In the Dermaptera[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 the Strepsiptera the folds are only longitudinal; a circumstance which, besides the form and neuration of the wing, sufficiently attests that its station is more near the Orthoptera and Coleoptera than the Diptera. We next come to the Orthoptera[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 of Blatta: 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, as Phasma, &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 the tegmina[1935]. In Gryllus Latr., 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 animal Gryllus 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 the Hemiptera Order, as in the Coleoptera, the Anal Area is turned under the wing and lies over the back of the insect; this is the only primary fold, but besides there are several longitudinal semifolds or secondary ones, in which one part of the surface forms an obtuse angle with another; and in Tettigonia, &c., these folds ramify in the wings as well as in the tegmina at the margin: a number of semifolds also, sometimes transverse and sometimes oblique, run in pairs from each side of every nervure of the disk of both tegmina and wings in the genus last named, the use of which has been before mentioned[1938].
We now come to those Orders that have four membranous wings: first, I shall consider the Lepidoptera. With respect to the position of their wings in repose some variations take place. In the majority of the day-fliers (Papilio L.), when the animal reposes the wings are applied to each other by their upper surface so as to be vertical; but in the skippers (Hesperia), the secondary wings assume a horizontal position, while the primary are vertical but applied to each other. In the Crepuscular tribes (Sphinx L.) the upper wings are incumbent on the lower, and deflexed. In the night-fliers (Phalæna L.) the types of position are various. In some Attacus, Saturnia, Noctua, &c., the wings cover each other, and are a little inclined from a horizontal position; in Gastropacha, Odenesis, and some other Bombycidæ, they are deflexed, and the anterior margin of the under wing projects beyond that of the upper: in some of the Tineæ L., as Crambus, 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: in Noctua, Geometra, &c., the wings usually cover the abdomen, and are nearly horizontal. With regard to the folds of their wings, the Anal Area of the secondary is the only part that has any striking one; in Papilio Hector and affinities it turns up so as to defend the sides and part of the back of the abdomen; in Morpho Teucer it turns down, and meeting that of the opposite wing, forms a semitube which receives and shelters that part below. In the Crepuscular and Nocturnal Lepidoptera this fold, especially in the former, is very slight. With respect to semifolds in the Diurnal, 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 many Noctuæ, Arctiæ, &c., have many longitudinal semifolds.
In the Neuroptera Order several variations take place with regard to the position of these organs in repose: thus, in Æshna, Libellula, &c., they continue expanded; in Argion they are applied to the body; in Myrmeleon the upper are horizontally incumbent on the lower; in Hemerobius they incline to the horizon. With regard to their folds in Æshna, &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, in Ascalaphus, Myrmeleon, &c.; in Panorpa every nervure is the ridge of a slight fold; in Termes, on the contrary, it forms its bottom. In the Trichoptera, 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 the Hymenoptera. In this Order the wings, as to their position in repose, are usually incumbent upon each other, and cover the abdomen; in the Vespidæ, 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 lately observed[1939] with regard to Jurine's bullæ (bubbles), but which are really the joints of 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 of Andrena cineraria; sometimes, however, instead of a bulla, 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 (Tenthredo L.), whose wings Linné terms tumidæ, by which term he would indicate the elevation of the whole surface produced by this structure; in the under wings of these, and Scolia, Bembex, &c., the Anal Area is turned under the wing, as in many preceding tribes[1940]: in Sirex, &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 the Ichneumonidæ. Several apical semifolds, marked by a pellucid streak, distinguish Tiphia F., and in Bombus, Bembex, &c., an infinity of branching ones, like those before described in Coleoptera, corrugate the apical margin. In the Vespidæ the upper wings are folded longitudinally into three nearly equal portions, but in the under ones the Anal Area only forms the fold.
In the Diptera Order, as to their position when at rest, the wings are mostly incumbent one on the other; but in Psychoda they are deflexed, so as to form a kind of penthouse. With regard to their plication, in some, Tipula oleracea, &c., a slight oblique semifold runs from the stigma to the apical margin, and the Anal Area has two, as it has in many Muscidæ, 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 many Stratyomidæ they are oblique, and run from the disk to the posterior margin; and in Asilus, Bombylius, &c., they are wavy.
5. We are next to say something upon the shape of wings: this, though apparently extremely various in the different Orders and tribes, may I think be traced in every wing to one original prototype, a triangle with the largest angle rounded and subtended by the anterior or costal margin: in some, as the Coleoptera, Orthoptera, &c., this type of formation is a right-angled triangle[1941]; and in others, as in the Hymenoptera, Diptera, &c., the majority of the Neuroptera, &c., it is an obtusangled one[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 the Coleoptera, Orthoptera, &c., were wider at the base than at the apex[1943]. The wings in the former Order, and in several of the Heteropterous Hemiptera, as Gerris, Velia, &c., may in general, as to their shape, be termed semicordate or semiovate[1944]; in the Dermaptera they incline to an oval figure[1945]: in the Strepsiptera, Orthoptera, most Homopterous and many Heteropterous Hemiptera, they approach to the quadrant of a circle; in a considerable portion of the Lepidoptera the 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 many Neuroptera the primary wings may be called oblong or linear-oblong, while the secondary betray more evidently the right-angled or obtusangled triangle; in the Hymenoptera this latter form is every where conspicuous, with little deviation, except in the rounding of the angles[1947]; and, finally, in the Diptera this 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 many Lepidoptera; thus in Attacus 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 the primary wings there are eight of these sinuses, and in the secondary, where they are most usual, seven; some are remarkable for the long tails which distinguish their secondary wings; those in Papilio are usually an elongation of the fifth, from the anterior margin, of the prominences before mentioned, into a spathula-shaped diverging process, varying in length and width[1952]: but in P. Ulysses it does not diverge; and in P. Podalirius it is linear. They are found also in other subgenera; thus in Urania Patroclus there are two; in U. Riphæus three; in Erycina Cupido five; and in E. Endymion six of these tails; in some, as in E. Dorylas, the whole wing seems to form the tail; in others again, as in Hesperia Proteus and Bombyx Luna, it is an elongation of the anal angle. Other wings in this Order are divided into lobes resembling feathers, as you may see in Pterophorus hexadactylus, &c.[1953]
6. We are next to consider the clothing of wings: these, in the Orders in which they are covered by elytra, tegmina, or hemelytra, are generally naked, except that the spots in those of Fulgora laternaria, serrata, &c., and the whole wing in Flata, 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 the Lepidoptera are the most remarkable for the clothing of their wings, I shall leave them till the last, and begin with the Neuroptera. If you lightly pass your finger over the wing of any dragon-fly (Libellula F., Æshna F.), 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 contribute to fix the atmospheric fluid when the wings are depressed in flight, while it glides over them as they rise[1955]; in Ascalaphus, 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; in Panorpa, besides these bristles, short hairs, pointing the same way, are thickly planted in the membrane of the wing; and in Hemerobius the margins of the wing are fringed; in the Ephemerina, Corydalis, &c., the wings are naked. In the Trichoptera Order, as their name imports, they are covered with minute decumbent hairs, less easily seen but still existing in the secondary pair. In the Hymenoptera in general the wings are covered with minute hairs or bristles; but in Tiphia, Scolia—with the exception of S. Radula and affinities in which they are hairy—and others, the wings are nearly naked; in Pompilus, Pepsis, &c., the hairs are infinitely numerous and very short; in the Sphecidæ, 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 of Diptera, under the present head, may be viewed with regard to the hairs that are implanted in the membrane of the wing, in its nervures, and in its margin. In the first view, in Stratyomis and immediate affinities the wing is nearly naked; but in Xylophagus, 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, in Anthrax, Bombylius, &c., is often remarkably bristly at the base, with hairs intermixed; in Œstrus 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; in Œ. Equi the whole costa is covered with short decumbent hairs or bristles; in Musca pagana F., 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 the Muscidæ, some of which have two and others more of these spurs. The little moth-like midges (Psychoda Latr., Hirtæa F.) 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, the third circumstance 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 the Lepidoptera in the covering of their wings: in the common gnat, when the wings are not rubbed, the nervures are adorned by a double series of scales, and the marginal fringe also consists of them[1957]; and in a Georgian genus, which appears in some degree to connect Culex with Anthrax &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.
The Order, the clothing of whose organs of flight excites the admiration of the most incurious beholder, is that to which the excursive butterfly belongs, the Lepidoptera. The gorgeous wings of these universal favourites, as well as those of the hawk-moths and moths, owe all their beauty, not to the substance of which they are composed, but to an infinite number of little plumes or scales so thickly planted in their upper and under surface, as in the great majority entirely to conceal that substance. Whether these are really most analogous to plumes or scales has been thought doubtful. De Geer is inclined to think, from their terminating at their lower end in little quills and other circumstances, that they resemble feathers as much as scales[1959]; Reaumur on the contrary suspects that they come nearer to scales[1960]. Their substance, approaching to membrane, seems to make further for the former opinion, and their shape and the indentations that often occur in their extremity, furnish an additional argument for the latter. Their numbers are infinite; Leeuwenhoek found more than 400,000 on the wings of the silk-worm moth (Bombyx Mori)[1961]; and in those of some of the larger moths and butterflies the number must greatly exceed this. You will observe however that in many Lepidoptera the wings are partially, and in some instances generally, transparent: thus in Hesperia Proteus, a butterfly before noticed for the long tail that distinguishes its secondary wings, there are many transparent spots; in Attacus Atlas, one of the largest of moths, and its affinities, there is as it were a window in each wing formed by a transparent triangular space; in A. Polyphemus, Paphia, &c., the pupil of the ocellus is transparent, which in the former is divided by a nervure. In several of the Heliconian butterflies, and in Zygæna F., &c., the greater part of both wings is transparent, with scales only upon their nervures, round their margin, or forming certain bands or spots upon them; in Parnassius Apollo, Mnemosyne, &c., the scales are so arranged as not wholly to cover the wings, which renders them semidiaphanous; and in some (Nudaria) the wings are intirely denuded. With regard to size, the scales vary often considerably in different tribes; in Heliconia they appear to be more minute than in the rest; and in Castnia they are the largest and coarsest; the extremity of the wings of Lepidopterous insects in general is fringed with longer scales than their surfaces, and even those of the last in the same wing; sometimes vary in magnitude. The little seeming tooth that projects from the middle of the posterior margin in the upper wings of Notodonta, a subgenus of Bombyx L., is merely produced by some longer diverging hairs. The shape and figure also of scales are very various—some being long and slender; others short and broad; some nearly round; others oval, ovate, or oblong; others spathulate; others panduriform or parabolical; some again almost square or rhomboidal; many triangular; some representing an isosceles triangle, and others an equilateral one; lastly, some are lanceolate and others linear; again, some have a very short pedicle and others a very long one: with regard to their extremity; some are intire, without projecting points or incisions, while others are furnished with them: of these some terminate in a single long mucro, others have several shorter ones; some are armed with teeth, varying in number from two to thirteen in different species[1962]. Many other forms might be enumerated, but these are sufficient to give you a general notion of the infinite variety of this part of the works of the Creator. I must next say a word or two upon their arrangement on the wing. In most instances this is in transverse lines, which sometimes vary a little from a rectilinear course, and the extremity of the scales of one row reposes on the base of those of the succeeding one, so that in this respect their arrangement is like that of tiles in a roof: in some cases it is not so regular: thus the minute scales on the wings of Parnassius Apollo, and others with subdiaphanous wings, are arranged without order; in Pieris and other Diurnal Lepidoptera, and many of the Crepuscular and Nocturnal, there appears to be a double layer of scales on both sides of the wing; the under layer usually consisting of white ones. If you denude the wings of any butterfly, which you may easily do by scraping it lightly on both sides with a penknife, you will be amused to trace the lines in which the scales were planted, consisting of innumerable minute dots: the lines of the under side, in some cases, so cut those of the upper side, as by their intersection to form lozenges. With regard to the position of the scales on the wing, they usually lie flat, but sometimes their extremity is incurved: in the beautiful Argynnis Vanillæ a very singular appearance of numerous transverse ridges is produced by the extremity of those scales that cover the longitudinal nervures of the primary wings, except at the base, being recurved.
But though the general clothing of the wings of Lepidoptera consists of these little scales, yet in some cases they are either replaced by hairs or mixed with them. Thus, in the clear parts of the wings of Heliconians, Attaci, &c., short inconspicuous hairs are planted; in a large number of the Orders the upper side of the Anal Area of the secondary wings is hairy; in several Crepusculars (Sphinx Phœnix, &c.), where there is a double layer as before mentioned, the upper one consists of dense hairs, except at the apex, and the lower one of scales; and in most of them the scales of the primary wings are piliform, and the secondary are covered by what approach very near to real hairs; many of the Attaci are similarly circumstanced: the four wings of A. Cytherea are also covered externally with hair.