Fig. 108.—Anechura scabriuscula. Himalaya. A, Outline of the Insect; B, tegmina, t, and tips of wings, w, showing their similar sculpture.
These forceps are, in the case of the common earwig—and they have not been studied from this point of view in any other species—remarkable, because of the great variation in their development in the male, a character which again reminds us of the horns of Lamellicorn beetles: in the female they are comparatively invariable, as is also the case in the few species of Lamellicornia, which possess horned females. A and B in Figure 109 represent the forceps of different males of the common earwig, C showing those of the other sex. The subject of the variation of the male callipers of the earwig has been considered by Messrs. Bateson and Brindley,[[131]] who examined 1000 specimens captured on the same day on one of the Farne islands off the coast of Northumberland; 583 of these were mature males, and the pincers were found to vary in length from about 2½ mm. to 9 mm. (A and B in Fig. 109 represent two of the more extreme forms of this set of individuals.) Specimens of medium size were not, as it might perhaps have been expected they would be, the most common; there were, in fact, only about 12 individuals having the forceps of the medium length—4¾ to 5¼ mm., while there were no less than 90 individuals having forceps of a length of about 7 mm., and 120 with a length of from 2¾ to 3¼. Males with a medium large length of the organ and with a medium small length thereof were the most abundant, so that a sort of dimorphism was found to exist. Similar relations were detected in the length of the horns of the male of a Lamellicorn beetle examined by these gentlemen. In the case of the set of earwigs we have mentioned, very little variation existed in the length of the forceps in the female sex.
Fig. 109.—Forceps of the common earwig: A, of large male; B, of small male; C, of female.
In many earwigs—including F. auricularia—there may be seen on each side of the dorsal aspect of the true fourth, or of the fourth and neighbouring segments of the hind body a small elevation, called by systematists a plica or fold, and on examination the fold will be found to possess a small orifice on its posterior aspect. These folds are shown in Figs. 105 and 108; they have been made use of for purposes of classification, though no functional importance was attached to them. Meinert, however, discovered[[132]] that there are foetid glands in this situation, and Vosseler has recently shown[[133]] that the folds are connected with scent-glands, from which proceed, in all probability, the peculiar odour that is sometimes given off by the earwig. The forms destitute of the folds, e.g. Labidura, are considered to have no scent glands. There is a very peculiar series of smooth marks in the earwigs on the dorsal aspect of the abdominal segments, and these are present in the glandless forms as well as in the others.
The internal anatomy has been to some extent investigated by Dufour and Meinert. Dufour dissected F. auricularia and Labidura riparia, and found[[134]] that salivary glands exist in the latter Insect (called by him Forficula gigantea), though he was unable to discover them in the common earwig. According to Meinert,[[135]] there are, however, salivary glands affixed to the stipes of the maxillae in F. auricularia, while (in addition?) L. riparia possesses very elongate glands seated in the middle or posterior part of the breast. The alimentary canal is destitute of convolutions, but oesophagus, crop, and gizzard all exist, and the intestine behind the stomach consists of three divisions. The Malpighian tubes are numerous, 30 or 40, and elongate. The respiratory system is not highly developed. Earwigs—the European species at least—have, as already mentioned, very small powers of flight; the tracheal system is correspondingly small, and is destitute of the vesicular dilatations that are so remarkable in the migratory Locusts.
Fig. 110.—Labidura riparia, male. Europe.
The three thoracic spiracles[[136]] are readily observed in living individuals. There are seven pairs of abdominal spiracles, which, however, are very minute, and can only be found by distending the body as shown in Fig. 103. The ventral chain consists of nine ganglia (the sub-oesophageal centre is not alluded to by Dufour); the three thoracic are equidistant and rather small; the hindmost of the six abdominal ganglia is considerably larger than any one of the other five.
The ovaries of Labidura riparia and Forficula auricularia are extremely different. In L. riparia there are on each side five tubes, each terminating separately in an obliquely directed lateral part of the oviduct. In F. auricularia there is but one tube on each side, but it is covered by three longitudinal series of very short sub-sessile, grape-like bodies, each of the two tubes being much dilated behind the point where these bodies cease.