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.