is not touch. Were this the case, it would be common to them all.
As to their structure, antennæ consist in general of a number of tubular joints; each of which having separate motion, the animal is thereby enabled to give them every flexure necessary for its purposes. The scape, or first joint, by means of the bulb inosculates in the torulus, or is suspended to it; and the others, sometimes by a similar, though less pronounced knob at their base, inosculate in the preceding one; but in some cases the inosculation seems not so perfect, the joints being simply suspended by ligament. In pectinated or lamellated antennæ, the branch is usually a lateral process of the joint from which it issues; but in Phengodes (Lampyris plumosa L.) its involute plumose branches appear to articulate with the apex of each joint[1534]. I have a specimen of one of the Cleridæ, of a genus undescribed, in which each branch is forked. In some tribes of the Capricorn-beetles (Stenocorus, &c.) the antennæ are often armed at their apex with spines, sometimes on the upper side and sometimes below. In some aquatic beetles (Gyrinus, Parnus) they are furnished with an auricle at their base, which, like the lid of a box, shuts them in when unemployed, and protects them from the water[1535].
The portions into which antennæ may in general be considered as divided, have been sufficiently explained to you above; but it may not be amiss to add here a few words on the principal variations in their structure that I have had an opportunity of observing. The scapus[1536] or first joint, which includes the bulbus, is usually the most conspicuous joint in the antenna (exclusive, I mean, of the capitulum, in those in which that organ terminates in a knob), it being thicker and often longer than the succeeding ones. In the Capricorn and Darkling beetles, indeed (Cerambyx and Tenebrio L.), the third joint is the longest, but the scape is still the thickest; and in the stag-beetles (Lucanus L.), many of the weevil tribes (Curculio L.), and those of the bees (Apis L.), except in the males, it is as long nearly as the remainder of the antennæ, which forms an angle with it. In shape it is generally somewhat curved and subclavate, or increasing in size from the base to the summit; but it is sometimes straight and filiform, at others oblong or square, at others again triangular, in several instances three-sided: in one (Cetonia cruenta F. Genuchus K.) it is, as it were, broken, the upper part forming nearly a right angle with the lower; in Cerocoma Schæfferi it is foliaceous; and it is occasionally suborbicular: and probably many other forms might be enumerated.
The Pedicellus[1537] is the second, and may be deemed the least conspicuous joint of the antennæ. Though more slender than the scape, it is generally thicker than that which immediately follows it. In broken antennæ it is the hinge or pivot on which the clavola or upper member turns: it is usually very short, campanulate or bell-shaped, or obconical; but in a species of bug (Tetyra, from New Holland—T. pedicellata Kirb. MS.) it is nearly as long as all the rest of the joints taken together. In those species of Lycus, a genus of beetles related to the glow-worm, that have flattened antennæ (as L. reticulatus, fasciatus, &c.), this joint is almost received into the socket of the scape, so that their antennæ appear at first to have only ten joints, but in those which have those organs filiform (as L. minutus, Aurora, &c.) it is more conspicuous.
The Clavola[1538], or remaining joints of the antennæ taken together, constitutes the principal part of the organ, which, especially at its extremity, exercises its functions of touch, or any other sense. The principal variations, as to form and structure, that occur in this part will be mentioned in another place. I shall only here observe, that in many instances the first joint of this part is longer than the rest; but in Tetyra pedicellata just mentioned, it is by far the shortest, and shaped like the pedicel of most insects. In the Libellulina, the Homopterous Hemiptera, and those flies whose antennæ terminate in a bristle, the clavolet is represented by the bristle. But in the flies which have a lateral bristle, on the last joint, and those with triarticulate antennæ that have no bristle, the terminal joint represents it. The clavolet often terminates in a knob, or in several joints thicker than that which precedes them. This varies greatly, not only in its form, but also in the number of joints of which it is composed. Thus in Paussus, Platypus, and many Calandræ, it consists of only a single joint[1539]; in Anthrenus, Ditoma, &c. of two; in Nitidula, Geotrupes, &c. of three[1540]; in Tetratoma, the Silphidæ, of four[1541]; of five in Scaphidium[1542]; of six in one species of Languria, of seven in the common cockchafer (Melolontha vulgaris[1543]); of eight in Diaperis Boleti, in which the whole clavolet forms the club[1544]; of nine in Oenas; and ten in Cerapterus[1545]. All the above, you will observe, are beetles. In the other orders there are eleven joints in the knob of some butterflies; twelve in that of Ascalaphus[1546] and Myrmeleon; and lastly, fourteen in Trachelus[1547].
Under structure also, the number of joints of which antennæ in general consist, should be considered. If you examine the insects belonging to the different orders, you will find remarkable variations in this respect. Let us run through them:—In the Coleoptera the natural number of joints is eleven; but this rule is not without many exceptions. Thus, many have fewer than the prescribed number: Paussus has only two[1548], Claviger and Platypus five, Dorcatoma and Calandra eight[1549], Geniates K. and Phanæus MacLeay nine[1550], and lastly Melolontha ten[1551]. Others, again, have more than eleven joints: Cebrio grandis, Chrysomela stolida, some Saperdæ, and several others, have twelve. In Prionus imbricornis the female has nineteen, and the male twenty[1552]. Rhipicera marginata has thirty-two; and in a New Holland species of this genus I counted thirty-eight. In the Orthoptera I can trace no general law in this respect. In Locusta Leach in some species you may count fourteen joints, in others sixteen, and in others twenty-five. In one, which appears to be a pupa, I found only thirteen. In Mantis they exceed thirty; but in Blatta, from between thirty and forty, they reach nearly one hundred and fifty; often varying in number in different individuals of the same species. The order Hemiptera exhibits two peculiar types of antennæ, which, with some exceptions, distinguish the two natural sections into which M. Latreille has judiciously divided it. In the Heteropterous section they are without a bristle at their end; and in the Homopterous one, with the exception of Aphis, Thrips, &c. they have one. In the genera of both these tribes, the number of joints varies in these organs. Thus, exclusive of the seta, in Flata and Cixius there are only two joints; in Galgulus, Fulgora, and Cercopis, there are three; in Lygæus, Coreus, &c. there are four; in Tetyra, Pentatoma, Tettigonia, there are five[1554]; in Aleyrodes there are six; in Aphis seven; in Thrips eight; in Psylla ten, the last of which is terminated by two bristles[1555]; and in Coccus eleven. The Neuroptera order, as it stands at present, is regulated by no general rule with regard to the number of joints in the antennæ of the insects that compose it. Several types of form in these organs distinguish its discordant tribes. The first is that of the Ephemeræ, in which the antennæ consist of two short joints, crowned by a short, tapering, unjointed bristle. The second is that of the Libellulina, similar to the above, but with a jointed bristle. The third is that of Psocus, in which the antenna has two short thick joints at the base, terminated by a long filiform bristle, consisting of seven or eight joints, and finer than a hair. Perhaps these three may be regarded as belonging to a common type. The fourth type is presented by the short filiform antennæ of Termes; the fifth by the setaceous ones of Corydalis, Hemerobius, &c.; and the sixth and last by the clavate and capitate ones of Myrmeleon and Ascalaphus. In the Lepidoptera and Trichoptera orders the antennæ, though varying in their general form in the three tribes of which Linné formed his genera Papilio, Sphinx, and Phalæna, with the exception of Hepialus, in which the joints are few, are always multiarticulate:—we will therefore, without further delay, proceed to the Hymenoptera. In Latreille's tribe Aculeata the general rule is, that the females shall have twelve joints and the males thirteen. In his Ichneumonides the law seems to be, that the antennæ shall be multiarticulate and setaceous; but in most of the other tribes of the order, even those that in other respects are most nearly related,—as in his Tenthredinetæ,—the number of joints of these organs varies without end. Thus in Hylotoma there are only three joints[1556]; in Cimbex læta[1557] five; in C. axillaris and Perga Leach[1558], six: and so on to twenty-five or more[1559]. The same fluctuation in this respect runs throughout the rest of the order. In the Diptera there are two general types of antennæ:—those of the Tipulariæ Latr., consisting usually of from fourteen to sixteen joints, in the males often resembling beautiful plumes; and those of the remainder of the order, in which they do not exceed three joints[1560]: though the last, or patella, is often further divided into obsolete or indistinct ones[1561]. These antennæ may be further subdivided into filatæ and aristatæ, or those without and those with a bristle, either lateral or terminal.
The clothing of antennæ also merits attention, since it is often not a little remarkable. By clothing I understand the down or hairs of every kind with which they are either generally or partially covered. A great number of filiform and setaceous antennæ of Predaceous beetles (Cicindela L., Carabus L.) have the first two, three, or four joints naked, and the rest covered with a fine down. In insects that have a knob at the end of these organs, whether lamellated or perfoliate, this down is often confined to it, or to its intermediate joints, and seems intermixed with nervous papillæ. These are particularly visible in the flabellate antennæ of Rhipicera, Lampyris Latreillii[1562], Elater flabellicornis[1563], &c. covering both surfaces of the processes of the joints. In some male bees these papillæ are inclosed in hexagonal spaces into which the antennæ are marked out[1564]. It is to be observed, that in many antennæ the joints of the clavolet have one or two bristles or more at their apex, one above perhaps, and one below; the lower angle in those of the serrated antennæ of Elater is usually so furnished, and sometimes the upper. In many Capricorn-beetles and various insects the antennæ are clothed, instead of down, with stiffish hairs or short bristles. Other insects have these organs, at least the clavolet, beset with longer hairs standing out from them on all sides: of this kind are those of a singular beetle (Sarrotrium muticum) sometimes found in this country[1565]. Again, there are some that have only their underside bearded with longer hairs; as Lamia curculionoides, speculifera K., and other Capricorns[1566]. In another of this tribe, Saperda hirsuticornis, the three intermediate joints are ornamented with branches of long black hairs, which give them an elegant and feathery appearance[1567]. In Callichroma alpina the apex of the slate-coloured joints of its antennæ is bearded with black hairs. In Lamia reticulata, and Saperda fasciculata and plumigera, all also Capricorns, a single bunch of hairs, resembling the brush of a bottle-cleaner, signalizes the middle of the antenna[1568]: in Saperda scopulicornis K. this is star-shaped[1569]. Sometimes the scape is externally bearded, as in Trox, a beetle found in horns and bones; and in many other Lamellicorns[1570]. In this last tribe the two exterior leaves of the knob of the antennæ are often set with short bristles[1571]; and in a minute beetle called by De Geer Dermestes atomarius, the hairs of this part are said to form a brush[1572].
When insects, I mean more particularly Coleoptera, are about to move from any station where they have been at rest, the first thing they usually do, before they set a step, is to bring forward and expand their antennæ, which have either been carefully laid up in a cavity fitted to receive them, or back upon the body: if they terminate in a lamellated knob, they separate the lamellæ as far as possible from each other; or if it is perfoliate, the joints of it mutually recede. The object of this is evidently to collect notices from the atmosphere, since the papillose part of these joints cannot be applied to surfaces. When the animal begins to move, in many cases the antennæ do the same, and continue their motion till it stops and returns to a state of repose. In the parasitic tribes of the Hymenoptera (Ichneumon L.) they are kept in an almost constant vibration. Many other insects move them in all directions without any order or regularity; and others, when they elevate one depress the other, and so proceed as if balancing themselves by means of these organs like a rope-dancer. I have before stated to you how by motions of their antennæ, ants and bees communicate their wants or discoveries to each other, or make inquiry concerning any thing they wish to know[1573]. But as I shall have occasion to make some further remarks upon this subject, when the senses of insects are under discussion, I shall for the present take my leave of it.
I shall conclude what I have to communicate to you relative to the organs of which we are treating, with a few observations with respect to their station when the insect reposes. In the Capricorn-beetles, Eucera and other insects with long antennæ, they are merely turned back or on one side with no particular cavity for their reception when unemployed, but probably the apex passes under the body. In the Predaceous and Darkling beetles (Carabus L., Tenebrio L.) their station is usually under the sides of the prothorax, and in the Tortoise beetles (Cassida), under its anterior margin. In the Elastic beetles (Elater) they are received into a groove between the under margin of that part and the fore-breast (antepectus). In Anthrenus, when the animal reposes or counterfeits death, the antennæ are concealed in a cavity of the underside of the prothorax, at right angles with the throat[1574]. In the kindred genus Byrrhus, another simulator of death, a large cavity is excavated under the same part, to receive both the forelegs and antennæ, a narrow space being left between the angle of the prothorax and fore-breast exactly admitting the base of the latter, which are quite concealed under the former. In Cryptocephalus and Chlamys, kindred beetles, when at rest they are withdrawn, except their scape and pedicel, with the head within the cavity of the prothorax. In others they are turned under the head, without any particular cavity for their reception; as in many moths, Apion, &c. In most of the Lamellicorn beetles their station is in the cavity formed by the eye and the throat, the knob forming an angle with the rest of the antenna. In Heterocerus they follow the contour of the eye[1575]. In Brentus, a genus of weevils remarkably long and slender, they are turned back and received by a slight longitudinal cavity of the rostrum; but in those of this tribe (Curculio L.) in which the clavolet forms an angle with the long scape, this latter part, bending back, is laid up in an oblique channel of that part; and the former, pointing in the contrary direction, is folded upon it. In many flies (Muscidæ) a vertical frontal cavity receives the antennæ, which point downwards during repose[1576]. Cryptocerus, a very remarkable ant, has on its head a singular square plate, the sides of which form a deep longitudinal cavity: in this cavity the antennæ, quite concealed, repose in safety. A cavity equally remarkable is exhibited by the water-scorpions, particularly Belostoma, in which is a very deep kidney-shaped box, between the eye and throat, to receive and defend its singular antennæ[1577]; which, when they are reposing, is closed by the exterior harder joints, and from which it seems as if they turned out, like a sentinel out of his box. In some aquatic genera of beetles, as Gyrinus, Parnus, &c. they are withdrawn within a lateral cavity of the same part, and are defended from the water externally by the auricle at their base[1578]. The flabellated and lamellated antennæ, previous to their being folded for repose, close all their plates; which in action are as widely expanded as possible, so as to form a knob; and in some the middle piece is entirely concealed, as if in a box. In broken antennæ, or those in which the clavolet forms an angle with the scape, the former is folded upon the latter, with its point downwards.
II. Subfacies.—Having dispatched the Facies, or upper side of the head, I am next to consider the Subfacies, or under side: but as the principal parts that occupy this side have been already considered, I shall have no occasion to detain you long.
i. Jugulum[1579].—This part, which may be regarded as analogous to the throat in vertebrate animals, lies between the cheeks; from which it may usually be distinguished by being more lubricous and tumid, and often separated by an impressed line. It is particularly conspicuous and elevated in the Lamellicorn beetles, and calculated by its lubricity for easy motion in the lower side of the cavity of the chest. Its apex is the base in which the mentum sits. It is not necessary to enlarge further upon it, as it seldom exhibits striking characters.
III. Collum[1580].—In a large proportion of insects the head inosculates in the trunk without the intervention of a neck, or a constriction of the head behind. In the Orders Orthoptera, Trichoptera, Lepidoptera, Hymenoptera, and Diptera, no instance of it that I recollect occurs: in the Coleoptera there are many. In the Predaceous beetles, though several have no distinct neck, yet others, as Anthia, &c. have a short and thick one; and some few, as Colliuris, Agra, &c. one more pronounced. Latreille has named a tribe in this Order Trachelides, from the circumstance of their having a neck: in this tribe you will find the blister-beetles (Cantharis and Mylabris) both of the moderns and the ancients. In the Hemiptera order the water-scorpions Nepa, &c. have a thick short neck; and Zelus, (a kind of bug,) one longer and more slender; and, like Raphidia, the snake's-head fly, which is similarly circumstanced, has the air of a serpent. Other Neuroptera, likewise, have a neck; as Hemerobius, Corydalis, &c. This part presents no other features that merit notice.
IV. Myoglyphides[1581].—The Myoglyphides, or muscle-notches, are sinuses, some shallow and some deeper, in the posterior margin of the upper side of the head, to which the levator muscles are affixed. They seem principally confined to the Coleoptera; though, in some cases at least, they may be traced in the Heteropterous Hemiptera. These notches vary in number and depth in different insects. Thus in Buprestis there is only one deep one[1582]: in Copris there are two shallow ones, in a deep sinus separated by a small prominence[1583]: in Elater and Lamia there are also two not in a sinus; and in Calandra Palmarum there are four, two on each side, with a prominent lobe between them[1584]. To each of these notches, at its under margin, below the ligament that unites the occiput to the trunk, a muscle to raise the head is usually attached.
As the head of insects is the principal seat of the organs of sensation, so is the trunk of those of motion; and in it are contained the muscles by which they are moved: it may therefore be regarded as the great centre of motion, and as the main support and prop of the two other primary sections of the body—the head and abdomen, between which it is situated—it may be deemed the most important part of the insect, the key-stone of the whole structure. In treating upon it, for the greater clearness, I shall consider its substance, general form, proportions, composition, internal processes, and members. It will first, however, be necessary to assign my reasons for the nomenclature of its parts that I have adopted.
Had the entomological world been universally agreed upon this subject, and there was an established system of Orismology[1585], I should have proposed no alteration without great reluctance, and the fullest conviction of the absolute necessity of some change; but as the standard of language in our science is still unsettled, and different terms are used by different writers, there seems full liberty left to me to select those that appear upon the whole most appropriate; and where proper and significant terms seem wanting, to invent new ones. M. Latreille, in a late Essay[1586], has proposed many changes of this kind, and seems to hesitate concerning the adoption of some of those recently coined in France for the parts of the trunk[1587]; it may therefore, I think, be permitted me to labour a little in this hitherto imperfectly cultured field, and to suggest such improvements as the subject may seem to require or admit.
Linné called the part we are now considering the trunk, its upper-side he usually denominated the thorax, and its under-side the breast: he notices also the scutellum and sternum[1588]. As the prothorax and scutellum are the only apparent parts of the back of the trunk in his first Orders (Coleoptera, Hemiptera L.), the rest being covered, in noticing these he puts the part for the whole, calling the prothorax the thorax, but which strictly was not synonymous with what he called by the same name in the other Orders. Linné's phraseology with regard to the trunk of insects was adopted by Fabricius and other Entomologists, till Illiger employed the term thorax to designate the whole of the trunk[1589], calling the upper part thorax superior and the lower thorax inferior. M. De Blainville, M. Latreille, and other French writers, improved upon this, naming the upper part the back (dorsum), and the lower the breast (pectus); and dividing the trunk, or according to them thorax, into three sections, each bearing a pair of legs. But I see no sufficient reason for this alteration—the terms trunk, thorax, and breast, in the common acceptation are well understood, and lead to no confusion or glaring impropriety; I shall therefore adhere to the old phraseology, especially as French Entomologists in popular language still do the same.
As to the division of the trunk into segments by M. Latreille and others, it has been regarded as consisting of three primary ones, which have been called in the order of their occurrence, reckoning from the head—prothorax, mesothorax, metathorax. The first of these segments, however—and the learned Entomologist just named seems to hint as much[1590]—is usually more distinct from the other two, than they are from each other. If this idea be correct, the trunk is properly resolvable into two primary segments, the first bearing the arms or fore-legs, and the other the proper legs and the organs of flight. M. Chabrier calls the latter tronc alifère, or wing-trunk;—a happy term, which I have adopted and latinized, calling it the alitrunk (alitruncus): the first segment, because it bears the fore-legs, I have named manitrunk (manitruncus). I adopt likewise the terms above mentioned, prothorax, mesothorax, metathorax, to signify the three segments into which the thorax of Linné, or the upper side of the trunk, is resolvable; and those of the breast I denominate antepectus, medipectus, and postpectus. If terms be thought necessary to designate the two intire segments into which the alitrunk is resolvable, the first may be the meditrunk (meditruncus), and the other the potrunk (potruncus).
I. Substance.—With regard to its substance, the trunk in general is softer than the head, and harder than the abdomen, especially as to its upper surface; but in some cases, where it is not protected by the elytra, as in the rove-beetles (Staphylinus L.), the abdomen appears as hard as the trunk. Though usually not very different from the elytra in this respect, in Meloe, Lytta, and other vesicatory beetles, it is of a firmer consistence.
II. General Form.—In the Coleoptera Order the only part of the trunk that is visible on its upper-side is the prothorax: the mesothorax, with the exception of the scutellum, and the metathorax, being entirely concealed by it and the elytra; so that, with regard to shape, it may nearly be considered as merging in the prothorax. Below it is more visible, and may be stated as more or less quadrangular; in oblong beetles inclining to a parallelogram, and in shorter or hemispherical ones to a square. In the majority it is more convex below than above, except in the case of the hemispherical or gibbous beetles (Coccinella, Erotylus, &c.), in which the under-side is flat and the upper-side very convex. In the Diurnal Lepidoptera the trunk approaches to a cubical shape, in the Nocturnal it is more spherical. A similar difference obtains in the Hymenoptera and Diptera: in the bees, wasps and flies, the trunk approaching to the figure of a sphere; in the ants, Scoliæ, crane-flies, &c. to that of a cube. The upper part of it in many Ichneumonidæ, female ants, &c. is very elevated, forming an arch, and sloping towards the abdomen. In general it may be observed with respect to the remaining Orders, that the form of the trunk merges in that of the whole body, the tendency of which is often to a three-sided figure.
III. Proportions.—The trunk is usually longer and larger than the head and longer than the abdomen, but not wider: but there are exceptions to both these rules. In Colliuris, Mantis, &c., it is more slender; and in Atta megacephala and some neuter ants, it is shorter than the head; in Atractocerus, many Staphylinidæ, Phasma, the Libellulina, the Lepidoptera, and various Hymenoptera, it is shorter, and in the Mantidæ more slender than the abdomen. The greatest disproportion between it and the last part is exhibited by the genus Evania, parasitic upon the Blattæ, in which the abdomen appears merely as a minute and insignificant appendage of the trunk. The vertical diameter of this part, almost without exception, is greater than that of either head or abdomen. When we consider that it contains the muscles that move both the organs of flight and the legs, we see clearly the reason why the Creator gave it greater volume.
IV. Composition.—I lately intimated to you that the trunk, though resolvable into three segments, in most cases properly consists of only two primary ones. Whoever examines the perfect insects of every Order, except the Aptera[1591], will find this distinction strongly pointed out, not only by the different direction of the first pair of legs from that of the two last, but also in a large proportion by a deep incisure; and in all it is further manifested by the anterior segment having a motion distinct from that of the rest of the trunk, and separating readily from it; and this not only where it is large, as in insects that have a thoracic shield, but also in those in which the prothorax is less apparent: whereas the other two pedigerous segments have little or no distinct motion, will not readily separate from each other, and in some cases exhibit no pectoral suture between them. Sometimes, however, these two last segments are more prominently distinguished: in Lytta, Mylabris, and other vesicatory beetles, they are separated below by an incisure, or rather the first or mid-leg segment, is not nearly so elevated as that of the hind-legs. In some ants (Atta Latr.), in the neuters, there is no distinction of segments in the trunk; but in others (Formica Latr.) it follows the general law, and consists of three. In the Arachnida, with the exception of Galeodes, in which the head is distinct, and the three segments of the trunk may be traced, these parts together form only a single segment. Induced by these reasons, I consider the trunk as consisting in general of two primary segments, the manitrunk and alitrunk: the latter resolvable into two secondary ones.
* Manitruncus[1592].—The manitrunk, then, is the anterior section of the trunk, which bears the arms and contains the muscles that move them. This part has free motion, or a motion independent of that of the rest of the trunk. This indeed seems a necessary result of the direction and uses of the arms. It consists of an upper and lower part—the prothorax and antepectus.
i. Prothorax[1593].—The upper part of the manitrunk in the Coleoptera, Orthoptera, and Hemiptera, is by far the most conspicuous part of the trunk, but in the other Orders it is less so. With respect to it, insects may be divided into two classes—those that have and those that have not a prothorax. In the Coleoptera Order it is remarkable both for size and variations in its shape and sculpture. In the Orthoptera, though less various, it is almost equally conspicuous, especially in Blatta. In the Homopterous section of the Hemiptera, in many genera it has become extremely short; while in the Heteropterous section its dimensions are not much reduced. In the majority of the Neuroptera, likewise, it is comparatively large; in the Libellulina much shorter, and in the Trichoptera and Lepidoptera nearly evanescent[1594].—In the Hymenoptera and Diptera, with very few exceptions, the thoracic shield altogether disappears, at least if I am correct in an idea, which I shall hereafter explain, that the collar usually regarded as the analogue of the prothorax, is really a part of the alitrunk. In these last Orders, though there is no true prothorax, the manitrunk still remains under the form of an antepectus, bearing the fore-legs, and containing the muscles that move them.
The prothorax of insects may in general be considered with respect to its parts, margin, appendages, shape, sculpture, clothing, and proportions.
1. The prothorax, regarded as a whole, distinct from the antepectus or fore-breast, consists commonly of two pieces—the shield, or upper part[1595], and the ora, or under part[1596]. In the shield you are to observe its apex[1597], base[1598], sides[1599], limb[1600], and disk[1601]. The apex is the part next the head; the base that next the abdomen; the limb the circumference, and the disk the central part. In many Orthoptera and Heteropterous Hemiptera, the shield appears further to consist of two pieces, an anterior and posterior one. The ora is a continuation of the shield below the lateral margin, turned downwards and inwards towards the fore-breast and the legs, but separated from the former in most cases by a suture, as in Carabus L.; and in others merely by an impressed line, as in Blaps F.; but in Curculio and Cerambyx L., &c. there is no ora, the shield being without a lateral margin, and forming one piece with the antepectus. The part we are now considering varies in different genera. Sometimes it is very narrow, as in Scarites; at others very broad, as in Buprestis, Nepa, &c. In Lampyris, except L. italica, and affinities, it projects posteriorly into a lobe or tooth, which forms a right angle with the rest of the ora, and becomes the lower part of the cavity that receives the head; and in Dermestes this part is excavated into an anterior and posterior one which admits the antennæ and arms when folded for repose.
2. The margin of the prothorax is a ridge, either defining its sides or whole circumference. In many cases this margin is broad and dilated, but in others it is merely a thread or bead that separates the shield from the ora. Though generally terminating the upper surface, it sometimes, as in Staphylinus, dips below it. In many insects, however, as I just observed, the thoracic shield has no lateral margin whatever.
3. Various and singular are the appendages with which the prothorax of numerous insects is furnished. Many of these are sexual distinctions, and have been before described to you[1602]; but there are others common to both sexes, the most remarkable of which I shall notice.—Some are distinguished by a long horn which overhangs the head, as Membracis cultrata, ensata, &c.[1603]; in others it stands upright, as in Centrotus spinosus[1604]; C. Taurus has a pair of thoracic horns like those of a bull, only dorsal[1605]; in Ledra aurita they are flat, and represent ears[1606]; in some species of Tingis (T. Echii, Pyri, &c.) a kind of reticulated hood, resembling lace, is elevated from the anterior part of the prothorax, which receives and shelters the head[1607]. In Centrotus globularis and clavatus F., especially the former, the part in question is armed by a most singular and wonderful apparatus of balls and spines,—in one case standing erect[1608], and in the other being horizontal[1609],—which gives these animals a most extraordinary appearance. In many of the species here quoted the prothorax is producted posteriorly into a long scutelliform horizontal horn, which more or less covers the wings and abdomen; a circumstance which also distinguishes the genus Acrydium F. (Tetrix Latr.). This horn seems to have been sometimes regarded by Linné and Fabricius as a real scutellum, and sometimes only as a process of the prothorax: but that it is merely the latter will be evident to you, if you examine carefully any insect furnished with this appendage; for if you remove that part, you will discover the true scutellum and other parts of the trunk concealed beneath it. A very remarkable prothoracic appendage is exhibited by some species of Mantis. In general the part we are treating of in this tribe is very slender; but in M. strumaria, gongyloides, &c., it appears dilated to a vast width, and assumes, either partially or generally, a subrhomboidal form; but if it is more closely examined, it will be found that the form of the prothorax is really similar to that of the rest of the tribe, but that this part is furnished on each side, either on its whole length or anteriorly, with a large membranous flat subtriangular appendage resembling parchment[1610]. Perhaps this kind of sail may be useful to the animal in flight. In Prionus coriarius &c. its sides are armed with teeth, and in many Lamiæ, Cerambyces, and other Capricorn beetles, and often in various bugs (Pentatoma Latr.) with sharp fixed spines. But the prothorax has moveable as well as fixed appendages; of this kind are those spines (umbones), whose base is a spherical boss moving in an acetabulum of the thoracic shield of the Capricorn subgenus Macropus Thunb. If I might hazard a conjecture, I should say that these organs were given to this animal by an all-provident Creator, to enable it to push itself forward, when in the heart of some tree it emerges from the pupa, that it may escape from its confinement. Another kind of moveable appendages are attached to the thorax of Lepidoptera, usually in the form of a pair of concavo-convex scales covered externally with a tuft of hairs[1611]. M. Chabrier, who examined these scales in recent insects, describes them as vesicles, which appeared to him full of a liquid and of air, and capable of being alternately inflated and rendered flaccid; he regards them as accessories to a pair of spiracles, which he looks upon as vocal[1612], opening into the manitrunk just above the insertion of the arms. These organs are quite distinct from the tegulæ that cover the base of the primary wings of insects of this Order[1613], and are what, borrowing a term from Mouffet[1614], I have called in the table patagia, or tippets. Under this head I may include the caruncles at the anterior angles of the prothorax of a genus of beetles with soft elytra, named by Fabricius Malachius. When pressed, says De Geer of these insects, a red inflated soft vesicle, of an irregular shape, and consisting of three lobes, emerges from the thorax and from each side of the anterior part of the abdomen, which re-enters the body when the pressure is removed[1615]. M. Latreille seems to think that these vesicles have some analogy with the poisers of Diptera and the pectens of scorpions; and that they are connected with the respiration[1616].
4. We are next to say something upon the shape of the prothorax. The forms of the thoracic shield, especially in the Coleoptera, are so various, that it would be endless to aim at particularizing all; but it may be useful to notice a few of the most remarkable. The prothorax of Moluris, a darkling-beetle, approaches the nearest of that of any insect to a spherical form, from its remarkable convexity; in the wheel-bug (Reduvius serratus) it is compressed, and longitudinally elevated into a semicircular serrated crest: it is crested, also, in many Locustæ and Acridæ, in some having two parallel ridges; but, generally speaking, its surface is more depressed. In Necrodes it is nearly circular, in Blatta petiveriana semicircular, in Nilion and some Coccinellidæ crescent-shaped, in Carabus obcordate, in Cantharis and Sagra approaching to a square, in Languria to a parallelogram; in many Cimicidæ, Belostoma, &c., it is triangular, with the vertex truncated; it is trapezoidal in Elater, in Ateuchus rather pentagonal, and exhibiting an approach to six angles in some other beetles[1617]: but the prothorax most singular in form is that of some species of M. Latreille's genus Helæus[1618], as H. perforatus, Brownii, &c.: in these its anterior angles are producted, and curving inwards, lap at the end one over the other, so as to form a circular orifice for the head, which otherwise would be quite covered by the shield. Thus the upper portion of the eyes can see objects above, as well as their lower portion those below. I might enumerate many other forms, but these are sufficient to give you some notion of the variations of this part.
5. The prothorax is equally various in its sculpture; but since in the Orismological table almost every instance of it has its place, I shall here only notice it as far as it is common to the whole tribes, genera, or subgenera. The Scarabæidæ of Mr. W. S. MacLeay are distinguished by a small excavation on each side of this part, which, as has been before remarked[1619], furnishes an elevated base for an internal process with which the anterior coxæ ginglymate. In Onitis and Phanæus, to these excavations are superadded a pair impressed in the base of the prothorax, just above the scutellum; in Carabus L. a longitudinal channel divides the thoracic shield into two equal portions; and many genera of that great tribe have in addition, at the base on each side, one or two excavations or short furrows. Elophorus F. has on this part several longitudinal channels, alternately straight and undulated. Generally speaking, in Carabus L. the prothorax has no impressed points; but in one or two subgenera of Harpalidæ (Chlænia &c.) it is thickly covered with them. In numbers of Locusta Leach, the part we are considering is what Linné terms cruciate, being divided into four longitudinal portions by three elevated lines, the intermediate one being straight, and the lateral ones diverging from it both at their base and apex, so as to form a sinus or angle[1620]. In certain Acridæ K. (Locusta F.) there are only two of these lines or ridges, but notched or toothed; and in some of the genus first named only one[1621]; in Locusta Dux and affinities the prothorax has several transverse channels or rather folds[1622], with corresponding ridges on its internal surface.