7.Colour.Orthopterous insects are seldom remarkable fortegminaof brilliantcolours; 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 ofBlatta Petiveriana; one hasfourwhite spots, and the other onlythree; but as one laps over the other, the symmetry of the arrangement is preserved: the HomopterousHemipteraare more distinguished in this respect, and some of theFulgoridæimitate theLepidopteraboth by their ocelli and spots:Fulgora laternaria,Candelaria,serrata, andDiadema, sufficiently exemplify this remark, as do severalFlatælikewise[1834].
We may observe here—thattegminaare more calculated for flight thanelytra, both from their thinner substance, and from the angle that their Anal Area, and often the Costal, forms with the rest of thetegmen; 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 HeteropterousHemiptera, are calledhemelytra, or half-elytra:—this term was also formerly employed, but certainly incorrectly, to denotetegmina. 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 theirsubstance, they must be separately considered with regard to their base and apex. In various instances thebase, or part consisting of the three areas, is almost corneous, as inCydnus Morioandbicolor, bugs not uncommon with us, and many others[1836]; in these cases it is lined with ahypodermalike elytra; and in many the points, which are impressed upon it, also perforate thehemelytrum, and seem to act as pores: but inLygæus,Reduvius,Capsus,Miris, and the majority of the HeteropterousHemiptera, the organs in question being soft and flexible,may be stated as rather resembling leather than horn;—on this account this part of ahemelytrumis denominated thecorium. InScutellerathe portion covered by the scutellum is membranous; and inAcanthia paradoxa, and the cucullated species ofTingis, 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 themembrana. I sayalmost, because inAradusand theHydrocorisæLatr., this part, though rather thinner than the rest of theHemelytrum, is also coriaceous; in the latter tribe usually with a very narrow membranous edge; and in manyReduviiandZelithere is scarcely any difference in the substance of the base and apex.
2. As to thearticulationofHemelytrawith the trunk, it seems not strikingly different from that oftegmina: 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'shumerusshaped like a swan's head and neck[1837].
3. Thecompositionof these organs differs from that oftegminain more respects than one: in the first place, they consist, as was lately observed, offourinstead ofthreeareas; in the next, they appear to have, at least several of them, a part, which I suspect to be analogous to that above described inColeoptera, supposed to represent thephialumof wings[1838]. I shall first speak of theareas. In some apterous species related to the bed-bug,Lygæus brevicollisLatr.[1839], &c., there is no trace of the usual areas,and themembranais a very narrow strip; inL. apterusthe 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 theHemelytrum, while in the two former it is at its base. InLygæus compressipes(RhinuchusK. MS.) the Anal Area is cultriform; and in most of theHydrocorisæit has an angle in the middle of its posterior margin. The proportion that themembranaorapical areabears 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 athirdof thehemelytrum; inLygæus compressipes,cruciatus, &c., fullhalf; inAlydus calcaratus,two-thirds; inReduvius, nearlythree-quarters[1844]; and inAradus depressusthe corium,—divided, however, though indistinctly, into the three areas,—is driven to thebaseof the wing: two ends are answered by this structure—as this insect lives under bark, its thinhemelytratake 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 usuallyruns obliquely from the vertex of the Anal Area to the base of the Costal.
4. As to theirposition and folding in repose,Hemelytraare usually nearly or altogetherhorizontal; but inNotonectaandPleathey aredeflexedand 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 inScutellera,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. InNotonecta, in which thehemelytraare deflexed, at the apex of themembranais a fissure which permits the two sides to form an angle with each other, and to apply exactly to the body. InPlea, in which there is no apical area, the posterior margins of thetegmina, as they ought rather to be termed, unite, but do not lap over each other. With regard to the appearance of something like aphialum, if you examine thehemelytraof 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 ofBelostoma grandis, where the structure is most conspicuous, or even the commonNepa 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 theirshape, with few exceptions,hemelytramore or less represent a wedge, being wider at their apex, wherethey are usually obliquely truncated, than at the base; but inPleaLeach they are obtusangular, with the angle in the sutural margin; inNotonecta, on the contrary, an obtusangular sinus distinguishes that part; inNaucoristhey are curvilinear and every where of equal width; inRanatrathey are linear and straight; inAradusthey 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 thehemelytrumhad 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 thehemelytra; but by this compensating contrivance of Providence, the gap is filled, the above lobe fitting exactly into it.
6. Theneurationof these organs will not occupy us long, since thecoriumor harder part, though in some species there are traces of nervures, is often without them. Those of the cucullated species ofTingisresemble manytegminain being ornamented by them with a kind of network, which looks like the finest lace; in severalLygæi,Edessa, and someReduvii, 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 inAradus, in others running parallel to each other, nearly to the end of this area, as inBelostoma grandis,where they are met by a traversing nervure; the object of this is doubtless to strengthen the membrane.
7. Bothtegminaandhemelytraare most commonly naked, yet very shorthairsare found on those of some species ofCercopis, and in many more instances in those of the latter description, as inNotonecta, severalLygæiandReduvii, &c.
8.Coloursinhemelytraare very various, and in many instances are peculiar to families; in certainLygæi(L. Hyoscyami, &c.) black and red; inLygæus compressipesand affinities a dingy black; in someReduviiblack with a large white spot;—but it is needless to enlarge further on this subject.
9. Thathemelytraare 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 thePentatomæLatr., the corium of thehemelytrumis 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 toflight, and therefore are properly denominatedwings. These in the Orders that haveelytra,tegmina, orhemelytra, are the pair that correspond with the secondary wings of the other Orders. It may be said, indeed, that in several instances bothtegminaandhemelytrado 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 thearticulation of these organs with the trunk; in general it may be stated that this, as integminaandhemelytra, 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 theDynastidæ, in theColeoptera; the first thing that will strike you, upon examining the base, will be the plate before mentioned called by Chabrier thehumerus, 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'shumerusby 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 saidhumerus; and at its posterior end is united to thepostfrænum[1848], with which it forms a right angle. In theOrthopteraOrder the structure is not very different, but the axes and other plates of the base of thewing 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 thepostfrænum[1850]. In the HomopterousHemipterathe three axes may be readily traced, but thehumeralplate, with which they all are connected, is more irregular in shape, and inFulgoralongitudinal, with an angular surface; in this Order the nervure, in some cases consisting of cartilaginous rings[1851], in which thefrænumandpostfrænumterminate in thetegminaand wings, is attached posteriorly to the ligament of the Anal Area. In the Heteropterous section the three axes are evident, but thehumeralplate is not easily made out. In theLibellulinathe 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 thefrænum, and in the secondary with thepostfrænum. 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 minuteHymenopterawhose wings have solely the costal nervures; probably in these there is onlyoneaxis. In theLepidopteraandHymenopteraa circumstance connected with the present head is observable, which is not to be discovered in the other Orders: these are thetegulæor base-covers, which appear intended to defend the base of the anterior wings. They are concavo-convex scales, which in theLepidopteraare largeand of an irregular shape[1852], but in theHymenopteraare smaller and semicircular[1853].
2. Wings, with regard to theirsubstance, may generally be termedmembranous; but they vary in this respect, some being much thicker than others, eitherpartiallyortotally: in spotted wings, as in those of manyLibellulina,TettigoniæF., &c., the dark opaque parts are denser than those that are transparent: in severalOrthopterousinsects, as inPhasma, someMantes, &c., the Costal Area or covering part of the wing is of a substance equally firm with that of thetegmen. 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 ofDivine Wisdomis exhibited by thosePterophyllæK. (LocustaF.) whosetegminaresemble the leaves of plants (Pt. laurifolia, &c.); in these the tip of the wings when folded being longer, is not covered by thetegmina, 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 mostColeoptera,Orthoptera,Hemiptera, andThereva coleoptrata, in theDiptera, &c., are of a firmer substance than those of the other Orders; in manyLocustæLeach,Fulgoræ, &c., they are nearly as firm as thetegmina; and inAscalaphus italicus, except at their base, thesecondarywings are less membranous than theprimary. M. Chabrier has observed[1855]that the wings of insects in general diminish in thickness from theirbase 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 thecompositionandneurationof wings, had M. Jurine, who has written so ably on those ofHymenoptera, undertaken a survey of the organs of flight in every Order of insects: but as his views were confined to onlytwoof 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 totegminaandhemelytra, 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 havewingsonly, 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]. Youhave already got a tolerably good idea of these areas from what has been said upon the subject undertegminaandhemelytra; but I shall now more particularly state to you how they are circumstanced inwings. I shall first explain the general law as to their limits. TheCostal Area[1857]is all that longitudinal portion of the wing that lies between the anterior margin and the postcostal nervure; theIntermediate Area[1858]is all that longitudinal portion of the wing that lies between the postcostal and the anal nervures; and theAnal 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 posteriorfoldof the wing; inColeopteraturned under when in repose; inOrthopterafolded like a fan; inLepidoptera, in somePapilionidæ, forming an arch over the abdomen. Again, inBlatta, the Costal Area is distinguished chiefly bylongitudinalnervures; the Intermediate byobliqueones; and the Anal byradiatingones; and in both this tribe and theMantidæthis last Area is marked out from the Intermediate by a marginal notch, which is not present inPhasma, but is found in both sections of theHemiptera. InLocustaLeach the notch is between the Costal and Intermediate Areas: inPhasmathe nervures of the Intermediate Area are branches of the externo-medial, while those of the Anal, as they do in all theOrthoptera, divergefrom the base of the wing: in many, as inPterophyllaK., the part of the wing lately alluded to, that is longer than thetegmen, 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 HomopterousHemipteraand theLibellulina, 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 theNeuropteraand theLepidoptera, 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, inCorydalisalmost a right angle, with the Intermediate: inHemerobiusand affinities this part is distinguished by areolets formed bytransversenervures, while those of the rest of the wing arelongitudinal[1860]: but if the posterior branches are included, the Costal Area will be more ample: a similar observation applies to theHymenopteraandDiptera; in these, in all cases, the areolets adjoining the anterior margin, which follow thestigma, should be regarded as belonging to the Area in question[1861]. In those tribes of the former Order, whose wings are withoutnervures, the areas are often marked byfolds.
M. Chabrier has observed that inColeopterathe 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 thebrachial, here called thecostalandmediastinalnervures, 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 thePhialum; we have before seen that it exists also inElytraand someHemelytra[1863]; but I have not detected it in any otherwings.
I have before given you a sufficiently full account of thealulæor winglets ofDiptera[1864]; and shall here only observe that they are not confined tooneparticular tribe, as has been usually imagined; but though sometimes extremely minute, simple, and not easily detected, are anuniversaldistinction of the Order.
Having thus endeavoured to elucidate thelargerAreas into which wings appear to be divided; I shall next say something on thesmallerones 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 thinkimproperly, cellules (cellulæ), was published; I therefore retain thepriorterm. 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 certainHymenoptera, that I must not pass without notice. If you examine attentively with a microscope against the light the wing of anyNomadaorAndrena, 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 itwas 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, namedbullæ, which he thus further describes:—"When the tube (of the nervure) arrives at the spot where abullais 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 slightfoldof the wing that cuts the nervure exactly at thebullæ, 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 thanbullæ(bubbles), they should be denominatedarticulations. A similar construction, but on a larger scale, may be observed in the wings ofColeoptera[1867]and some others, asPsocus, where the folds traverse the nervures. I shall next make a few observations on the principal nervures; and first a word upon theirnames. M. Jurine, being of opinion that a striking analogy exists between the wings ofinsectsand those ofbirds, in which M. Chabrier seems to agree with him, has named the nervures in the anterior margin of the wings of the former,radiusandcubitus, as corresponding with the bones so named in the fore-arm of the latter, and the plate which often terminates these nervures inHymenoptera, he names thecarpus; it may look like presumption to differ from two such weighty authorities, but as their observations seem to have been toolimited, in one case to theHymenopteraandDipteraonly; and in various Orders there is nothing analogous to thestigmaorcarpus, 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, thefore-legconverted into an organ of flight, but, like the wing of theDraco, an organsuperaddedto 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.
TheCosta[1870]is usually the strongest of the nervures, and that upon which the wing seems to be built; but in some cases, as inBlatta,Scutellera,Cynips, &c., it is represented by the mere membrane of the anterior margin; in someColeoptera, as inGeotrupes,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. Thestigmaorcarpus[1871], though most conspicuous in theHymenopteraOrder, may be traced in someColeoptera, HeteropterousHemiptera, theLibellulina, &c.; but it has no representative in theOrthoptera,Lepidoptera,Trichoptera, &c. Themediastinalis usually avery slender nervure, placed between thecostaandpostcosta, sometimes terminating in the former[1872], and at others in the latter[1873]: in theOrthoptera,Lepidoptera, &c., however, and some others, it is a very conspicuous and principal one[1873]; in theHymenopterait is obsolete, merging in those nervures[1874]. ThePostcostais the principal nervure of the wing inScutellera, but inStaphylinusit is wanting; inChalcis sispesit is the only true nervure of that organ, the others being represented by spurious ones[1875]. Theexternomedialandinternomedialare sometimes distinct at their origin, but more frequently are branches from a common stem.
Having made thesegeneralremarks, I shall now considerparticularlytheneurationof the wings in the different Orders, beginning with theColeoptera. 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 noclosed areolets, except in the Costal Area, where they are inconspicuous; inDytiscus marginalis, indeed, andTenebrio Molitorone or two may be found, but in general there are none. In many of this tribe thepostcosta, which terminates at the joint of the wing, becomes recurrent, so as to form a hook, which perhaps represents thestigma, as inDynastes[1881]; inCreophilusK., a rove-beetle, there is no hook but a broad plate adjacent to thecosta. In theStrepsipteraOrder 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 theOrthoptera. In theDermapterathis 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 theOrthopteraOrder may be calledradiate, the longitudinal nervures for the most part diverging from the base of the wing likerays: 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 HeteropterousHemipterathe type of neuration, as to the wing, seems borrowed from theColeoptera, a further proof that these are the analogues of that Order amongst theHaustellataClairv. In these the nervures usually are few and dispersed, and seldom form any closed areolets. If you examine anyScutellera,Pentatoma, orLygæus, you may trace the uncinated, forked, serpentine, and insulated nervures of Coleopterous insects; inGerrisandVeliathere is an approach to the neuration of someHomopterousspecies, and inBelostoma&c. the wing is reticulated byspuriousnervures. In the Homopterous section there are several types of neuration; thus theFulgoræresemble theOrthopterain this respect; while theTettigoniæF., &c., approach nearer to theHymenopteraandDiptera, and have their apical areolets circumscribedwithinthe margin by a traversing nervure; inFlata, &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 theLepidoptera. In this last-named Order there are some variations with regard to their neuration—thus, amongst thebutterfliesinUrania, &c., there is no closed areolet in any of the wings, and almost all the nervures diverge from the base[1888]; inMorpho, &c., there is only one in the primary wing[1889]; inHeliconia, &c., there is one in both wings; amongstthemoths, in theBombycesL., this is divided intotwo, and inCossus labyrinthicusDon. intothreeareolets: in some butterflies (Lycæna) there is one insulated nervure[1890], and in others (Hesperia) there are two[1891]; in these two last, andHeliconia,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 manyNeuroptera; and at the base of this Area, inMorpho, 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 theNeuropteraOrder the general type of neuration is borrowed from theOrthoptera; but inOsmylus,Termes, &c., there is an approach to that ofFlatain the HomopterousHemiptera, and inPsocusto 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 theHymenoptera; and I only regret that his labours were directed to so small a portion of the ClassInsecta, and in that portion only to a part of the upper wing; I say only apart, because all those areolets of the posterior part of the wing, in some cases amounting tofive[1895], that lie behind hiscubital cellules, are not employed by him as diagnostics, and are left without a name. By dividing the areolets of theIntermediate Area of these wings intothreeportions, thebasal,medial, andapical[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 termAreolets, you will find it easy to describe any given areolet and its place in the wing; those of the base may be called theanterior,intermediate, andposterior, wherethreeoccur; and the first and last of these terms will suffice where there are onlytwo; the apical areolets, or those that are open to the margin, may be called,first,second, andthirdin 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 inPsilusonly thecostalnervure and thestigmaare to be traced[1897]; inChalcisthepostcostalandstigma[1898]; inCodrusandLeucospisthecostal,postcostal,stigma, and a nervure representing theexterno-medialandinterno-medialcoalescing into one[1899]; inOmalusthebasilarareolets appear[1900]; inCrabrobothbasilarandmedial[1901]; inCynips basilar,medial, andapical[1902]; and inHylotomathe wing is filled with its greatest complement of areolets[1903]. The medial areolets of the Intermediate Area, as you will see in the definitions, formthreedistinct series; these may be called theprotomesal,deuteromesal, andtritomesal, reckoning from the postcostal areolets; the first of these corresponds with the cubital cellules of Jurine. These series may beexpressed, according to the number of their areolets; by figures, the protomesal standing first. They vary much in this respect in the different genera. Thus inCyclostomaK.[1904], reckoning the didymous areolet as two, the numbers will stand 4:2:1; inHylotoma, &c., 3:2:1[1905]; inAulacus, &c., 2:2:1[1906]; inBracon, &c., 2:1:1[1907]; inChelonus, 2:0:1[1908]; inCynips erythrocephalusJur., 2:0:0[1909]; inFormica, 1:1:1[1910]; inOxybelus, 1:0:1[1911]; inChrysis, 0:1:1[1912]; and inCynips RubiK., 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 theapical, or those open to the margin; the most usual number of them, excluding the postcostal areolets which belong to the Costal Area, isthree; but inSirexthere is an approach tofour[1914]; inEvaniathere are onlytwo[1915]; and inPhilanthusthere are none[1916]; in many, asProsopis,Nomada,Andrena[1917], though there is the usual number, they are incomplete and do not reach the margin. Thebasalareas are of little importance in assisting to determine genera; they are most commonlytwoin number, but inCynips, &c., there is onlyone[1918]. The shape and other circumstances of the areolets vary considerablyin 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 theDipteraOrder as to their neuration. These are not so easily made subservient to a general plan. Thebasilarareolets are now reduced considerably in length, occupying merely the base of the wing[1919]; themedialare become less numerous and important[1920]; and theapical, in a variety of instances, are the most conspicuous[1921]; in some wings, as in those ofPenthetria, the Intermediate Area has no nervures or areolets, or only spurious ones; inPsychodathe 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 inHymenoptera[1923]; these are often crowned by a single small one, as in theStratyomidæ,Tipula, &c., from which numerous branches proceed to the margin[1924]; but inMuscatwo large ones approach the margin, the anterior one having an angle open to it[1925]; in theHippoboscidæalmost 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 thepositionof wings inreposeand theirfolding. 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 presenta 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 intotwoclasses—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 theColeoptera.
There aretwoprincipal folds of the wing in this Order, which may be named theanaland theapical: theformeris 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 lesslongitudinal: thelatter, the apical fold, is by means of thecommissuraor joint of thepostcostalately mentioned: which inHister,Staphylinus, &c., for obvious reasons[1928]is nearer the base of the wing; inNecrophorusin the middle; inDynastes Aloeusbeyond the middle[1929]; inTenebrio Molitornear the apex; and inDytiscus marginalisthere appears to be no joint at all; but the fact is, that in this insect thepostcosta,—the termination of which really forms the joint, thecostaitself 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 anangle, 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 inGeotrupes stercorariusand other Lamellicorns: inStaphylinus, &c., there are several transverse and longitudinal folds, and thus the wing is more easily packed under the short elytra; inMolorchus,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; inAtractocerusthe wing appears to be onlylongitudinallyfolded; and inBuprestis vittataonly theanalfold 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 thetegminaofFlata[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].
In theDermaptera[1933], at least the common earwig, there is a triple transverse fold of the wing, and besides this it has numerous longitudinal ones like those of a fan, each of the diverging nervures representing one of the sticks. In theStrepsipterathe folds are only longitudinal; a circumstance which, besides the form and neuration of the wing, sufficiently attests that its station is more near theOrthopteraandColeopterathan theDiptera.We next come to theOrthoptera[1934]; in these the folds in general are longitudinal; and those of the Anal Area in particular, either in whole or in part, exact counterparts of a fan: wherever there is a straight nervure, there is usually a fold or a tendency to it; this is the case even with the short oblique ones observable in the Intermediate Area ofBlatta: in this tribe the Anal Area, or a considerable portion of it, is folded under the rest of the wing, and the whole lies on the back of the animal, so that in this wing there are only two primary folds; but in those with a narrower body, asPhasma, &c., there are more, and the Anal Area, folded like a fan, lies horizontally on the back; the Costal is vertically applied to the sides, and the Intermediate is between both, as in thetegmina[1935]. InGryllusLatr.,Gryllotalpa, &c., when the wings are folded, the end of the Anal Area projects so as to present the appearance of two tails[1936]; and in that remarkable Chinese animalGryllus monstrosus, in which these tails are very long, they are convolute like those of some quadrupeds[1937]. It is to be observed that in the secondary folds of these wings the angles of the folds are surmounted by a nervure.
In both sections of theHemipteraOrder, as in theColeoptera, the Anal Area is turned under the wing and lies over the back of the insect; this is the onlyprimaryfold, but besides there are several longitudinal semifolds orsecondaryones, in which one part of the surface forms an obtuse angle with another; and inTettigonia, &c., these folds ramify in the wings as well as in thetegminaat themargin: a number of semifolds also, sometimes transverse and sometimes oblique, run in pairs from each side of every nervure of the disk of bothtegminaand wings in the genus last named, the use of which has been before mentioned[1938].
We now come to those Orders that havefourmembranous wings: first, I shall consider theLepidoptera. With respect to the position of their wings in repose some variations take place. In the majority of theday-fliers(PapilioL.), when the animal reposes the wings are applied to each other by their upper surface so as to bevertical; but in the skippers (Hesperia), the secondary wings assume ahorizontalposition, while the primary areverticalbut applied to each other. In theCrepusculartribes (SphinxL.) the upper wings are incumbent on the lower, and deflexed. In thenight-fliers(PhalænaL.) the types of position are various. In someAttacus,Saturnia,Noctua, &c., the wings cover each other, and are a little inclined from a horizontal position; inGastropacha,Odenesis, and some otherBombycidæ, they are deflexed, and the anterior margin of the under wing projects beyond that of the upper: in some of theTineæL., asCrambus, the wings are convoluted, and in others,Galleria, they are applied close to the sides of the body, and being elevated at the apex, terminate, to use a French term—en queue de coq: inNoctua,Geometra, &c., the wings usually cover the abdomen, and are nearly horizontal. With regard to thefoldsof their wings, the Anal Area of the secondary is the only part that has any striking one; inPapilio Hectorand affinities it turnsup so as to defend the sides and part of the back of the abdomen; inMorpho Teucerit turns down, and meeting that of the opposite wing, forms a semitube which receives and shelters that part below. In theCrepuscularandNocturnalLepidoptera this fold, especially in the former, is very slight. With respect to semifolds in theDiurnal, there is one originating in the disk, between each of the nervures, that goes to the margin of the wing; likewise the under wings, particularly of manyNoctuæ,Arctiæ, &c., have many longitudinal semifolds.
In theNeuropteraOrder several variations take place with regard to thepositionof these organs in repose: thus, inÆshna,Libellula, &c., they continue expanded; inArgionthey are applied to the body; inMyrmeleonthe upper are horizontally incumbent on the lower; inHemerobiusthey incline to the horizon. With regard to theirfoldsinÆshna, &c., the longitudinal nervures alternately form the summit or the bottom of a semifold, as do those branches that terminate in the posterior margin; this kind of plicature may be observed, but in a less degree, inAscalaphus,Myrmeleon, &c.; inPanorpaevery nervure is the ridge of a slight fold; inTermes, on the contrary, it forms its bottom. In theTrichoptera, the under wing being much more ample than the upper, the Anal Area forms a fold under the wing, and there seem longitudinal secondary folds besides.
We now come to theHymenoptera. In this Order the wings, as to their position in repose, are usually incumbent upon each other, and cover the abdomen; in theVespidæ, however, they are placed parallel to the body, but do not cover it. Before I notice the plicature of these wings, I must recall your attention to what I latelyobserved[1939]with regard to Jurine'sbullæ(bubbles), but which are really thejointsof the nervures, as they are to be found only where the folds pass; and where they exist they are an index by which the folds, or rather semifolds, may be traced. I counted eleven of these little joints in the upper wing ofAndrena cineraria; sometimes, however, instead of abulla, a nervure stops short to admit the fold. Wings in this Order have often three longitudinal semifolds more or less conspicuous; these you may trace in the saw-flies (TenthredoL.), whose wings Linné termstumidæ, by which term he would indicate the elevation of the whole surface produced by this structure; in the under wings of these, andScolia,Bembex, &c., the Anal Area is turned under the wing, as in many preceding tribes[1940]: inSirex, &c., that Area of the upper wing turns upwards, forming an acute angle with the rest of the organ; the same circumstance distinguishes the under wing in theIchneumonidæ. Several apical semifolds, marked by a pellucid streak, distinguishTiphiaF., and inBombus,Bembex, &c., an infinity of branching ones, like those before described inColeoptera, corrugate the apical margin. In theVespidætheupperwings are folded longitudinally intothreenearly equal portions, but in theunderones the Anal Area only forms the fold.
In theDipteraOrder, as to theirpositionwhen at rest, the wings are mostly incumbent one on the other; but inPsychodathey are deflexed, so as to form a kind of penthouse. With regard to theirplication, in some,Tipula oleracea, &c., a slight oblique semifold runs from thestigmato the apical margin, and the Anal Area has two, as it has in manyMuscidæ, itself forming nearly a right angle with the rest of the wing; besides these it is corrugated with minute transverse semifolds, which are observable also in several other Dipterous insects; in manyStratyomidæthey are oblique, and run from the disk to the posterior margin; and inAsilus,Bombylius, &c., they are wavy.
5. We are next to say something upon theshapeof wings: this, though apparently extremely various in the different Orders and tribes, may I think be traced in every wing to one original prototype, atrianglewith the largest angle rounded and subtended by the anterior or costal margin: in some, as theColeoptera,Orthoptera, &c., this type of formation is aright-angledtriangle[1941]; and in others, as in theHymenoptera,Diptera, &c., the majority of theNeuroptera, &c., it is anobtusangledone[1942]; it may be further observed, that in receding from these forms wings very often assume that of the half or quadrant of some regular figure, as we shall see when we consider those of the different Orders. Another general observation I shall first mention,—that these organs are universally narrowest at their base and widest at the apex, provided we consider as the apex the termination outwards of the three Areas; otherwise we might say that wings in theColeoptera,Orthoptera, &c., were wider at the base than at the apex[1943]. The wings in the former Order, and in several of the HeteropterousHemiptera, asGerris,Velia, &c., may in general, as to theirshape, be termed semicordate or semiovate[1944]; in theDermapterathey incline to an oval figure[1945]: in theStrepsiptera,Orthoptera, most Homopterous and many HeteropterousHemiptera, they approach to the quadrant of a circle; in a considerable portion of theLepidopterathe two under wings, if united at their posterior margin, approach a circular form; the upper ones vary a little from the prototype of the under ones, forming an obtusangled triangle[1946]; in manyNeuropteratheprimarywings may be called oblong or linear-oblong, while thesecondarybetray more evidently theright-angledorobtusangledtriangle; in theHymenopterathis latter form is every where conspicuous, with little deviation, except in the rounding of the angles[1947]; and, finally, in theDipterathis form shades off again into an oblong, ovate, or linear shape, the wing being most commonly attenuated at the base into a kind of footstalk[1948]. Some singular variations with respect to the termination or marginal processes of the wings are exhibited by manyLepidoptera; thus inAttacus Atlas, &c., the primary wings are falcated or hooked at their apex[1949]; and in great numbers both wings are there scolloped into alternate bays and capes, if I may so speak, varying in depth and length[1950]. There is usually a sinus between every pair of nervures, each of which terminates in the adjoining prominence, as a fold does in the sinus[1951]. Where present, in theprimarywings there areeightof these sinuses, and in thesecondary, where they are most usual,seven; some are remarkable for the longtailswhich distinguish their secondary wings; those inPapilioare usually an elongation of the fifth, fromthe anterior margin, of the prominences before mentioned, into a spathula-shaped diverging process, varying in length and width[1952]: but inP. Ulyssesit does not diverge; and inP. Podaliriusit is linear. They are found also in other subgenera; thus inUrania Patroclusthere aretwo; inU. Riphæusthree; inErycina Cupidofive; and inE. Endymionsixof these tails; in some, as inE. Dorylas, the whole wing seems to form the tail; in others again, as inHesperia ProteusandBombyx Luna, it is an elongation of the anal angle. Other wings in this Order are divided into lobes resembling feathers, as you may see inPterophorus hexadactylus, &c.[1953]
6. We are next to consider theclothingof wings: these, in the Orders in which they are covered by elytra, tegmina, or hemelytra, are generally naked, except that the spots in those ofFulgora laternaria,serrata, &c., and the whole wing inFlata,Aleyrodes, and others, are covered with a kind of farinaceous powder; but in all the remaining Orders, hairs or scales are more or less implanted in these organs: as theLepidopteraare the most remarkable for the clothing of their wings, I shall leave them till the last, and begin with theNeuroptera. If you lightly pass your finger over the wing of any dragon-fly (LibellulaF.,ÆshnaF.), from the apex towards the base, you will find that the longitudinal nervures are, as it were, serrulated with very minute bristles, which point towards the extremity; if you next move the finger across the wing, from the posterior to the anterior margin, a similar circumstance will strike you. M. Chabrier conjectures that, amongst other uses[1954], these hairs may contributeto fix the atmospheric fluid when the wings are depressed in flight, while it glides over them as they rise[1955]; inAscalaphus,Myrmeleon,Nemoptera,Hemerobius, &c., the nervures are more visibly bristled; the bristles diverging on each side from the longitudinal ones, but all pointing towards the apex from the connecting or transverse ones; inPanorpa, besides these bristles, short hairs, pointing the same way, are thickly planted in the membrane of the wing; and inHemerobiusthe margins of the wing are fringed; in theEphemerina,Corydalis, &c., the wings are naked. In theTrichopteraOrder, as their name imports, they are covered with minute decumbent hairs, less easily seen but still existing in the secondary pair. In theHymenopterain general the wings are covered with minute hairs or bristles; but inTiphia,Scolia—with the exception ofS. Radulaand affinities in which they are hairy—and others, the wings are nearly naked; inPompilus,Pepsis, &c., the hairs are infinitely numerous and very short; in theSphecidæ,Mutilla, &c., they are more distinct, longer, and less numerous; in the humble-bee (Bombus) and many others the apex of the wing is darkened by a large number of more conspicuous hairs, each of which seems to spring from a minute tubercle: as these tubercles are in a part of the wing that is strengthened by few nervures, they may probably be intended to supply their place, in giving firmness and tension to this part. The wings ofDiptera, under the present head, may be viewed with regard to the hairs that are implanted in themembraneof the wing, in itsnervures, and in itsmargin. In the first view, inStratyomisand immediate affinities the wing is nearly naked; but inXylophagus,Beris, and the great majority of the Order, the membrane of the wings is thickly planted with innumerable very minute bristles, not to be seen but under a powerful lens, often black, and seemingly crowning a little prominence, and giving the wing an appearance of the finest net-work. As to the clothing of the nervures, the costal, inAnthrax,Bombylius, &c., is often remarkably bristly at the base, with hairs intermixed; inŒstrus Ovis, in the inner margin or edge of this nervure, is a single series of bristles, or rather short spines, like so many black points; inŒ. Equithe wholecostais covered with short decumbent hairs or bristles; inMusca paganaF., just at the apex of the costal areolet, that nervure is armed with a spur or diverging bristle larger than the rest, which is also to be found in many others of theMuscidæ, some of which have two and others more of these spurs. The little moth-like midges (PsychodaLatr.,HirtæaF.) at first appear to have the whole surface of their wings covered with hairs; but upon a closer examination it will be seen that they are planted in the nervures, from each of which they diverge, so as under a lens to give it a very elegant appearance[1956]. This fly has its wings beautifully fringed with fine hairs, thethirdcircumstance to be attended to under this head; in the Tipulidans, and many others of this Order, the apex and posterior margin are also finely fringed with short hairs. Some Dipterous insects make a near approach to theLepidopterain thecoveringof their wings: in the common gnat, when the wings are not rubbed, the nervures are adorned by a double series ofscales, and themarginal fringe also consists of them[1957]; and in a Georgian genus, which appears in some degree to connectCulexwithAnthrax&c., there are scales scattered upon the membrane as well as upon the nervures; besides, its antennæ[1958]and abdomen are also covered with them.