THE GENUS DENDROCTONUS. 29 



Prephragma. The prephragma is strongly flexed beneath the 

 median area, but the anterior arms, in conjunction with the anterior 

 angles of the prescutum, are prominent and strongly produced. 



Scutum. The oblique, impressed, triangular section situated 

 beneath the posterior margin of the prescutum evidently represents 

 the scutum. 



Scutellum. The hornlike process situated at the apical angle of 

 the prescutum apparently belongs to the scutellum and corresponds, 

 perhaps, to the structure which forms the scutellar groove of the 

 metathorax. This, with the posterior lateral section, represents the 

 median and lateral sections of the scutellum. 



PostscuteUum. The postscutellum apparently is not represented 

 by an external piece, but by an evaginated fold beneath the scutellum 

 and by the lateral arms of the postphragma (fig. 19). 



Postphragma. The postphragma is represented by the posterior 

 invagination or fold beneath the scutellum and by the lateral arms, 

 as evidenced by the attachment of the scutal muscle to the arm and 

 the connection of the arm with the invaginated phragma. 



MESOPLEURA. 



The episternum, preepisternum, epimerum, and postepimerum are 

 all represented and together occupy an area greater than that of the 

 sternal and slightly greater than that of the tergal. 



Preepisternum. The preepisternum occupies the area in front of 

 a transverse impression and is quite prominent. The anterior mar- 

 gin bears the preepisternal process (fig. 19, j), which is common to 

 most of the Khynchophora, and is more or less represented in certain 

 other Coleoptera, such as Carabidae, Cicindelidae, Scarabseidae, etc., 

 but it is surprising to find that it is not represented in Ips ( Tomicus) 

 and allied genera of the Scolytidae. It is peculiar to the mesothorax 

 and is progressively modified in character and function through 

 various groups of Coleoptera from an obscure accessory of the clavi- 

 cle disk to a prominent process. In Dendroctonus it is fused with 

 the anterior margin of the preepisternum and has an arm extending 

 to the dorsal angle to form an accessory to the wing process. It 

 bears a set of powerful muscles, which are attached to the inner wall 

 of the postscutellar and scutellar areas of the pronotum, thus form- 

 ing a powerful muscle connection between the two segments. The 

 mesothoracic spiracle is situated in the angle between this process 

 and the sternal area and is covered by the epicranial area of the 

 prothorax. 



Clavicle disk. The slender plate situated in front of the arm of 

 the preepisternal process appears to correspond with the clavicle 

 disk of the metapleurum. It is connected by a ligament to the head 

 of the elytra, and its muscle is attached to the rudimentary ante- 

 coxal piece. Both the preepisternal process and the clavicle plate, 



