Volume 1 — LETTER XXXIV. (continued 3)

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.