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.
Having made these general remarks, I shall now consider particularly the neuration of the wings in the different Orders, beginning with the Coleoptera. The first thing that strikes the physiologist in surveying a wing belonging to an insect of this Order, is the general arrangement of the nervures[1876]; which are so placed that the required degree of tension may be given to every part of this organ: thus some are nearly straight[1877]; others run in a serpentine direction[1878]; others are forked with one branch recurrent and another proceeding onwards[1879]; others again are insulated, or do not originate from the base of the wing, or from other nervures, but are merely placed to strengthen an open space of it[1880]: these nervures are also usually broader and more substantial than those of the wings of the subsequent Orders. Another striking circumstance with regard to them is that the nervures form few or no closed areolets, except in the Costal Area, where they are inconspicuous; in Dytiscus marginalis, indeed, and Tenebrio Molitor one or two may be found, but in general there are none. In many of this tribe the postcosta, which terminates at the joint of the wing, becomes recurrent, so as to form a hook, which perhaps represents the stigma, as in Dynastes[1881]; in Creophilus K., a rove-beetle, there is no hook but a broad plate adjacent to the costa. In the Strepsiptera Order the neuration is extremely simple, the nervures, except one insulated one, diverging from the base of the wing[1882]: in this respect, as well as in the form of that organ, an approach is made to the Orthoptera. In the Dermaptera this approach is still more evident; in the common earwig[1883], the diverging nervures become numerous; between each is an insulated one, taking its origin in the middle of the wing, and running to the margin; a little nearer to the latter all the nervures are dilated into a plate; those of the anal area are angular[1884], and the exposed part of the costal is as hard as the elytra. The neuration in the Orthoptera Order may be called radiate, the longitudinal nervures for the most part diverging from the base of the wing like rays: in some few instances[1885], but not often, I believe, an insulated nervure intervenes between each; traversing or connecting nervures, cutting the longitudinal ones in various directions, ornament these wings with an infinity of areolets, causing them to resemble fine gauze or beautiful lace or net-work; very often these areolets are quadrangular, sometimes rhomboidal, frequently nearly circular, and differing occasionally, as has been before observed[1886], in the different areas: it sometimes occurs that there are no traversing nervures[1887], when the wing of course is without areolets. In the Heteropterous Hemiptera the type of neuration, as to the wing, seems borrowed from the Coleoptera, a further proof that these are the analogues of that Order amongst the Haustellata Clairv. In these the nervures usually are few and dispersed, and seldom form any closed areolets. If you examine any Scutellera, Pentatoma, or Lygæus, you may trace the uncinated, forked, serpentine, and insulated nervures of Coleopterous insects; in Gerris and Velia there is an approach to the neuration of some Homopterous species, and in Belostoma &c. the wing is reticulated by spurious nervures. In the Homopterous section there are several types of neuration; thus the Fulgoræ resemble the Orthoptera in this respect; while the Tettigoniæ F., &c., approach nearer to the Hymenoptera and Diptera, and have their apical areolets circumscribed within the margin by a traversing nervure; in Flata, &c., the areolets are mostly formed, not by traversing nervures, but by the branching of the longitudinal ones; in this respect they are not unlike the Lepidoptera. In this last-named Order there are some variations with regard to their neuration—thus, amongst the butterflies in Urania, &c., there is no closed areolet in any of the wings, and almost all the nervures diverge from the base[1888]; in Morpho, &c., there is only one in the primary wing[1889]; in Heliconia, &c., there is one in both wings; amongst the moths, in the Bombyces L., this is divided into two, and in Cossus labyrinthicus Don. into three areolets: in some butterflies (Lycæna) there is one insulated nervure[1890], and in others (Hesperia) there are two[1891]; in these two last, and Heliconia, Urania, &c., the end of the Costal Area is divided into several areolets by oblique nervures[1892], which gives them some analogy to the wings of many Neuroptera; and at the base of this Area, in Morpho, is a roundish areolet[1893]. In this Order the externo-medial and interno-medial nervures coalesce into one, and are only represented separately by their first and third branches[1894]. In the Neuroptera Order the general type of neuration is borrowed from the Orthoptera; but in Osmylus, Termes, &c., there is an approach to that of Flata in the Homopterous Hemiptera, and in Psocus to others of that section; in the second of these genera the nervures, except those of the costal margin, are spurious.
I now come to the Order in which M. Jurine has laboured with so much success, I mean the Hymenoptera; and I only regret that his labours were directed to so small a portion of the Class Insecta, and in that portion only to a part of the upper wing; I say only a part, because all those areolets of the posterior part of the wing, in some cases amounting to five[1895], that lie behind his cubital cellules, are not employed by him as diagnostics, and are left without a name. By dividing the areolets of the Intermediate Area of these wings into three portions, the basal, medial, and apical[1896], I have endeavoured to remedy this defect, and by naming each set of areolets in the middle portion, as you will see in the Orismological Definitions, under the term Areolets, you will find it easy to describe any given areolet and its place in the wing; those of the base may be called the anterior, intermediate, and posterior, where three occur; and the first and last of these terms will suffice where there are only two; the apical areolets, or those that are open to the margin, may be called, first, second, and third in the order of their occurrence, reckoning from the anterior or costal margin.
In this Order it is curious to trace the progress of neuration in the wings of different genera. Thus in Psilus only the costal nervure and the stigma are to be traced[1897]; in Chalcis the postcostal and stigma[1898]; in Codrus and Leucospis the costal, postcostal, stigma, and a nervure representing the externo-medial and interno-medial coalescing into one[1899]; in Omalus the basilar areolets appear[1900]; in Crabro both basilar and medial[1901]; in Cynips basilar, medial, and apical[1902]; and in Hylotoma the wing is filled with its greatest complement of areolets[1903]. The medial areolets of the Intermediate Area, as you will see in the definitions, form three distinct series; these may be called the protomesal, deuteromesal, and tritomesal, reckoning from the postcostal areolets; the first of these corresponds with the cubital cellules of Jurine. These series may be expressed, according to the number of their areolets; by figures, the protomesal standing first. They vary much in this respect in the different genera. Thus in Cyclostoma K.[1904], reckoning the didymous areolet as two, the numbers will stand 4:2:1; in Hylotoma, &c., 3:2:1[1905]; in Aulacus, &c., 2:2:1[1906]; in Bracon, &c., 2:1:1[1907]; in Chelonus, 2:0:1[1908]; in Cynips erythrocephalus Jur., 2:0:0[1909]; in Formica, 1:1:1[1910]; in Oxybelus, 1:0:1[1911]; in Chrysis, 0:1:1[1912]; and in Cynips Rubi K., 1:0:0[1913]. The most natural number is 3:2:1. The next in importance to the medial areolets of the Intermediate Area are the apical, or those open to the margin; the most usual number of them, excluding the postcostal areolets which belong to the Costal Area, is three; but in Sirex there is an approach to four[1914]; in Evania there are only two[1915]; and in Philanthus there are none[1916]; in many, as Prosopis, Nomada, Andrena[1917], though there is the usual number, they are incomplete and do not reach the margin. The basal areas are of little importance in assisting to determine genera; they are most commonly two in number, but in Cynips, &c., there is only one[1918]. The shape and other circumstances of the areolets vary considerably in different genera and species: upon these however I shall not enlarge further, but proceed in the next place to consider very briefly the wings of the Diptera Order as to their neuration. These are not so easily made subservient to a general plan. The basilar areolets are now reduced considerably in length, occupying merely the base of the wing[1919]; the medial are become less numerous and important[1920]; and the apical, in a variety of instances, are the most conspicuous[1921]; in some wings, as in those of Penthetria, the Intermediate Area has no nervures or areolets, or only spurious ones; in Psychoda the nervures diverge from the base almost without branching, so as to form no closed areolets[1922]; in many, the lower medial areolets are very long, resembling the basilar in Hymenoptera[1923]; these are often crowned by a single small one, as in the Stratyomidæ, Tipula, &c., from which numerous branches proceed to the margin[1924]; but in Musca two large ones approach the margin, the anterior one having an angle open to it[1925]; in the Hippoboscidæ almost the whole of the wing is occupied by the apical areolets[1926]; though in some cases they are incomplete[1927].
4. I am next to consider the position of wings in repose and their folding. The most important object of this is that when unemployed they may occupy less space, be less in the way of the insect, and be most effectually protected from injury. Another end is also served by this structure,—that wings can thus be very ample, and present a large surface to the action of the atmosphere without incommoding the insect when it has not occasion to use them.
With respect to this head, insects may be divided into two classes—namely, those whose wings in repose are covered by wing-cases harder than the wings themselves, and those that have no such protection. In the former the wings, though the rule admits several exceptions, have more folds than in the latter. As the different mode of folding the wings has been assumed for a characteristic of the earlier Orders, I shall explain to you with as much brevity as possible how each is circumstanced in this respect, beginning as usual with the Coleoptera.
There are two principal folds of the wing in this Order, which may be named the anal and the apical: the former is when the Anal Area or part of it is folded on the under surface of the base of the wing; this fold is always more or less longitudinal: the latter, the apical fold, is by means of the commissura or joint of the postcosta lately mentioned: which in Hister, Staphylinus, &c., for obvious reasons[1928] is nearer the base of the wing; in Necrophorus in the middle; in Dynastes Aloeus beyond the middle[1929]; in Tenebrio Molitor near the apex; and in Dytiscus marginalis there appears to be no joint at all; but the fact is, that in this insect the postcosta,—the termination of which really forms the joint, the costa itself being only flexible at that point,—stands at a greater distance from the latter at its end. Well, at this joint the above fold is made, the apex of the wing, being first folded longitudinally, turning under and inwards, and forming an angle, more or less acute, with the joint or costal margin, so that the fold is not quite but nearly transverse: this at least is the case in Geotrupes stercorarius and other Lamellicorns: in Staphylinus, &c., there are several transverse and longitudinal folds, and thus the wing is more easily packed under the short elytra; in Molorchus, Necydalis, &c., in which it is left uncovered, except at its base, the anal fold takes place, and the apical in some degree; a short portion near the apex forming an obtuse angle with the margin; in Atractocerus the wing appears to be only longitudinally folded; and in Buprestis vittata only the anal fold is to be detected. Besides these transverse and longitudinal folds these organs, in many beetles, have an infinity of fine corrugations, which ramify like the nervures of the tegmina of Flata[1930], &c., proceeding from the Costal Area or the disk of the wing to the posterior margin; the object of these plicatures is doubtless to present a more ample surface to the action of the atmosphere in flight[1931]. When all these folds have been made in a Coleopterous wing, the apex of the one at its posterior margin crosses or rests upon that of the other[1932].