The definition of them by Linné and Fabricius as smooth, shining, elevated or hemispheric puncta, conveys a very inadequate idea of them; for, except in a very few instances, they are perfectly clear and transparent, and their appearance is precisely the same as that of the simple eyes of Arachnida &c., under which head they might very well have been arranged; but as the last are primary eyes, and the stemmata secondary, it seemed to me best that they should stand by themselves. The structure of both is probably the same, and their internal organization that of one of the lenses of a compound eye, and both are set in a socket of the head.
Though a large number of insects have them, they are by no means universal, since some Orders, as the Strepsiptera, Dermaptera, and Aptera, are altogether without them. The Coleoptera, also, have been supposed to afford no instance of species furnished with them; but in the last number of Germar and Zincken Sommer's Magasin, it is affirmed that they are discoverable in Gravenhorst's genus Omalium, but not in the kindred genera Micropeplus and Anthophagus[1504]. Upon examining the former genus, I find, that although Omalium planum and affinities, O. striatulum, and some others, appear not to have them, yet with the aid of a good magnifier they may be discovered in most species of that genus; as likewise in Evœsthetus Grav. I find them also very conspicuous in A. Caraboides and other Anthophagi, but some species appear to want them. In these insects they are two in number, situated in the vertex a little behind the eyes but within them, and either at each end of a transverse furrow, or at the posterior termination of two longitudinal ones. Nor are they found in all the genera of the other Orders. In the Orthoptera, the Blattidæ, unless a white smooth spot on the inner and upper side of the eyes may be regarded as representing them, have them not; but in all the other genera of that Order they are to be found[1505]. In the Hemiptera all the Cicadiadæ are gifted with them; as are likewise Tetyra, Pentatoma, with many other Cimicidæ, and the Reduviadæ very remarkably; but many others in both sections of this order, as Thrips, Coccus, Aphis, Capsus, Miris, Naucoris, Nepa, and Notonecta, &c. are deprived of them[1506]. Of the Neuroptera the Libellulina add stemmata to their large eyes, in the anterior angle of which they are stationed[1507]; but many other genera of that Order are without them; as Myrmeleon, Ascalaphus, Hemerobius, &c. The Trichoptera and Lepidoptera universally have them; though in the latter, except in Castnia and the Sphingidæ, they are not easily seen. In the Hymenoptera they are usually very conspicuous, but in Larra and Lyrops, two genera of this order, the posterior pair are scarcely discernible; and in the neuter ants they are quite obsolete. In the Diptera, though many genera are furnished with them, yet many also want them; amongst the rest Latreille's Tipulariæ, and all the horse-flies (Tabanus L.). The Pupiparæ (Hippobosca L.) usually have none; but in Ornithomyia avicularia, one of that tribe, though extremely minute they are visible, arranged in a triangle, in the polished space of their vertex.
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.