178 THE SCOLYTID BEETLES. 



questions of distinction and definition and should be considered in all 

 detailed studies of single genera and allied groups of genera. The 

 scutellum has been extensively used by systematists, and a detailed 

 comparative study of this element in a wide range of genera and 

 species should be made to determine its real significance in taxonomy. 

 The pleurum (Part I, fig. 19) is also exceedingly variable in the form 

 and sculpture of the epimeron, episternum, and preepisternum, while 

 the remarkable structure designated as the preepisternal process 

 (Part I, p. 29) is of special taxonomic importance since its presence 

 or absence is peculiar not only to groups and divisions of the Scoly- 

 toidea but to many other groups of Coleoptera, and evidently represents 

 an extreme in progressive modification which, in connection with 

 other progressively modified elements, will doubtless serve as an index 

 to the systematic position of genera and species. The elements of 

 the sterna (Part I, fig. 18) are also variable, but, like the prosterna, 

 their modification conforms to the variable form of the body, and they 

 are of less importance than the more independent structures like the 

 scutellum and preepisternal process. 



METATHORAX. 



The metathorax (Part I, figs. 20, 21) is quite variable in general 

 characters and especially so in some of the elements of the tergum, 

 probably due to the variable form cf the body and the requirements 

 of flight. There is considerable variation of this element within the 

 species of a genus and often there are wide differences m allied genera. 

 Perhaps the element subject to the greatest variation is the postscutel- 

 kim, which ranges from obscure or rudimentary to almost the length 

 of the combined anterior elements. The scutellar groove, the trans- 

 verse sutures, and the entothoracic ridges or apodemes are also quite 

 variable. The writer has examined the metatergum of quite a large 

 number of species and it would appear that there may be in it some 

 important characters peculiar to minor groups of genera, but the 

 taxonomic value of the va,riations is largely lunited to the species. 

 It would therefore be difficult to trace correlated lines of progressive 

 modiflfcation. Nevertheless, a special comparative study of the 

 metathorax should be made of a very large number of examples repre- 

 senting all of the genera to determine whether or not there are special 

 taxonomic elements or recognizable lines of modification. The epi- 

 sternum (Part I, fig. 20) is quite variable m length, width, sculpture, 

 and vestiture and is of considerable taxonomic importance. The 

 metasterna are also variable in conformity Math the variable form of 

 the body. The sternum always occupies the greater part or nearly 

 all of the area. The greatest range of variation between the elements 

 of the metathorax is to be found in the short and broad forms of the 

 stouter species of the Ipidse and Scolytidse and the exceedingly long 

 and narrow forms peculiar to the Platypodidae. 



