68 THE SCOLYTID BEETLES. 



Front of head grooved to convex. 

 Body spines small to coarse. 



Body simple, without chitinous plates or hairs, to distinct chitinous plates and more 

 prominent hairs. 



Eighth and ninth abdominal segments without chitinous plates to with plates, these 

 last unarmed to armed. 



Spiracles simple to complex, smooth to tuberculate. 



Epipleurites without tubercle, to prominent tubercle. 



GALLERIES. 



Long and winding to short and straight. 



Eggs isolated to grouped and massed. 



Larval mines hidden to exposed and short to long. 



From one genus to many genera, and from one species to many species. 

 D. brevicomis is found in pine only, while D. valens infests Pinus, Picea, and Larix; 

 D. simplex infests Larix only, and D. pseudotsugse infests Pseudotsuga and Larix. 



DISTINCTION OF MAJOR AND MINOR DIVISIONS OF THE GENUS. 



In a comparative study of the species of the genus to determine 

 their relative positions, as indicated by degrees of resemblance or dif- 

 ference, they are found to fall according to progressive modification of 

 characters into major and minor divisions, which may be designated as 

 divisions, subdivisions, sections, subsections, series, and subseries, to 

 the smallest practicable minor division of the genus, viz, the species. 



In this classification of the genus the rank of a primary division may 

 be that of the subgenus of some authors and the lower series of closely 

 alHed species may be recognized by some systematists as occupying 

 the rank of subspecies, races, or varieties; but the writer has been 

 guided by the belief that the principle of a less restricted range of 

 ■generic and more restricted range of S'pecific distinction will contribute 

 toward a more correct knowledge of the forms of life than if the 

 reverse principle is followed. 



The classification of the species of a genus into major and minor 

 divisions is necessarily arbitrary, and is subject to changes as may 

 be suggested by increased knowledge and the addition of species. 

 To a more limited extent, the designation of a species is arbitrary 

 and with additional material and information is subject to revision; 

 but since the species, next to the individual, is the constant or unit 

 of classification and investigation, it should represent the lowest prac- 

 tical division of a genus that is recognizable from a description of a 

 typical form or by comparison with the type on which the description 

 was based. 



