The group Forficulidae seems to be most rich in species in warm and tropical regions; several unwinged species are met with in the mountainous districts of Europe; indeed, in some spots their individuals are extremely numerous under stones. In Britain we have a list of six species, but only two of these are to be met with; the others have probably been introduced by the agency of man, and it is doubtful whether more than one of these immigrants is actually naturalised here. One of these doubtfully native species is the fine Labidura riparia (Fig. 110), which was formerly found near Bournemouth. Altogether about 400 species of earwigs are known at the present time, and as they are usually much neglected by Insect collectors, it is certain that this number will be very largely increased, so that it would be a moderate estimate to put the number of existing species at about 2000 or 3000. None of them attain a very large size, Psalis americana being one of the largest and most robust of the family; a few display brilliant colours, and some exhibit a colour ornamentation of the surface; there are two or three species known that display a general resemblance to Insects of other Orders. The remarkable earwig represented in Fig. 102 (and which appears to be a nondescript form—either species or variety—closely allied to P. marmoricauda) was found by Baron von Hügel on the mountains of Java; the femora in this Insect have a broad face which is turned upwards instead of outwards, the legs taking a peculiar position; and it is curious that this exposed surface is ornamented with a pattern. The feature that most attracts attention on inspecting a collection of earwigs is, however, the forceps, and this is the most marked collective character of the group. These curious organs exhibit a very great variety; in some cases they are as long as the whole of the rest of the body, in others they are provided with tynes; sometimes they are quite asymmetrical, as in Anisolabis tasmanica (Fig. 113); in Opisthocosmia cervipyga, and many others they are curiously distorted in a variety of ways. The classification of the earwigs is still in a rudimentary state; the number of joints in the antennae, the form of the feet, and (in the terrestrial forms) the shape of the rudimentary wing-cases and wings being the characters that have been made most use of by systematists; no arrangement into sub-families or groups of greater importance than genera is adopted.
The only particulars we have as to the embryological development of the earwig are due to Heymons.[[144]] The forceps spring from the eleventh abdominal segment, and represent the cerci of other Orthoptera. An egg-tooth is found to be present on the head for piercing the egg-shell. The embryo reverses its curved position during the development, as other Orthoptera have been observed to do, but in a somewhat different manner, analogous to that of the Myriapods.
Several fossil Forficulidae are known; specimens belonging to a peculiar genus have been described from the Lower Lias of Aargau and from the Jurassic strata in Eastern Siberia, but the examples apparently are not in a very satisfactory state of preservation. In the Tertiary formations earwigs have been found more frequently. Scudder has described eleven species of one peculiar genus from the Lower Miocene beds at Florissant in Colorado; some of these specimens have been found with the wings expanded, and no doubt that they were fully developed Forficulidae can exist. The fossil species of earwigs as yet known do not display so remarkable a development of the forceps as existing forms do.
Fig. 113.—Anisolabis tasmanica ♂.
Brauer and others treat the Forficulidae as a separate Order of Insects—Dermaptera—but the only structural characters that can be pointed out as special to the group are the peculiar form of the tegmina and hind wings—which latter, as we have said on p. [206], are considered by some to be formed on essentially the same plan as those of other Orthoptera—the imbrication of the segments, and the forceps terminating the body. The development, so far as it is known, is that of the normal Orthoptera. Thus the Forficulidae are a very distinct division of Orthoptera, the characters that separate them being comparatively slight, though there are no intermediate forms. Some of those who treat the Dermaptera as a sub-Order equivalent to the rest of the divisions of the Order, call the latter combination Euorthoptera.
Fam. II. Hemimeridae.
Apterous, blind Insects with exserted head, having a constricted neck, mouth placed quite inferiorly; the thoracic sterna large, imbricate. Hind body elongate, the segments imbricate, the dorsal plates being large and overlapping the ventral; the number of visible segments being different according to sex: a pair of long unsegmented cerci at the extremity. Coxae small, widely separated. Development intra-uterine.
Fig. 114.—Hemimerus hanseni, female. Africa. (After Hansen.)