Fig. 103.—Lateral view of Forficula auricularia L. Female abdomen distended showing spiracles, S, and the small 8th and 9th dorsal plates (7 and 8 in Fig.).
The structure of the abdomen in the earwig has given rise to considerable discussion. In Fig. 103 we reproduce Westwood's diagram of it as seen fully distended in a female specimen; in this state the minute spiracles can be detected, though in the normal condition of the body they cannot be seen, being placed on the delicate membranes that connect the chitinous plates. Westwood's interpretation of the structure was not, however, quite correct, as the part which he considered to be the first dorsal plate is really the second; so that the segments numbered 7, 8, 9 in our figure are really 8, 9, 10. The common earwig is interesting as exhibiting, in an imperfect state, the union of the first dorsal plate of the abdomen with the thorax; a condition which is carried to so great an extent in the Hymenoptera as to quite obscure the nature of the parts, and which has consequently given rise to much perplexity and discussion. We represent this structure as seen in the common earwig in Fig. 104, where a represents the pronotum, b the mesonotum, c the metanotum, d the first, f the second abdominal segment; e being a delicate membrane of considerable size that intervenes between the two, and which is more exposed than are the corresponding membranes connecting the subsequent rings; a condition similar to that which is found in Cimbex, Cephus, and some other Hymenoptera.
Fig. 104.—Dorsal portions of the middle segments of body of Forficula auricularia (tegmina and wings removed).
On the under surface of the abdomen of the earwig the full number of 10 plates cannot be superficially distinguished; but it is found by dissection that in the female the short eighth and ninth dorsal rings are joined on the ventral aspect by a delicate membrane, while the tenth ventral is of a less delicate nature, and forms a triangular plate at the base of each half of the forceps. Between the branches of the forceps there is a perpendicular plate, the pygidium of Orthopterists, possibly the unpaired terminal portion of the body seen in some embryos, and called the telson. The pygidium is a separate sclerite, though it looks as if it were only a portion of the large tenth dorsal plate bent downwards, and in some descriptive works is erroneously described as being such.
Fig. 105.—Chelidura dilatata, male. Pyrenees.
Fig. 106.—Tegmina and wings (visible in part or invisible) of apterous earwigs. 1, Chelidura sp.; 2, Chelidura dilatata; 3, Anisolabis moesta; 4. A. maritima. a, First thoracic segment; b, second; c, third; d, basal portion of abdomen.