These, however, are only exceptions to the rule; but in the Diptera, unless we consider the alulæ, the representatives of the secondary wings[1782], as a distinct pair, there are never more than two wings, and one instance is known in which an insect of this Order has none[1783]. Certain genera or individuals of the Tetrapterous Orders are also furnished with alulæ: besides Dytiscus, Blatta, Phalæna hexaptera, which have been before noticed[1784], they may be detected in miniature in Ammophila K. and affinities; these all may be regarded in some slight degree as insects with six wings.
ii. Kinds. Under this head we may consider the organs of flight as to their situation and as to their substance. As to their situation, usually the first pair are attached to the mesothorax, and the second to the metathorax; but in one instance, as has been before observed[1785], in the Strepsiptera K., the anterior pair belong to the manitrunk, and the posterior to the mesothorax. As to their substance, they take the several denominations of elytra, tegmina, hemelytra, and wings, for the most part according to its variations, as will be seen more at large hereafter. Under this head I shall only further observe, that in many instances the organs of flight appear to be mere abortions or rudiments, which serve to exemplify what has been more than once stated, that the CREATOR has seen it good to approach to new organs gradually as well as to new forms. Thus elytra are mere rudiments that do not serve to protect the wings in Atractocerus; tegmina in some species of Phasma, Acrydium, &c.; hemelytra in the bed-bug[1786]; wings in many female moths, in Cryptus hemipterus a Hymenopterous insect, &c.
iii. Composition. The structure of wings has been before explained to you[1787], and I shall again have occasion to allude to it; but here I wish to call your attention to a circumstance that has not hitherto, that I recollect, been adverted to; I mean that all kinds of organs of flight, and it may be traced as we shall soon see even in elytra, are divided longitudinally into three areas or folds; the first or external one I call the Costal Area[1788] from its beginning with the costal nervure; the second is the Intermediate Area[1789]; and the third is the Anal Area[1790].
Having made these observations with respect to the organs of flight in general, I shall now proceed to consider more at large the elytra, tegmina, hemelytra, and wings.
i. Elytra. These are the wing-covers of the Coleoptera Order, distinguished from tegmina by the absence of nervures, from hemelytra by the want of the membrane at the apex, and from both by their uniting in almost every instance at the suture. I shall consider them as to their substance; articulation with the trunk; expansion; parts; shape; appendages; sculpture; clothing; colours, and uses.
1. Substance. The firmness of the substance of elytra is usually regulated by that of the crust of the insect to which they belong; in hard insects they are hard, and in soft ones they also are soft. The most impenetrable ones that occur to my recollection are those of Illiger's genus Doryphora, and the softest and most flexile those of Telephorus, Meloe and affinities. With regard to individuals, they are mostly as hard as the prothorax, and harder than the back of the abdomen. Elytra also, as far as my observation goes, are never diaphanous.
2. Articulation with the trunk. This is by means of a process of the base of the elytrum which I call the axis[1791] or pivot, attached by elastic ligaments, and certain little bony pieces (osselets Chabr.) in the socket under the side of the anterior angle of the dorsolum[1792]. You may easily remove the elytra attached to the mesothorax from Geotrupes stercorarius, which will enable you to see the mode of articulation with little trouble[1793].
3. Expansion. It is by means of the bony pieces just mentioned that the organs in question are opened and shut[1794] under the action of the antagonist muscles. In opening for flight the two elytra recede from each other, and are elevated so as not to retain their horizontal position, which would interfere probably with the play of the wings, but form an angle with the body. When they return to a state of rest, the sutures usually meet and coincide longitudinally; but in some cases when closed, as in Necydalis, &c., they diverge from each other at the apex; and in Meloe, like the Orthoptera, to which that genus approaches, one laps over the other.
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.