36 



THE SCOLYTID BEETLES. 



Abdominal Tergites. 



The eight abdominal tergites are normally covered by the elytra. 

 The apparent difference in the relative proportions, as indicated by 

 figures 3 and 22, is due to the flexible intersegmental membrane and 

 the fact that figure 20 is from a balsam mount. The integument 

 of 1 to 6, inclusive, is more or less membranous, while that of 7 and 8, 

 with the exception of the finely sulcate membranous lobes of 7, is 

 chitinous. In the female, 8 is covered by 7, and forms the so-called 



Fig. 23. — Dendroctonus valens: Male, abdominal tergites 7 and 8. ^,Tergite 8 (pygidium), ventral; £,same, 

 dorsal; C, same, ventral, showing position of stemite 8; D, tergite 7 (propygidium), dorsal; E, same, 

 ventral; a, anterior arm of epipleurite 7; 6, posterior arm of epipleurite 7; c, membranous lobe; d, 

 epipleural process; e, hypopleural arm of stemite 8; /, pleural opening; g, anal space; h, ventral fold; i, 

 rudimentary spiracle 8; j, epipleural disk; fe, apical spine; I, stridulating process; m, sensory bristles; 

 n, enlarged section of lateral area; o, enlarged section of membranous lobe; p, stridulating scraper. 

 (Original.) 



"pygidium," while in the male 8 is distinct and together with 7 forms 

 the so-called '^ divided pygidium." 



Pygal tergites of the male. — The pygal tergites of the male are shown 

 in figure 23, A, B, C, D, E. Tergite 7 is the propygidium and as a 

 bearer of generic and sexual characters is the most important of the 

 entire series. In the male the posterior margins between the epi- 

 pleurites converge toward the apex, which is produced into a bifid 

 process and supports the stridulating scrapers. The posterior area 

 of the tergite is thickened and strengthened to meet the requirements 

 of stridulation. There is a broad ventral fold (fig. 23 E, K) of the 

 integument which may serve a similar function to that of a sounding 

 board. 



