Chapter XXIII: Letter XXXV (3)

4. _Parts._ The parts to be considered in an elytrum are the _areas_, the _axis_, the _suture_, the _margin_, the _epipleura_, the _base_ and _apex_, the _angles_, and the _hypoderma_. At first it should seem as if an _elytrum_ was not like other wings divided into areas; but I think upon examination it will be found that, though often nearly obsolete, these are represented in it; for the _epipleura_[1795] with the recurved part of the external margin seems to me analogous to the _Costal_ Area; the inflexed part adjoining the scutellum and often going beyond it to the _Anal_, and the rest of the organ to the Intermediate. All this you may see in the dung-chafer, _Geotrupes stercorarius_. The _axis_[1796] or pivot by which the elytrum articulates with the trunk is generally placed about the middle of its base, but nearer the scutellar than the humeral angle, and varies in length and shape in the different tribes, but not so as to merit particular notice; it may be regarded as composed of _three_ parallel pieces, one belonging to each area, that of the _costal_ being the longest. In many these pieces are marked by no line of distinction, but in _Macropus_, &c., they may be readily traced[1797]. The _suture_[1798] is the _internal_ margin of the elytrum from the point of the scutellum to the end. In many beetles the right hand suture, looking from the anus to the head, has a lower ledge or margin, and the other, one more elevated, which when they are closed lies upon the former; in some _Dynastidæ_ there seems a kind of ginglymous structure in this part, each suture being fitted with a kind of ridge which is received by a channel of the other; in these the suture is generally marked out by an adjacent channel: but the most remarkable structure of this part distinguishes the genuine species of the genus _Chlamys_, in which both the sutures, except at their base, are armed with little teeth, alternating with each other like the cogs of a mill-wheel. In apterous beetles the elytra are often _connate_, or have both sutures as it were soldered together. The margin[1799] or external edge of the elytra is generally formed by a bead or ridge, which, except in the case of the truncated ones, in which it is straight, curves more or less from the base to the apex; this ridge is often recurved so as to form a kind of channel between it and the disk of the elytrum, as may be seen in the _Dynastidæ_; in some there are two parallel ridges, as in _Copris_; in _Silpha_ the margin is dilated; in _Helæus_ and _Cossyphus_ it is remarkably so and recurved, so that, in conjunction with those of the _prothorax_ which are similarly circumstanced, they give the animal some resemblance to a small model of a barge. Though the margin of elytra is most commonly intire, yet in some beetles, as _Gymnopleurus_ Illig., a sinus is taken out of it; in _Cetonia_ it often projects at the _base_, and in _Cryptocephalus_ in the _middle_, into a lobe; in _Phoberus_ MacLeay it is denticulated, and in many _Buprestes_ more or less serrulated; sometimes it terminates before it reaches the apex of the elytrum in a tooth, as in many _Carabi_ Latr. The _epipleura_[1800] or side-cover is that part of the organ in question, below the margin, with which it usually forms an angle, being more or less inflexed, that covers the sides of the body. It varies in different tribes, being sometimes obsolete, as in the weevils (_Curculio_ L.); in the Capricorn beetles it is very narrow; in _Carabus_, &c., dilated at the base; in many Heteromerous beetles, as _Blaps_, _Pimelia_, &c., it is very wide and conspicuous; in _Cossyphus_ it stands out a little from the abdomen, so as to form a kind of fence round it. Its _shape_ generally approaches that of a scythe, being incurved and growing more slender towards the apex[1801]; but it is sometimes straighter and shorter. In _Geotrupes_ and many other Lamellicorns, the _base_ of the elytrum is nearly vertical, forming a right angle with the rest of it; it is usually transverse and straight; but in _Calandra Palmarum_ and many _Cassidæ_ it slants to the scutellum; in _Chlamys_ it is sinuate, and in _Elater_ it has a deep cavity above the axis which receives the points of the _phragma_ mentioned before[1802]. The _apex_ of elytra is usually acute, the angle being formed by the confluence of a curving and straight line: but there are many exceptions; for instance, in _Mylabris_ it is rounded; in _Hister_ obliquely, and in _Necrophorus_ transversely, truncated; in many Capricorns it is emarginate; in others, as _Macropus longimanus_, it is bidentate; in some _Prioni_, _P. cinnamomeus_, &c., it terminates in a mucro at the internal angle; and in _Cerambyx Batus_, _horridus_, &c., at the _external_; and, to name no more, in some species of _Necydalis_ it ends in a long acumen. The _scutellar angle_ in insects that have a large scutellum, as _Macruspis_ MacLeay, is obliquely truncated to admit it, but where it is small it is generally rectangular, with the angle rounded; in _Buprestis vittata_ it is obtusangular; and in _Dytiscus marginalis_, &c., it is emarginate. In _Cassida spinifex_, _perforata_, &c., the _humeral_ angle is producted into an acute lobe that stretches beyond the head, and in _C. bicornis_ and _Taurus_ it forms a horn at right angles with the elytrum. In general it is either rectangular or rounded, with a prominence of the elytrum within it. The _sutural_ and _anal angles_ exist only where the elytra are truncated at the apex. In this case the _sutural_ is generally rectangular, and the _anal_ rather obtusangular or rounded. The _Hypoderma_ is the fine soft membrane before noticed[1803] that lines the underside of the elytra, the use of which is probably to prevent injury to the wings from friction with their usually hard substance; this membrane is commonly of either a pallid or brownish colour; but in some insects, as _Staphylinus hybridus_, _murinus_, &c., _Buprestis grandis_, it is of a beautiful green or blue; and it exhibits the _puncta_, _striæ_, and other modes of sculpture of the elytra very distinctly, the pores of which usually perforate this membrane[1804]. Just under the shoulders of these organs you may observe an oblong and sometimes roundish spot, occasioned by the _hypoderma_ in that part being particularly tense, and covering a cavity or pocket which appears to be connected with the axis by the hollow part, which I regard as representing the Costal Area; this pocket is evidently the analogue of a part in the _wings_ noticed by M. Chabrier[1805], and named by me the _phialum_: from its connexion with the axis by a channel, this part in elytra should also seem destined to receive a fluid to add to the weight of the margin and its means of resistance.

5. _Shape._ The shape of elytra is various; taken together, in which case, in describing insects, they are denominated _coleoptra_, their most common form is more or less oblong, or forming more or less a considerable portion of an ellipse; taken separately, it inclines to that of an isosceles triangle, with the exterior side curvilinear: truncated elytra are generally quadrangular, sometimes presenting a trapezium, at others nearly a parallelogram, and at others a square. With regard to their proportions they vary considerably, but the most general law seems to be that the length shall exceed _twice_ the width; in some, as _Buprestis grandis_, it is more than _thrice_; in many _Staphylinidæ_ they are as wide as they are long and sometimes wider; they are generally narrower at the apex than at the base, but in some species of _Lycus_, as _L. fasciatus_, &c., the reverse takes place; in _Telephorus_ they are nearly of the same width every where: with regard to their surface they are sometimes very convex, as in _Moluris_; at others very flat, as in _Eurychora_, _Akis_, &c.

6. _Appendages._ These, though not so remarkable as those of the head and prothorax of beetles, ought not to be overlooked. In many Capricorns, as _Lamia Tribulus_, _speculifera_, &c., the disk and sides are armed with short sharp spines; in others (_Stenocorus_, &c.) the sutural and anal angles or one of them terminate in a spine or tooth; sometimes the whole surface, as in _Hispa atra_, &c., is covered, like a porcupine, with a host of slender spines, or its sides defended by spinose lobes, as in _H. erinacea_: the humeral prominence is armed with a spine pointing to the head in _Macropus longimanus_, and forming a right angle with the elytrum in some _Curculionidæ_, as _Rhynchites spinifex_; but the most remarkable appendage of this kind is exhibited by _Cassida bidens_ and its affinities,--from the centre of the sutures of the elytrum rise perpendicularly a pair of long, slender, sharp processes internally concave, which both apply exactly to each other, so as together to form a single horn which rises, like a mast from a ship, from the body of the animal[1806]. Besides the appendages here mentioned, the elytra exhibit a variety of tubercles and other elevations of various form and size, which it would be endless to particularize.

7. _Sculpture._ The _sculpture_ of the organs in question is very various and often very ornamental: but as almost every kind of it will be noticed in the _orismological_ tables, it will not be necessary to enlarge upon it here, especially since I have endeavoured upon a former occasion to explain how it may be useful and important as well as ornamental to the animal[1807]. I shall therefore only notice a few instances, amongst many, in which a particular kind of sculpture distinguishes particular tribes. Amongst those that are Predaceous the _Cicindelidæ_ have elytra without striæ or furrows, while the majority of the subsequent terrestrial tribes of this section are distinguished by them: the _Dynastidæ_ in the Lamellicorn section are remarkable for a single crenated furrow next the suture; in the weevil tribes the numerous species of the genus _Apion_ are ornamented by furrowed elytra with pores in the furrows, which give them the appearance of neat stitching; in many of those beetles that have soft elytra, as the glow-worms (_Lampyris_), the blister-beetles (_Cantharis_, _Mylabris_), and still more in _Œdemera_, two or three slight ridges generally run longitudinally from the base to the apex, and are visible also on the under-side; as the _furrows_ probably lighten a _hard_ elytrum, these ridges may serve to strengthen a _soft_ one, and it is by these that the first approach is made to the reticular structure of _tegmina_ or the wing-covers of _Orthoptera_: _Lycus palliatus_, &c., in its elytra exhibits a direct resemblance of the reticulations of nervures.

8. _Clothing._ To what I have before said on this subject in general[1808] I shall here add a few remarks, which, though they more properly belong to _elytra_, may in many cases be extended to the whole body of a beetle. In various instances it happens that the beautiful markings of these organs, as in _Macropus longimanus_, whose elytra when denuded are black, are produced by short decumbent hairs; in some these variegations are the effect of scales resembling those of _Lepidoptera_, often of a metallic lustre; from these scales is derived all the brilliancy of the diamond-beetle (_Entimus imperialis_, Germ.); in some the scales are so minute as to resemble the pollen of flowers, as the white marks observable on the green elytra of the rose-chafer (_Cetonia aurata_).

9. _Colour._ The organs of flight in the majority of the Orders with respect to _colour_ are usually the most gaily decorated part of insects; I therefore deferred the notice of that subject till I came to treat of them. In general the colour of insects is either inherent in the substance of their crust, or produced by the hairs or scales that either partially or totally cover it. To confine myself to the _Coleoptera_, of whose elytra we are treating, it may be observed, I think, in general, that the majority of those that feed upon _putrescent_ substances, the _saprophagous_ tribes of Mr. W. S. MacLeay, are commonly of a more dark and dismal aspect and colour than those which feed upon such as are living and fresh, denominated _thalerophagous_ by the same learned author; this you may see exemplified in his _Scarabæidæ_ and _Cetoniadæ_. Again, in the Predaceous beetles a similar contrast of colours is often observable. How brilliant and gay are the fierce _Cicindelæ_! those tigers of insects, as Linné calls them; how black as to colour, how horrible in aspect is their near relation the _Manticora_: what difference exists in the economy of these animals is not known, except, as I learn from Mr. Burchell, that the latter is subterraneous, whereas the former seek the sunbeam and fly rapidly. I shall now point out a few instances in which the colours of their _elytra_ distinguish tribes or families. Amongst the Predaceous beetles a large family of the _Cicindelidæ_ are distinguished by a middle angular white band, and several white dots on their green or brown elytra, as in _C. sylvatica_; a family of _Brachinus_, and the majority of _Mylabris_, _Lamia capensis_ and _fasciatus_, &c., by black elytra, with yellow or red bands; _Carabus violacea_ and affinities by the violet margin of these organs; _Calliochroma_ Latreille by their _sericeous_, and _Eumolpus_ by their _metallic_, lustre. These instances will be sufficient to turn your attention to this subject, which though not of _primary_ importance in discriminating genera &c., is not without its use in a _secondary_ view.

10. _Uses._ I must not quit this subject without saying something upon the ends which elytra seem designed to serve. Their first and most obvious use is the protection of the wings when unemployed, that they may not be lacerated or soiled, and rendered unfit for flight in the various retreats to which these animals betake themselves either for food, repose, or to lay their eggs; to promote this purpose more effectually, the wings are usually curiously folded and laid up under them; and where the elytra are very short, as in the _Staphylinidæ_, these folds are very numerous and complex. In some instances, however, as in _Molorchus_ F., _Atractocerus_, &c., the wings are only partially protected by the elytra and not folded under them; probably they are less in danger of laceration from their peculiar habits than the generality. Another use is to protect the upper-side of the alitrunk, which for reasons before assigned is usually softer than the under-side, and also of the abdomen, often above nearly membranous, from the injury to which they would otherwise be exposed; in the latter part also the spiracles in _Coleoptera_ are not covered by the inosculations of the segments, as is the case in most other Orders, and therefore probably require some covering when the insect is not flying. In the Apterous beetles this appears to be their principal use; where these organs are connate, or as it were soldered together, the back of the abdomen is a thin membrane; the appearance of _two_ elytra in these cases is given, doubtless, for the sake of symmetry and beauty, a subordinate attention to which may be traced in all the works of creation. If we consider the bulk and weight of many flying beetles, we may imagine that they want some assistance, more than the extent and dimension of their wings seem to promise, to support them in the air, and to enable them to move more readily in it; and although it seems clear from the state of their muscular apparatus that elytra do not move much in flight, yet by giving a broad and concave surface to the air, for then they are usually nearly vertical, they may assist in some measure as sails, and help them in flying traversely and before the wind[1809].

ii. _Tegmina_[1810]. By this name the learned Illiger has distinguished the upper organs of flight of the _Orthoptera_ and Homopterous _Hemiptera_[1811]. They may be considered under the same heads nearly as elytra.

1. _Substance._ _Tegmina_ differ very materially from _elytra_ in their _substance_, being generally more or less diaphanous, though in _Blatta Petiveriana_ the dark parts are as opaque as elytra, and those of the _Mantes_ that resemble dry leaves are only semidiaphanous. These organs are also of a less dense substance than elytra, something between coriaceous and membranous, which I shall express by the term _pergameneous_, as somewhat resembling parchment or vellum. Another circumstance relative to this head also distinguishes them,--they are not lined with membrane. In some instances, as in _B. Petiveriana_ just named, they approach nearly to the substance of elytra, and in _B. viridis_, some _Mantes_, and _Tettigonia_, &c., they are little different from wings in their substance; but this does not diminish their right to be considered as _tegmina_, since their structure is altogether the same.

2. _Articulation with the trunk._ I observed above that the axis of elytra may be regarded as formed of _three_ parts, one appertaining to each of the areas or their representatives[1812]; in _tegmina_, and indeed in wings in general, these parts are separate and may be more distinctly traced, the _axis_ of the _Costal_ Area being generally the longest, and that of the _Intermediate_ often the shortest; these _axes_ are suspended in the wing-socket by elastic ligaments, intermixed with hard bony plates, the principal one of which, called by M. Chabrier the _humerus_[1813], is connected both with the _tegmen_ and the trunk, and in some a little resembles the head and neck of a swan. This structure permits the animal to move the lateral areas in some degree separately, so that each, especially the _anal_, shall form an angle with the intermediate; as the motion of the latter is not wanted, its _axis_ often falls short of the base, or is obsolete, as in _Blatta_.

3. _Composition._ The three areas, traces of which we had discovered in _elytra_, are particularly visible in _tegmina_. If you take any cockroach (_Blatta_), you will at first sight see that in it they are divided into three larger portions by stronger nervures or folds; and if you also take a _Mantis_, or _Locusta_ Leach, a _Fulgora_ or _Tettigonia_, the same circumstance will strike you, only you will see that in these the intermediate portion terminates also in an axis; these are what I call the three _areas_. The external one or Costal is usually the longest and narrowest[1814]; the Intermediate one is commonly triangular, with its inner side curvilinear[1815]; and the interior one, or _Anal_ area, in the _Orthoptera_ is rather oblong; in _Fulgora_ angular, and in _Tettigonia_ it presents an isosceles triangle; with its vertex to the apex of the wing[1816]. The first of these may be defined as that portion of the wing that lies between the _costal_ and _postcostal_ nervures; and perhaps, in some cases, as in _Mantis_, for there is the fold of the _tegmen_, the _mediastinal_ may be regarded as its limit; the Intermediate Area is that which lies between the _postcostal_ or _mediastinal_ nervure and the _anal_ fold of the wing; and the Anal Area is the remainder. These areas may perhaps best be made out by tracing each to its axis. To study them carefully in _tegmina_ and _hemelytra_ is of considerable importance; for in them we find the first outline of the general plan upon which the wings of insects are constructed, and which, as we shall see hereafter, more or less enters into the composition of them all.

4. _Position, and folding in repose._ With regard to their position when not expanded, _tegmina_ vary somewhat in the different tribes. In the _Coleoptera_ we have seen that, except in a few instances, the elytra unite at their suture. Something like this takes place in _Fulgora_, _Cercopis_ and affinities, in the Homopterous _Hemiptera_; in these, though the union is not near so exact, yet the _tegmina_ do not lap over each other; they are usually more or less deflexed, with scarcely any portion in a horizontal position: in _Tettigonia_ F., _Chermes_, _Aphis_, &c., the middle part only of these organs meets, from which point they diverge both towards their base and apex[1817]. In the _Orthoptera_ the position is quite different, for one tegmen more or less lies over the other. In _Blatta_, in which the _tegmina_ are nearly horizontal, the left hand one covers almost half the other[1818]: in the other tribes of the Order, with little variation, the Anal Area of the _tegmen_ is horizontal, and covers the back of the animal, and the Intermediate and Costal are vertical and cover its sides; the former, however, in some cases, only forms the angle between them. Sometimes in these the right-hand one is laid upon the left, as in _Acheta_; and sometimes the reverse of this takes place, as in _Acrida_ K. With regard to the folding of the _tegmina_, the most remarkable instance that occurs is that of _Acheta monstrosa_, in which the ends of both these organs and the wings, in repose, are folded like a fan, and then rolled up like a serpent[1819].

5. _Shape._ The shape of _tegmina_ is various. In the _Blattæ_ and some _Mantes_ they are more or less _oblong_; in _Mantis precaria_, _strumaria_[1820], and others, they incline to _elliptical_; in _Phasma grandis_ and _Acheta monstrosa_ they are rather _panduriform_[1821]; in _M. gongyloides_ they are _semi-cordate_[1822]; in _Pterophylla trapeziformis_ they are _rhomboidal_[1823]; in _Conocephalus erosus_ they are _sinuated_; in _Locusta_ Leach they are usually _linear_ or linear-oblong[1824]; in _Pterophylla_ K. they generally terminate in a short mucro[1825]; and in some of those _Mantidæ_ whose _tegmina_ simulate arid leaves, in a recurved one[1826]. In the Homopterous _Hemiptera_ the shape of these organs is less various. In the _Fulgorellæ_ Latr. they incline to a trapezium, sometimes to a pentagon[1827]; in the _Tettigoniæ_ F. they approach to an obtuse-angled triangle; and in others of the tribe they are nearly wedge-shaped[1828].

6. _Neuration._ The circumstance that most strikingly distinguishes _tegmina_ from _elytra_ is their _neuration_ or veining; which adds much to their strength, without increasing their weight so much as to render them unapt for flight. To look at these organs in _Blatta Petiveriana_, you would imagine them at first to be deprived of this distinction; but if you observe them attentively, particularly their white spots, you will soon detect their nervures; and if you further examine their lower surface, you will find them very visible. The gibbous _Blattæ_ also, _Blatta picta_ and affinities, the analogues of _Erotylus_ amongst the _Coleoptera_, have _tegmina_ which, except at their apex, exhibit but faint traces of the nervures of their tribe, and approach to elytra besides by the innumerable minute impressed points that cover them. In the _Orthoptera_ and some Homopterous _Hemiptera_ the nervures may be divided into _longitudinal_ ones more or less ramified, and _traversing_ ones. In the _Blattæ_ the traversing nervures cut the longitudinal ones nearly at right angles, but not at regular intervals, so as to cover the tegmen with quadrangular areolets; in _Mantis precaria_ and affinities the longitudinal nervures of the Anal Area diverge from the base, and are traversed nearly as in _Blatta_, while those of the Costal diverge from the mediastinal nervure, but the traversing ones form innumerable irregular reticulations; in _Mantis sinuata_ K.[1829] the whole tegmen has such reticulations but less numerous; in _Locusta_ Leach it is regularly reticulated at the base, but the areolets of the apex are quadrangular; in the _Mantes_, with oblong wings, all are quadrangular; in _Pterophylla_ K. the longitudinal diverging nervures are not numerous, and the traversing ones cut them into quadrangular and triangular areolets, besides which they are covered by innumerable impressed points, so as altogether to exhibit a most exact resemblance of the leaf of some evergreen: in _Gryllotalpa_ the longitudinal nervures of the Anal Area rather converge towards the apex, are traversed by few transverse nervures, and those of the Costal Area which diverge from the mediastinal nervure by still fewer; the neuration of _Acheta_ F. has been before described[1830]; I shall only observe here, that the constructors of _stringed_ instruments of music might, perhaps, from the _tegmina_ of the male, the nervures of which probably modulate the sounds which it produces, take a hint for giving the strings in them a serpentine or convolute direction, and so might produce something new in that department, corresponding with the serpents and French-horns in _wind_ instruments. Of the Homopterous _Hemiptera_ in the _Fulgorellæ_ Latr., which are most analogous to the _Orthoptera_ of all that tribe, the longitudinal nervures are more numerous and branching, more especially toward the apex of the _tegmen_, and are traversed as much by transverse ones, sometimes reticulating the wing with roundish areolets, as in _F. laternaria_, and at others with quadrangular ones, as in _F. candelaria_; in some of these however, as _Otiocerus_ K., _Flata_ F., &c.[1831], there are no traversing nervures; and these lead to the _Cercopidæ_ and others in which the longitudinal nervures become few, and some are without any[1832], and these terminate those of this section of the Order in which the nervures in question are continued to the margin of the wing. We next come to those, _Darnis_, _Centrotus_, _Membracis_, &c., in which they are circumscribed a little within the apex by a traversing nervure, so that the tegmen ends in a margin of pure membrane, and thus some approach seems to be made to the _Hemelytra_, from _Tettigonia_, the most conspicuous genus of this tribe, in which the _areolets_, few in number, like those of _Lepidoptera_, are not formed, except the terminal ones, by traversing nervures, but by the ramifications of the longitudinal ones; in _Chermes_ the Intermediate Area, which is connected with the base of the wing by a single nervure, is the only part that has any areolets[1833].

7. _Colour._ Orthopterous insects are seldom remarkable for _tegmina_ of brilliant _colours_; there is in them none of that gilding or metallic lustre which so often distinguishes elytra: they are also frequently less ornamented in this respect than the wings, with which they usually form an agreeable contrast. Their reticulations and nervures, which are sometimes of a different colour from the rest of the tegmen, decorate them considerably: a remarkable circumstance belonging to this head attends the black tegmina of _Blatta Petiveriana_; one has _four_ white spots, and the other only _three_; but as one laps over the other, the symmetry of the arrangement is preserved: the Homopterous _Hemiptera_ are more distinguished in this respect, and some of the _Fulgoridæ_ imitate the _Lepidoptera_ both by their ocelli and spots: _Fulgora laternaria_, _Candelaria_, _serrata_, and _Diadema_, sufficiently exemplify this remark, as do several _Flatæ_ likewise[1834].

We may observe here--that _tegmina_ are more calculated for flight than _elytra_, both from their thinner substance, and from the angle that their Anal Area, and often the Costal, forms with the rest of the _tegmen_; a circumstance which, in wings, M. Chabrier thinks presents some facilities in that kind of motion.

iii. _Hemelytra_[1835]. The next species of wing-covers, which though varying in the substance of their base, terminate in a part distinct from the three areas, consisting in almost every case of mere membrane, peculiar to the Heteropterous _Hemiptera_, are called _hemelytra_, or half-elytra:--this term was also formerly employed, but certainly incorrectly, to denote _tegmina_. I shall consider them with respect to such of the particulars noticed under the former heads as apply to them, but without repeating them formally.

1. As to their _substance_, they must be separately considered with regard to their base and apex. In various instances the _base_, or part consisting of the three areas, is almost corneous, as in _Cydnus Morio_ and _bicolor_, bugs not uncommon with us, and many others[1836]; in these cases it is lined with a _hypoderma_ like elytra; and in many the points, which are impressed upon it, also perforate the _hemelytrum_, and seem to act as pores: but in _Lygæus_, _Reduvius_, _Capsus_, _Miris_, and the majority of the Heteropterous _Hemiptera_, the organs in question being soft and flexible, may be stated as rather resembling leather than horn;--on this account this part of a _hemelytrum_ is denominated the _corium_. In _Scutellera_ the portion covered by the scutellum is membranous; and in _Acanthia paradoxa_, and the cucullated species of _Tingis_, the wing-covers are entirely so. The apex of these organs is almost universally either membranous or coriaceo-membranous, on which account it is called the _membrana_. I say _almost_, because in _Aradus_ and the _Hydrocorisæ_ Latr., this part, though rather thinner than the rest of the _Hemelytrum_, is also coriaceous; in the latter tribe usually with a very narrow membranous edge; and in many _Reduvii_ and _Zeli_ there is scarcely any difference in the substance of the base and apex.

2. As to the _articulation_ of _Hemelytra_ with the trunk, it seems not strikingly different from that of _tegmina_: the point or base of the Intermediate Area, which falls short of that of the lateral areas, seems connected by a slender ligamentous piece, with its axis, which is thick; and I do not discern Chabrier's _humerus_ shaped like a swan's head and neck[1837].

3. The _composition_ of these organs differs from that of _tegmina_ in more respects than one: in the first place, they consist, as was lately observed, of _four_ instead of _three_ areas; in the next, they appear to have, at least several of them, a part, which I suspect to be analogous to that above described in _Coleoptera_, supposed to represent the _phialum_ of wings[1838]. I shall first speak of the _areas_. In some apterous species related to the bed-bug, _Lygæus brevicollis_ Latr.[1839], &c., there is no trace of the usual areas, and the _membrana_ is a very narrow strip; in _L. apterus_ the former are very faintly traced out, but they are present in all those that are furnished with wings; whence we may conjecture that they are of the same importance in flight with the folds observable in those organs[1840]. The three basal areas may be said most commonly to present three isosceles triangles, the Costal one being narrow and curvilinear[1841], the Intermediate the most ample[1842], and the Anal one the narrowest and shortest[1843], with its vertex towards the apex of the _Hemelytrum_, while in the two former it is at its base. In _Lygæus compressipes_ (_Rhinuchus_ K. MS.) the Anal Area is cultriform; and in most of the _Hydrocorisæ_ it has an angle in the middle of its posterior margin. The proportion that the _membrana_ or _apical area_ bears to the rest of the wing varies in the different tribes. In some, as before stated, it is obsolete, in others nearly so; in the majority, perhaps, it occupies about a _third_ of the _hemelytrum_; in _Lygæus compressipes_, _cruciatus_, &c., full _half_; in _Alydus calcaratus_, _two-thirds_; in _Reduvius_, nearly _three-quarters_[1844]; and in _Aradus depressus_ the corium,--divided, however, though indistinctly, into the three areas,--is driven to the _base_ of the wing: two ends are answered by this structure--as this insect lives under bark, its thin _hemelytra_ take less room; and as it flies, though it has only rudiments of wings, they are more fit to supply their place: the part we are speaking of usually runs obliquely from the vertex of the Anal Area to the base of the Costal.

4. As to their _position and folding in repose_, _Hemelytra_ are usually nearly or altogether _horizontal_; but in _Notonecta_ and _Plea_ they are _deflexed_ and cover the sides of the body; and the apical area of one wing precisely covers that of the other; where the scutellum does not intervene, as in _Scutellera_, _Pentatoma_, &c., the vertical angles of the Anal Area meet in the middle of the back, so as to exhibit the appearance of a cross. In _Notonecta_, in which the _hemelytra_ are deflexed, at the apex of the _membrana_ is a fissure which permits the two sides to form an angle with each other, and to apply exactly to the body. In _Plea_, in which there is no apical area, the posterior margins of the _tegmina_, as they ought rather to be termed, unite, but do not lap over each other. With regard to the appearance of something like a _phialum_, if you examine the _hemelytra_ of most species of bugs on the underside, you will see that the costal nervure at the base is inflexed and covers a kind of channel; if you next take one of _Belostoma grandis_, where the structure is most conspicuous, or even the common _Nepa cinerea_, you will find in the same situation, adjacent to the inflexed costal nervure, a hollow tube running from the base of the wing, and terminating, after proceeding about one-fourth of its length, in a hollow cavity, which, as it is covered by a membrane, appears to me to be a collapsed pouch. This circumstance is worthy of further and more general investigation.

5. In their _shape_, with few exceptions, _hemelytra_ more or less represent a wedge, being wider at their apex, where they are usually obliquely truncated, than at the base; but in _Plea_ Leach they are obtusangular, with the angle in the sutural margin; in _Notonecta_, on the contrary, an obtusangular sinus distinguishes that part; in _Naucoris_ they are curvilinear and every where of equal width; in _Ranatra_ they are linear and straight; in _Aradus_ they are oblong, usually with an external lobe or dilatation at their base: a remarkable instance of the intention of this is observable in a nondescript Brazilian species, in which the head, prothorax, and abdomen, are edged with a number of broad foliaceous appendages; if the base of the _hemelytrum_ had not been furnished with a similar appendage, the symmetry of the whole body would have been destroyed by the hiatus between the prothorax and abdomen, as may be seen by removing the _hemelytra_; but by this compensating contrivance of Providence, the gap is filled, the above lobe fitting exactly into it.

6. The _neuration_ of these organs will not occupy us long, since the _corium_ or harder part, though in some species there are traces of nervures, is often without them. Those of the cucullated species of _Tingis_ resemble many _tegmina_ in being ornamented by them with a kind of network, which looks like the finest lace; in several _Lygæi_, _Edessa_, and some _Reduvii_, there are a few diverging longitudinal nervures which occasionally by a ramification here and there form an areolet[1845], but there are seldom any traversing nervures. The Apical Area is usually most distinguished by nervures, in some forming several areolets, as in _Aradus_, in others running parallel to each other, nearly to the end of this area, as in _Belostoma grandis_, where they are met by a traversing nervure; the object of this is doubtless to strengthen the membrane.

7. Both _tegmina_ and _hemelytra_ are most commonly naked, yet very short _hairs_ are found on those of some species of _Cercopis_, and in many more instances in those of the latter description, as in _Notonecta_, several _Lygæi_ and _Reduvii_, &c.

8. _Colours_ in _hemelytra_ are very various, and in many instances are peculiar to families; in certain _Lygæi_ (_L. Hyoscyami_, &c.) black and red; in _Lygæus compressipes_ and affinities a dingy black; in some _Reduvii_ black with a large white spot;--but it is needless to enlarge further on this subject.

9. That _hemelytra_ are used in flight is evident not only from the large space allowed for their muscles[1846], but likewise from a circumstance noticed by M. Chabrier, that in flight, in the _Pentatomæ_ Latr., the corium of the _hemelytrum_ is fixed to the wing[1846]; in which case both must describe the same arc.

iv. _Wings._ We are next to consider organs which are exclusively appropriated to _flight_, and therefore are properly denominated _wings_. These in the Orders that have _elytra_, _tegmina_, or _hemelytra_, are the pair that correspond with the secondary wings of the other Orders. It may be said, indeed, that in several instances both _tegmina_ and _hemelytra_ do not differ at all in substance or use from the wings that they cover. This is true; but as their structure in other respects is the same with that of those that are more solid and less apt for flight, it was convenient to consider them under the same name.

1. To begin with the _articulation of these organs with the trunk_; in general it may be stated that this, as in _tegmina_ and _hemelytra_, is usually by the intervention of three axes, formed by the conflux of the nervures of the three areas at the base of the wing, which either immediately or by other pieces are implanted in the trunk, so as to receive from it the aërial and other fluids, necessary for its expansion and motions[1847]. Having given this general statement, I shall next apply it to the wings in some of the different Orders. If you carefully extract one from the stag-beetle (_Lucanus Cervus_) or any large species of the _Dynastidæ_, in the _Coleoptera_; the first thing that will strike you, upon examining the base, will be the plate before mentioned called by Chabrier the _humerus_, which is a stout transverse corneous piece, with a deep sinus towards the wing, filled with ligament: if you again follow the costal, mediastinal, and postcostal nervures, you will find them unite to form an axis, consisting of three parallel pieces, which connects by its intermediate internal piece with one end of this plate. The nervures of the Intermediate Area terminate also in a corneous axis at a greater distance from the base than the other two, which connects with Chabrier's _humerus_ by means of the ligament of the sinus just named. Those of the Anal Area are received by a ligament attached to a transverse plate, widest at its anterior end, which connects with the posterior part of the said _humerus_; and at its posterior end is united to the _postfrænum_[1848], with which it forms a right angle. In the _Orthoptera_ Order the structure is not very different, but the axes and other plates of the base of the wing are less distinct and rather cartilaginous; the nervures of the Anal Area often terminate in a transverse one that there forms the segment of a circle[1849]; the inner base of this circle is ligament connected with the _postfrænum_[1850]. In the Homopterous _Hemiptera_ the three axes may be readily traced, but the _humeral_ plate, with which they all are connected, is more irregular in shape, and in _Fulgora_ longitudinal, with an angular surface; in this Order the nervure, in some cases consisting of cartilaginous rings[1851], in which the _frænum_ and _postfrænum_ terminate in the _tegmina_ and wings, is attached posteriorly to the ligament of the Anal Area. In the Heteropterous section the three axes are evident, but the _humeral_ plate is not easily made out. In the _Libellulina_ the axes of the Costal and Intermediate Areas are the coloured broad plates at their base, formed by the dilatation of their nervures; that, however, of the Anal is not dilated, but forms one nervure, in the primary wing, with the _frænum_, and in the secondary with the _postfrænum_. Having given you this clue to trace the axes in those tribes in which they are most conspicuous, it will assist you in searching for them in the remaining Orders, in all of which they may be traced, except perhaps in those minute _Hymenoptera_ whose wings have solely the costal nervures; probably in these there is only _one_ axis. In the _Lepidoptera_ and _Hymenoptera_ a circumstance connected with the present head is observable, which is not to be discovered in the other Orders: these are the _tegulæ_ or base-covers, which appear intended to defend the base of the anterior wings. They are concavo-convex scales, which in the _Lepidoptera_ are large and of an irregular shape[1852], but in the _Hymenoptera_ are smaller and semicircular[1853].

2. Wings, with regard to their _substance_, may generally be termed _membranous_; but they vary in this respect, some being much thicker than others, either _partially_ or _totally_: in spotted wings, as in those of many _Libellulina_, _Tettigoniæ_ F., &c., the dark opaque parts are denser than those that are transparent: in several _Orthopterous_ insects, as in _Phasma_, some _Mantes_, &c., the Costal Area or covering part of the wing is of a substance equally firm with that of the _tegmen_. This is a compensating contrivance, that where the latter is shorter and smaller than the former, its membranous part, when folded, may be protected from injury. Another similar contrivance of DIVINE WISDOM is exhibited by those _Pterophyllæ_ K. (_Locusta_ F.) whose _tegmina_ resemble the leaves of plants (_Pt. laurifolia_, &c.); in these the tip of the wings when folded being longer, is not covered by the _tegmina_, and therefore exposed to injury; to prevent which this small piece, while the whole wing, as far as covered by those organs, is membranous, is of the same substance with them[1854]. The wings of most _Coleoptera_, _Orthoptera_, _Hemiptera_, and _Thereva coleoptrata_, in the _Diptera_, &c., are of a firmer substance than those of the other Orders; in many _Locustæ_ Leach, _Fulgoræ_, &c., they are nearly as firm as the _tegmina_; and in _Ascalaphus italicus_, except at their base, the _secondary_ wings are less membranous than the _primary_. M. Chabrier has observed[1855] that the wings of insects in general diminish in thickness from their base to their apex, and from their anterior to their posterior margin.

3. I should have had, it is probable, but little original matter to communicate under the head of the _composition_ and _neuration_ of wings, had M. Jurine, who has written so ably on those of _Hymenoptera_, undertaken a survey of the organs of flight in every Order of insects: but as his views were confined to only _two_ of the Linnean Orders, it is not wonderful that his system and set of terms should fail where a generalization is necessary; and I may stand acquitted of presumption and conceit if I attempt to substitute a system and body of terms more universally applicable. Had the plan of this able Entomologist led him to pay attention to _tegmina_ and _hemelytra_, their division into three longitudinal areas would have immediately struck him; and having acquired this outline of the greater natural divisions, he would have applied it to the Orders that have _wings_ only, and having discovered that it is to be traced in all, the result would have probably superseded my labors. Had his life been longer spared, perhaps something of this kind would have been effected by him; but as he, alas! is gone, and no abler hand seems to have undertaken the task, I will do what I can to give you satisfaction on this subject[1856]. You have already got a tolerably good idea of these areas from what has been said upon the subject under _tegmina_ and _hemelytra_; but I shall now more particularly state to you how they are circumstanced in _wings_. I shall first explain the general law as to their limits. The _Costal Area_[1857] is all that longitudinal portion of the wing that lies between the anterior margin and the postcostal nervure; the _Intermediate Area_[1858] is all that longitudinal portion of the wing that lies between the postcostal and the anal nervures; and the _Anal Area_[1859] is all that longitudinal portion of the wing that lies between the anal nervure and the posterior margin. But there are other helps to enable you to distinguish the areas in the different Orders. The Anal Area in all Orders forms the posterior _fold_ of the wing; in _Coleoptera_ turned under when in repose; in _Orthoptera_ folded like a fan; in _Lepidoptera_, in some _Papilionidæ_, forming an arch over the abdomen. Again, in _Blatta_, the Costal Area is distinguished chiefly by _longitudinal_ nervures; the Intermediate by _oblique_ ones; and the Anal by _radiating_ ones; and in both this tribe and the _Mantidæ_ this last Area is marked out from the Intermediate by a marginal notch, which is not present in _Phasma_, but is found in both sections of the _Hemiptera_. In _Locusta_ Leach the notch is between the Costal and Intermediate Areas: in _Phasma_ the nervures of the Intermediate Area are branches of the externo-medial, while those of the Anal, as they do in all the _Orthoptera_, diverge from the base of the wing: in many, as in _Pterophylla_ K., the part of the wing lately alluded to, that is longer than the _tegmen_, and of the same substance, points out the limit of the Costal Area; and in others this part terminates in a segment of a circle and is differently reticulated at the apex from the Intermediate: in the Homopterous _Hemiptera_ and the _Libellulina_, in which the areas at first seem indistinct, they may generally be easily traced by following them from the axes. The separation of the Costal from the Intermediate in the remaining Orders seems less easy on account of the branching of the nervures: in the rest of the _Neuroptera_ and the _Lepidoptera_, if the posterior branches of the postcostal nervure are not included, you will have a narrow Postcostal Area, which in most cases forms an angle more or less prominent, in _Corydalis_ almost a right angle, with the Intermediate: in _Hemerobius_ and affinities this part is distinguished by areolets formed by _transverse_ nervures, while those of the rest of the wing are _longitudinal_[1860]: but if the posterior branches are included, the Costal Area will be more ample: a similar observation applies to the _Hymenoptera_ and _Diptera_; in these, in all cases, the areolets adjoining the anterior margin, which follow the _stigma_, should be regarded as belonging to the Area in question[1861]. In those tribes of the former Order, whose wings are without _nervures_, the areas are often marked by _folds_.

M. Chabrier has observed that in _Coleoptera_ the specific weight of the margin of the wing, and its means of resistance, are augmented by a liquid which is introduced, at the will of the animal, into a long pocket under the _brachial_, here called the _costal_ and _mediastinal_ nervures, covered by a supple membrane, which in a state of repose becomes flaccid[1862]: it is easily detected, being of a paler colour than the nervures between which it lies; this is what I call the _Phialum_; we have before seen that it exists also in _Elytra_ and some _Hemelytra_[1863]; but I have not detected it in any other _wings_.

I have before given you a sufficiently full account of the _alulæ_ or winglets of _Diptera_[1864]; and shall here only observe that they are not confined to _one_ particular tribe, as has been usually imagined; but though sometimes extremely minute, simple, and not easily detected, are an _universal_ distinction of the Order.

Having thus endeavoured to elucidate the _larger_ Areas into which wings appear to be divided; I shall next say something on the _smaller_ ones produced by the intersection or ramification of the nervures; these had been named areolets (_areolæ_) several years before M. Jurine's work, in which he calls them, I think _improperly_, cellules (_cellulæ_), was published; I therefore retain the _prior_ term. The general structure of the nervures of the wings of insects having been before explained[1865], I shall not here repeat what I then said; but there is a curious circumstance connected with it, particularly visible in the wings of certain _Hymenoptera_, that I must not pass without notice. If you examine attentively with a microscope against the light the wing of any _Nomada_ or _Andrena_, you will discover little transparent points in some of the smaller transverse nervures that form the middle areolets, in which the nervure becomes white and looks as if it was interrupted, though in substance it seems continued: these little points, somewhat resembling minute air bubbles detained in the tubes, are what M. Jurine, who first discovered them, has, on that account, named _bullæ_, which he thus further describes:--"When the tube (of the nervure) arrives at the spot where a _bulla_ is to be formed, it extends itself on all sides in minute threads in the upper membrane of the wing, losing its colour and tubular structure, which it resumes immediately after the formation of the bulla[1866]." But if you look closely at them you will find that there is always a slight _fold_ of the wing that cuts the nervure exactly at the _bullæ_, and if the fold changes its direction they accompany it; their object, therefore, is clearly to relax the tension so as to admit a little motion where the fold is; consequently, rather than _bullæ_ (bubbles), they should be denominated _articulations_. A similar construction, but on a larger scale, may be observed in the wings of _Coleoptera_[1867] and some others, as _Psocus_, where the folds traverse the nervures. I shall next make a few observations on the principal nervures; and first a word upon their _names_. M. Jurine, being of opinion that a striking analogy exists between the wings of _insects_ and those of _birds_, in which M. Chabrier seems to agree with him, has named the nervures in the anterior margin of the wings of the former, _radius_ and _cubitus_, as corresponding with the bones so named in the fore-arm of the latter, and the plate which often terminates these nervures in _Hymenoptera_, he names the _carpus_; it may look like presumption to differ from two such weighty authorities, but as their observations seem to have been too limited, in one case to the _Hymenoptera_ and _Diptera_ only; and in various Orders there is nothing analogous to the _stigma_ or _carpus_, and all the other nervures of an insect's wing have no analogue in that of a bird, but more especially as M. Latreille seems to think with me on this subject[1868], I have retained Linné's term for the marginal nervure, and for most of the others have adopted those of the great French Entomologist just mentioned. I shall here only further observe,--and it seems to me an observation of prime importance, in the determination of the question of the analogy of the wings of insects,--that they are not, as in birds, the _fore-leg_ converted into an organ of flight, but, like the wing of the _Draco_, an organ _superadded_ to the legs; and, further, that the connection is not with the fore-legs, but, as has been before observed[1869], with the two posterior pairs.

The _Costa_[1870] is usually the strongest of the nervures, and that upon which the wing seems to be built; but in some cases, as in _Blatta_, _Scutellera_, _Cynips_, &c., it is represented by the mere membrane of the anterior margin; in some _Coleoptera_, as in _Geotrupes_, _Dytiscus_, &c., its structure, except at the base, appears to be annular or nearly so, at least a vast number of corrugations, running transversely, are observable on its upper and lower surfaces; it is thus capable of greater tension and relaxation, and more flexile. The _stigma_ or _carpus_[1871], though most conspicuous in the _Hymenoptera_ Order, may be traced in some _Coleoptera_, Heteropterous _Hemiptera_, the _Libellulina_, &c.; but it has no representative in the _Orthoptera_, _Lepidoptera_, _Trichoptera_, &c. The _mediastinal_ is usually a very slender nervure, placed between the _costa_ and _postcosta_, sometimes terminating in the former[1872], and at others in the latter[1873]: in the _Orthoptera_, _Lepidoptera_, &c., however, and some others, it is a very conspicuous and principal one[1873]; in the _Hymenoptera_ it is obsolete, merging in those nervures[1874]. The _Postcosta_ is the principal nervure of the wing in _Scutellera_, but in _Staphylinus_ it is wanting; in _Chalcis sispes_ it is the only true nervure of that organ, the others being represented by spurious ones[1875]. The _externomedial_ and _internomedial_ are sometimes distinct at their origin, but more frequently are branches from a common stem.