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.