30 THE SCOLYTID BEETLES. 



as here defined, are probably modifications of the episternal paraptera 

 of Audouin. 



Episternum. The episternum is the clearly defined, large, exposed 

 triangular pleurite situated between the posterior margin of the 

 preepisternum and the epimerum, with the dorsal angle dilated, pro- 

 duced, and flexed ventrally at the apex and with the apex of the 

 epimerum and preepisternal process forming the pleural claviculus 

 with its clavicle and coracoidal condyle for the articulation of the 

 elytra. The posterior angle of the episternum is acute, and the 

 suture between it and the sternum is obliquely sinuate. The epi- 

 sternal impression is clearly defined by the elevated posterior margin 

 of the preepisternum, and is usually covered by the posterior margin 

 of the prothorax. 



Epimerum. The epimerum is exposed at its posterior ventral 

 half and has its posterior margin fused with the metasternum and 

 metepisternum and the produced anterior dorsal angle with its 

 coracoidal condyle is covered by the episternum. 



Postepimerum. The postepimerum is represented by a small de- 

 clivous area beneath the posterior dorsal angle, where it covers the 

 metathoracic spiracle. 



MESOSTERNA. 



Presternum (fig. 18). The presternum is quite clearly represented 

 by the narrow, slightly elevated, anterior margin joined directly with 

 the anterior ventral angle of the preepisternum. 



Sternum. The sternum is short, flat, and subdeclivous, with the 

 posterior angle (exocoxal piece) extending around the coxal cavity to 

 its junction with the anterior angle of the metasternum. 



Sternellar area. The sternellar area appears to be represented by 

 the elevated and rather broad intercoxal piece, while the poststernel- 

 lum is apparently represented by a poststernellar piece. 



THE METATHORAX. 

 METATERGUM. 



From a systematic and taxonomic point of view, the metatergum 

 is by far the most important and interesting part of the thorax of 

 beetles. We find in it not only evidence of the four transverse 

 divisions, but examples of the possible extremes in modification to 

 meet the requirements of wing articulation and wing motion. 



Transverse sutures. By a comparison of the metatergum of repre- 

 sentatives of different orders of insects and of the larvae, pupae, and 

 adults of some insects, as in Dendroctonus, we find that the prevailing 

 principle of division involves three transverse external lines, sutures, 

 or impressions, and three corresponding entothoracic ridges, apo- 

 demes, or invaginated ectoderm, which define more or less clearly 

 the four divisions, viz, prescutum, scutum, scutellum, and postscu- 



