Chapter XIX: Letter XXXIV: External Anatomy of Insects (4)

The _situation_ and _arrangement_ of simple eyes are also various. In many they are imbedded, as usual, in the head; but in the little scarlet mite, formerly noticed[1446], (_Trombidium holosericeum_), they stand upon a small foot-stalk[1447]: the hairiness of this animal might otherwise have impeded its sight. In spiders they are planted on the back of the part that represents the head, sometimes four on a central elevation or tubercle, and the remaining four below it--as in _Lycosa_; sometimes the whole eight are on a tubercle, as in _Mygale_; and sometimes, as in the common garden-spider (_Epeira Diadema_), upon three tubercles, four on the central one and two on each of the lateral ones. Other variations in this respect might be named in this tribe. In the scorpions a pair are placed one on each side, on a dorsal tubercle, and the other four or six on two lateral ones of the anterior part of the head[1448]. In the _Phalangidæ_ the _frontal_ eyes of the scorpion cease, and only a pair of _dorsal_ ones are inserted vertically in the sides of a horn or tubercle, either bifid or simple, often itself standing upon an elevation which emerges from the back of the animal[1449]. If their eyes were not in a vertical and elevated position, the sight of these insects would be very limited; but by means of the structure just stated, they get a considerable range of surrounding objects, as well as of those above them. With regard to the _arrangement_ of the eyes we are considering, it varies much. Sometimes they are placed nearly in the segment of a circle, as in those spiders that have _six_ eyes only, before noticed[1450]; sometimes in two straight lines[1451]; at others in two segments of a circle[1452]; at others, in _three_ lines[1453], and at others in _four_[1454]. Again, in some instances they form a cross, or two triangles[1455]; in others, two squares[1456]; in others, a smaller square included in a large one[1457]; in others, a posterior square and two anterior triangles[1458]; sometimes a square and two lines. Though generally separate from each other, in several cases two of the eyes touch[1459]; and in one instance three coalesce into a triangle[1460]. But it would be endless to mention all the variations, as to arrangement, in the eyes of spiders.

2. _Conglomerate Eyes_[1461] differ in nothing from simple eyes, except that instead of being dispersed they are collected into a body, so as at first sight to exhibit the appearance of a compound eye:--they are, however, not hexagonal, and are generally convex. They occur in _Lepisma_, the _Iulidæ_, and several of the _Scolopendridæ_. In _Scolopendra forficata_ the eye consists of about twenty contiguous, circular, pellucid lenses, arranged in five lines, with another larger behind them, as a sentinel or scout, placed at some little distance from the main body. In the common millepede (_Iulus terrestris_) there are twenty-eight of these eyes, placed in seven rows, and forming a triangle, thus --the posterior row containing seven lenses, the next six, and so on, gradually losing one, till the last terminates in unity. Each of these lenses is umbilicated, or marked with a central depression. In _Craspedosoma_ Leach, you will find a similar formation. In _Glomeris zonata_, a kind of wood-louse that rolls itself into a ball, the lenses are arranged in a line curved at the lower end, with a single one by itself at the posterior end on the outside; they are oblong and set transversely, and their white hue and transparency give them the appearance of so many minute gems, especially as contrasted with the black colour of the animal[1462]. Between these eyes and the antennæ is another transverse linear white body, but opaque, seemingly set in a socket, and surrounded by a white elevated line, like the bezel of a ring. Whether it is an eye, or what organ, I cannot conjecture[1463]. Its aspect is that of a spiracle.

3. _Compound Eyes_[1464].--These are the most common kind of eye in hexapod insects, when arrived at their perfect state; in their larva state, as we have seen, their eyes being usually simple[1465]; except, indeed, those whose metamorphosis is _semicomplete_, which have compound eyes in every state.--In considering compound eyes, I shall advert to their _structure_, _number_, _situation_, _figure_, _clothing_, _colour_, and _size_.

As to their _structure_,--when seen under the microscope they appear to consist usually of an infinite number of convex _hexagonal_ pieces. If you examine with a good glass the eye of any fly, you will find it traversed by numberless parallel lines, with others equally numerous cutting them at right angles, so as apparently to form myriads of little squares, with each a lens of the above figure set in it. The same structure, though often not so easily seen, obtains in the eyes of _Coleoptera_ and other insects. When the eye is separated and made clean, these hexagons are as clear as crystal. Reaumur fitted one eye to a lens, and could see through it well, but objects were greatly multiplied[1466]. In Coleopterous insects they are of a hard and horny substance; but in _Diptera_, &c. more soft and membranous. The number of lenses in an eye varies in different insects. Hooke computed those in the eye of a horse-fly to amount to nearly 7,000[1467]; Leeuwenhoeck found more than 12,000 in that of a dragon-fly[1468]; and 17,325 have been counted in that of a butterfly[1469]. But of all insects they seem to be most numerous in the beetles of Mr. W. S. MacLeay's genus _Dynastes_. In the eyes of these the lenses are so small as not to be easily discoverable even under a pocket microscope, except the eye has turned white[1470]: it is not, therefore, wonderful, that Fabricius should call these eyes _simple_[1471]. In some insects, however, as in the _Strepsiptera_ Kirby, the lenses are not numerous: in _Xenos_ they do not exceed fifty, and are distinctly visible to the naked eye[1472]. These lenses vary in magnitude, not only in different, but sometimes in the same eyes. This is the case in those of male horse-flies and flies, those of the upper part of the eye being much larger than those of the lower[1473]. The partitions that separate the lenses, or rather bezels, in which they are set, are very visible in the eyes just mentioned, and those of _Xenos_; but in many insects they are only discernible at the intersecting lines of separation between the lenses. In hairy eyes, such as those of the hive-bee, the hairs emerge from these _septa_. Every single lens of a compound eye may be considered as a _cornea_, or a _crystalline humour_, it being convex without and concave within, but thicker in the middle than at the margin: it is the only transparent part to be found in these most remarkable eyes. Immediately under the _cornea_ is an opaque varnish, varying according to the species, which produces sometimes in one and the same eye spots or bands of different colours. These spots and bands form a distinguishing ornament of many of the _Tabani_ and other flies. And to this varnish the lace-winged flies (_Hemerobius_, &c.) are indebted for the beautiful metallic hues that often adorn them. When insects are dead, this varnish frequently loses its colour, and the eye turns white: hence many species are described as having _white_ eyes which when alive had _black_ ones. The consistence of this covering is the same with that of the varnish of the _choroid_ in the eyes of vertebrate animals; but it entirely covers the underside of the lens, without leaving any passage for the light. Below this varnish there are numbers of short white hexagonal prisms[1474], every one of which enters the concavity of one of the lenses of the cornea, and is only separated from it by the varnish just described: this may be considered as the _retina_ of the lens to which it is attached; but at present it has not been clearly explained how the light can act upon a _retina_ of this description through an opaque varnish. Below this multitude of threads (for such the bodies appear), perpendicular to the _cornea_, is a _membrane_ which serves them all for a base, and which consequently is nearly parallel with that part. It is very thin, of a black colour, not produced by a varnish; and in it may be seen very fine white _tracheæ_, which send forth branches still finer, that penetrate between the prisms of the _cornea_: this membrane may be called the _choroid_. Behind this is a thin expansion of the optic nerve, which is a true nervous membrane, precisely similar to the _retina_ of red-blooded animals. It appears that the white pyramidal threads which form the _retina_ of each lens are sent forth by this general _retina_, and pierce the choroid by a number of almost imperceptible holes[1475]. From this description it appears that the eyes of insects have nothing corresponding with the _uvea_ or _humours_ of those of vertebrate animals, but are of a type peculiar to themselves.

Having explained to you the wonderful and complex structure with which it has pleased the CREATOR to distinguish the organs of vision of these minute beings, proving, what I have so often asserted, that when animals seem approaching to nonentity, where one would expect them to be most _simple_, we find them in many cases most _complex_, I shall now call your attention to the next thing I am to consider--the _number_ of the eyes in question. Most insects have only _two_; but there are several exceptions to this rule. Those that have occasion to see both above and below the head, the eyes of all being immovable, must have them so placed as to enable them to do this. This end is accomplished in many beetles, for instance _Scarabæus_ L., _Helæus_ Latr., &c., by having these organs fixed in the _side_ of the head, so that part looks upward and part downward; but in others _four_ are given for this purpose. If you examine the common whirlwig (_Gyrinus Natator_) that I have so often mentioned[1476], which has occasion, at the same time, to observe objects in the air and in the water, you will find it is gifted with this number of eyes. _Lamia Tornator_ (_Cerambyx tetrophthalmus_ Forst.) and some others, of which I make a genus, under the appellation of _Tetrops_, are also so distinguished. In these insects, one eye is above and the other below the base of the antennæ; in fact, in these the _canthus_, instead of dividing the eye partially, as in the other Capricorn-beetles, runs quite through it at considerable width[1477]. In _Ryssonotus_ MacLeay (_Lucanus nebulosus_ K.) the eye appears also to be divided in two by the _canthus_. In the _Neuroptera_ Order there is more than one instance of the same kind. In _Ascalaphus_ there are two considerable eyes on each side of the head, which, though clearly distinct, meet like those of many male flies and the drone. The male, likewise, of more than one species of _Ephemera_, besides the common lateral eyes and the _stemmata_ on the back of the head, have a pair of compound eyes on the top of a short columnar process[1478]. In the _Hemiptera_ Order, also, an instance occurs of four eyes in the genus _Aleyrodes_[1479]. Amongst the vertebrate animals, there is an example of eyes with two pupils in _Anableps_, a genus of fishes[1480], but no vertebrate animal has _four_ of these organs. That many insects should have more than _two_ eyes, will not seem to you so extraordinary as that any should be found that, like the Cyclops of old, have only _one_. There is, however, an insect, before celebrated for its agility[1481] (_Machilis polypoda_ Latr.), which has a single eye in its forehead; or we may say, its eyes are confluent, without any line of distinction between them except a small notch behind. Now that I am treating of the _number_ of eyes, I must not forget to observe to you, that in some insects no eyes at all have been discovered. In _Polydesmus complanatus_, on each side of the head there is an eye-shaped portion separated by a suture, in which under a powerful lens I cannot satisfy myself that I can discern any thing like the facets that usually distinguish compound eyes. In _Geophilus electricus_, another myriapod, they certainly do not exist[1482]. Whence we may conclude, as was before observed[1483], that the faculty of emitting light is rather given it as a means of defence than to guide it in its path.

The _situation_ of compound eyes differs in different tribes. In some, as in the _Staphylinidæ_, they are planted laterally in the _anterior_ part of the head; in others, the _Carabi_ &c., in the _middle_; in others again, _Locusta_ Leach &c., in the _posterior_ part. In some, their station is more in the upper surface, either before or behind; so that a very narrow space separates them, or perhaps none at all. Instances of this position of the eyes occur in a minute weevil (_Ramphus_ Clairv.[1484]), and many _Diptera_, &c. Of those that form an union on the top of the head, some are placed obliquely, so as to leave a diverging space below them, as in many _Libellulina_[1485], the drone[1486], &c. Others, as _Atractocerus_, in which the eyes occupy nearly the whole head, and unite anteriorly, have this diverging space _above_ their conflux. In _Rhina barbirostris_ Latr., another kind of weevil, they are confluent _below_ the head, at the base of the rostrum, and a very narrow interval separates them above. In a large number of the _Heteromerous_ beetles, they are set _transversely_, in the _Capricorn_ ones _longitudinally_. Their surface, when they are lateral, has usually two aspects, one _prone_ to see below, the other _supine_ to see above. In general the eyes are situated behind the antennæ, so that their position, whether it shall be anterior or posterior, depends upon that of those organs. Often, indeed, as in the last-named beetles, part of the eye is behind and part before the antennæ; but except where there are _four_ eyes, as in _Tetrops_, they are never placed before or below them.

Though the eyes of insects are generally _sessile_, yet to give them a wider range they are sometimes, but it rarely occurs, placed, like those of many _Crustacea_, on a _footstalk_, but not a _moveable_ one. An instance of this in certain male _Ephemeræ_ has already been mentioned. In the _Hemiptera_ De Geer has figured two species of bugs (_Cimicidæ_) that are so circumstanced[1487]; as are also all the known _Strepsiptera_ K., though in these the footstalk is very short[1488]: but the most remarkable example of columnar eyes is afforded by that curious _Dipterous_ genus _Diopsis_, in which both eyes and antennæ stand upon a pair of branches, vastly longer than the head, which diverge at a very obtuse angle from its posterior part[1489].

In their _figure_ eyes vary much. Sometimes they are so prominent as to be nearly _spherical_: this is the case with some aquatic bugs, as _Ranatra_, _Hydrometra_, and several male _Ephemeræ_[1490]. Very often they are _hemispherical_, as in the tiger-beetles (_Cicindela_ L.), and the clocks or dors (_Carabus_ L.); but in a large number of insects they are flat, and do not rise above the surface of the head.--With regard to their _outline_, they are often perfectly _round_, as in many weevils; _oval_, as in various bees; _ovate_, as in other bees (_Andrena_ F.); _triangular_, as in the water-boatman (_Notonecta_). They are also often _oblong_, and occasionally narrow and _linear_; as in that singular beetle _Helæus_. In many of the _Muscidæ_ they form nearly a _semicircle_, or rather, perhaps, the quadrant of a sphere. The eyes of the Capricorn-beetles (_Cerambyx_ L.) have a sinus on their inner side, as it were, taken out of them; so that they more than half surround the antennæ, before which is the longest portion of them. An approach to this shape is more or less observed in the darkling-beetles (_Tenebrio_ L.); but in these the sinus is not so deep. I may under this head observe, that in those _Mantidæ_ that represent dry leaves, and some others, these organs usually terminate in a spine[1491].

Though not distinguished by the beauty and animation that give such interest to the eye of vertebrate animals, and exhibiting no trace of iris or pupil, yet from the variety of their _colours_ the compound eyes of insects, though most commonly black or brown, are often very striking. Look at those of one of the lace-winged flies that commit such havoc amongst the _Aphides_[1492], and it will dazzle you with the splendour of the purest gold, sometimes softened with a lovely green. The lenses of those of _Xenos_ blaze like diamonds set in jet[1493]. You have often noticed the fiery eyes of many horse-flies (_Tabanus_ L.) with vivid bands of purple and green[1494]. Others are spotted[1495]; and Schellenberg has figured one (_Thereva hemiptera_)[1496], that exhibits the figure of a flower painted in red on a black ground. These colours and markings are all most vivid and brilliant in the _living_ insect, and often impart that fire and animation to the eyes for which those of the higher animals are remarkable. Take one of the large dragon-flies that you see hawking about the hedges in search of prey, examine its eyes under a lens, and you will be astonished at the brilliance and crystalline transparency which its large eyes exhibit, and by the remarkable vision of larger hexagons which appear in motion under the _cornea_, being reflected by the _retina_--all which give it the appearance of a living eye. This moving reflexion of the hexagonal lenses in living insects was noticed long since in some bees (_Nomada_ F., _Cœlioxys_ Latr.)[1497]

Compound eyes differ greatly in their _size_. In some insects, as _Atractocerus_, the drone-bee, many male _Muscidæ_, &c., they occupy nearly the whole of the head; while in others, as numerous _Staphylinidæ_, _Locusta_ Leach, &c., they are so small as to be scarcely larger than some simple eyes of spiders: and they exhibit every intermediate difference of magnitude in different tribes, genera, and species.

Under this head I must say something of the _Canthus_ of the eye; by which I mean an elevated process of the cheek, which in almost all the genera of the Lamellicorn beetles enters the eye more or less, dividing the upper portion from the lower. Though usually only a _process_ of the cheek, yet in the _Scarabæidæ_ the whole of that part forms the _canthus_[1498]. It only _enters_ the eye in the _Rutelidæ_, _Cetonidæ_, &c.; it extends through _half_ of it in _Copris_; it goes _beyond_ the half in _Ateuchus_; and in _Ryssonotus_ MacLeay (_Lucanus nebulosus_ K.) it quite divides the eye into two[1499], as I before observed. In _Lucanus_, _Passalus_ &c. it projects before the eye into an angle; in _Lucanus femoralis_ nearly into a spine; but in _Lamprima_ and _Œsalus_ it does not exist. The part, also, that enters the eye in the Capricorn-beetles may be regarded as a kind of _canthus_, though it is merely a dilatation of the _front_.

4. _Stemmata_[1500].--Having given so full an account of the kinds and structure of the ordinary eyes of insects, you may perhaps expect that I should now dismiss the subject: you would, however, have great cause to blame me, did I not make you acquainted with a kind of auxiliary eyes with which a large portion of them are gifted; I mean those pellucid spots often to be found on the posterior part of the front of these animals, or upon the vertex, frequently arranged in a triangle. These, Linné, from his regarding them as a kind of coronet, called _Stemmata_. They have been of late denominated _Ocelli_; but as this latter term is also in general use for the eyelets on the wings of _Lepidoptera_, I have adhered to that of the illustrious Swede. Neither he nor Fabricius has expressed any opinion as to the _use_ of these organs; but Swammerdam and Reaumur were aware that they were real _eyes_. The former found that there are nerves that diverge to them though not easily traced, and that they have a _cornea_, and what he takes for the _uvea_[1501]; and the latter has supposed that the compound eyes and these simple ones have, the one the power of magnifying objects much, and the other but little, so that the former are for surveying those that are distant, and the latter those that are near[1502]. The same author relates some experiments that he tried with the common hive bee, by which he ascertained that the _stemmata_, as well as the compound eyes, were organs of vision. He first smeared the _latter_ over with paint, and the animals, instead of making for their hive, rose in the air till he lost sight of them. He next did the same with the _former_, and placing the bees whose _stemmata_ he had painted within a few paces of their hive, they flew about on all sides among the neighbouring plants, but never far: he did not observe that these ever rose in the air like the others[1503]. From this experiment it seems as if the compound eyes were for _horizontal_ sight, and the _stemmata_ for _vertical_.

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.

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.