A very large number of species of Forficulidae have the organs of flight undeveloped. Fig. 105 represents Chelidura dilatata, an apterous form that is very common in the Eastern Pyrenees. The condition of the meso- and meta-nota—the parts from which the tegmina and wings are developed, and to which they are attached when present—is very remarkable in these forms, and exhibits much variety. In Fig. 106 we represent the conditions of these parts in a few apterous forms. The tegmina or the segment from which they are developed (b), are seen in the shape of a plate which may extend all across the middle and be undivided (No. 4); in which case the appearance indicates entire absence of the tegmina; these are, on the contrary, evidently present in the form of slips grafted one to each side of the second thoracic segment in Anisolabis (No. 3); or they may look like short broad slips extending all across the body, and marking off a piece frequently called a scutellum, but which is really the mesonotum (some species of Chelidura, as No. 2); or, again, they may be nearly free tegmina, somewhat similar to those of the winged forms; this is the case with some species of Chelidura, as represented by No. 1. This last figure is taken from a species from the Sierra Nevada, apparently undescribed, allied to C. bolivari.
In the cases we are considering no analogous structures exist on the metanotum (the part of the body that in the winged forms bears the wings, and which is marked c in our diagrams, Fig. 106), so that the tegmina are to all appearance less rudimentary (or vestigial) than the wings. The metanotum forms a sort of flap, called by Fischer[[129]] "involucrum alarum"; he considered the part immediately behind this to be the metanotum; this piece is, however, no doubt really part of the abdomen (d in our Figure). This is apparently the view taken by Brunner.[[130]] The structure of these parts is important as bearing on the subject of the nature and origin of Insects' wings, a question to which no satisfactory answer has yet been given. The appearances we have remarked on are to some extent similar to the conditions existing in the immature state of the organs of flight in the common earwig (see Fig. 112, p. [212]), but whether the varieties presented by the wingless forms have parallels in the immature conditions of the various winged forms is quite uncertain, the life-histories of earwigs being almost unknown.
Fig. 107.—Wing of Forficula auricularia. A, Wing expanded, explanation in text; B, wing folded and packed.
The developed wings of earwigs are worthy of attention, both as regards their actual structure and the manner in which they are folded up in repose. When expanded they have a shape curiously suggestive of the human ear. The chief parts of the wing, as shown in Fig. 107, A, are a, b, two portions of the horny piece that forms the scale which covers the more delicate parts of the wing when it is folded, and which, according to Brunner, represents the radial and ulnar fields of the wings of Acridiidae and Locustidae (see Fig. 167); c is the small apical field limited below by the vena dividens; d is the vena plicata which runs along the under side of the scale as far as the apical field, where it gives off the axillary nerves; e is a vena spuria, or adventitious vein such as exists in many other Orthoptera with delicate wings. On the front part of the scale, a, and on a different plane so that it is not shown in our figure, there is a very delicate small band which is supposed to represent the marginal field of the wing of other Orthoptera. There are, however, grave difficulties in the way of accepting this view of the earwig's wing, amongst which we may mention the position of the vena dividens and its relation to the so-called radial and ulnar fields of the wing. The wings are remarkable for their delicacy; moreover, the way in which they fold up so as to be packed in the manner shown in B, Fig. 107, is very interesting, there being, in fact, no other Insects that fold up their wings in so complicated and compact a fashion as the earwigs do. The process is carried out somewhat as follows: the longer radii come a little nearer together, the delicate membrane between them falling into folds somewhat like those of a paper fan; a transverse fold, or turn-over, then occurs at the point where the radii, or axillary nerves, start from the vena plicata; then a second transverse fold, but in a reversed direction, occurs affecting the wing just close to the spots where the shorter radial nervures are dilated; then by a contraction close to the scale the whole series of complex folds and double are brought together and compressed.
It is quite a mystery why earwigs should fold their wings in this complex manner, and it is still more remarkable that the Insects very rarely use them. Indeed, though Forficula auricularia is scarcely surpassed in numbers by any British Insect, yet it is rarely seen on the wing; it is probable that the majority of the individuals of this species may never make use of their organs of flight or go through the complex process of unfolding and folding them. It should be remarked that no part of the delicate membranous expanse of the wing is exposed when the wings are packed in their position of repose; for the portion that projects from under the tegmina—and which, it will be remembered, is always present, for when wings exist in earwigs they are never entirely concealed by the tegmina—is, it is curious to note, of hard texture, and is frequently coloured and sculptured in harmony with the tegmen; in fact, one small part of the wing forms in colour and texture a most striking contrast to the rest of the organ, but agrees in these respects with the wing-covers. This condition is seen in Fig. 108, where B shows the sculpture of the tegmina t, and of the projecting tips of the wings w. There are numerous other instances in Orthoptera where one part of a wing or wing-case is exposed and the other part concealed, and the exposed portion is totally different in colour and texture from the concealed portion.
The wings of earwigs are attached to the body in a very unusual manner; each wing is continued inwards on the upper surface of the metanotum, as if it were a layer of the integument meeting its fellow on the mesial line; the point of contact forming two angles just behind the metanotum.
Some writers have considered that the tegmina of earwigs are not the homologues of those of other Orthoptera, but are really tegulae (cf. Fig. 56, p. [103]). We are not aware that any direct evidence has been produced in support of this view.
The pair of forceps with which the body is armed at its extremity forms another character almost peculiar to the earwigs, but which exists in the genus Japyx of the Thysanura. These forceps vary much in the different genera of the family; they sometimes attain a large size and assume very extraordinary and distorted shapes. They are occasionally used by the Insects as a means of completing the process of packing up the wings, but in many species it is not probable that they can be used for this purpose, because their great size and peculiarly distorted forms render them unsuitable for assisting in a delicate process of arrangement; they are, too, always present in the wingless forms of the family. Their importance to the creature is at present quite obscure; we can only compare them with the horns of Lamellicorn Coleoptera, which have hitherto proved inexplicable so far as utility is concerned. No doubt the callipers of the earwigs give them an imposing appearance, and may be of some little advantage on this account; they are not known to be used as offensive instruments for fighting, but they are occasionally brought into play for purposes of defence, the creatures using them for the infliction of nips, which, however, are by no means of a formidable character.
