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.