8 INTRODUCTIOIf. 



depressed, with a transverse elliptical cross-section ; the sides- 

 may be parallel (Labidwa, Elaunon), or gently dilated about the 

 middle {Anisolahis and certain species of Forjicula) ; in the 

 Apacliyklce and in Platylabia the body is remarkably depressed, 

 being as flat as cardboard. In tlie female the abdomen almost in- 

 variably tapers somewhat towards the hinder end. There are in all 

 ten segments ; excluding the last, there are nine, but in the female- 

 only seven are visible, as the eighth and the ninth are aborted and 

 reduced to mere specks of chitin which can only be detected upon 

 dissection. 



In Forcijmla the sides of certain segments are furnished with 

 ridges, keels or long spines ; in certain species of Psalince the 

 sides of the segments are produced posteriorly to a more or less 

 acute angle and are generally striated or furrowed. This struc- 

 ture affords useful specific characters. In Eparchus the sides of 

 the abdomen are studded with a series of knobs or tubercles. In the 

 Lahidtirince the posterior margins of the segments are milled. All 

 these structures are almost or entirely undeveloped in the females. 

 In many genera, as in the Forjiculidie, the Labiidce, and others,, 

 the second and third abdominal segments have at the sides 

 tubercular folds in the integument, which are stink-glands. The 

 segments are closely imbricated into each other ; when the abdomen 

 is distended, the membrane holding the chitinous plates together 

 may be seen with the minute perforation of the tracheal pores. 

 The texture of the chitin varies from smooth to granular. The 

 ventral surface is flat, and generally smoother and paler than the 

 dorsal surface. 



The last dorsal plate, or sdcrite, is large and ample, and as it 

 presents a great variety of form and of armature it offers valuable 

 discriminative characters ; it is invariably more simple and almost 

 always narrower in the female than in the male. 



The ■penidtimate ventral sderite is very large, so that the last 

 ventral sderite is almost or entirely hidden ; w hen disclosed by 

 the removal of the penultimate it is shown to be separated into 

 two portions by a median fissure so as to permit the passage of 

 the fseces and the reproductive cells. The outline of the penulti- 

 mate segment is usually slightly different in the two sexes and 

 offers useful generic and specific characters. 



The pygidium is a chitinous organ existing between the roots of 

 the forceps and the edges of the apical abdominal segments ; it is 

 sometimes hidden, but is as frequently protruded. It assumes 

 a great variety of shapes in both sexes, thus offering very useful 

 specific characters. 



In the Apadiyidce the last abdominal segment is produced 

 without any apparent suture or joint into a depressed lobe termed 

 the anal process ; as it appears to be a mere extension of the anal 

 segment, it is probably not homologous with the pygidium, which 

 is a distinct organ. 



The forcejys are the most characteristic organs of earwigs. They 

 consist of a pair of more or less elongate, hard unsegmented 



