10 THE SCOLYTTD BEETLES. 



ILLUSTRATIONS. 



The figures are intended to be sufficiently complete to leave little 

 to be added in the way of description, except to emphasize and 

 elucidate some of the more important features, or to call attention 

 to the variation within the genus or species. 



EXTERNAL CHARACTERS OF THE IMAGO. 



The structural details and general external anatomy and sculpture 

 are shown in figures 1, 2, and 3. The principal characters peculiar 

 to the genus are found in the large, prominent head, the epistomal 

 process (figs. 2, 3, 4, 6, 10) (referred to by Dietz as the median seg- 

 ment of the epistoma), the form of the antenna (figs. 11, 12, 13), 

 the approximate or subcontiguous anterior coxas (fig. 2), and the 

 strongly recurved hypopleural sutures 4, 5, and 6 of the abdominal 

 sternites (fig. 25). 



Length and relative proportions. — The length of the imago ranges 

 from 2.5 mm. in D. frontalis to 9 mm. in D. valens. There is con- 

 siderable range in length within the limits of some of the species, 

 while in others the length is more constant. The relative propor- 

 tions of the width of the head, width and length of the prothorax, 

 width and length of the elytra, or a composite of the ratios, serve 

 as a taxonomic index for the classification of the species, and, together 

 with other characters, serve to distinguish the major and minor 

 divisions and, to a certain extent, the species. The progressive 

 modification appears to be from a head nearly as broad as the pro- 

 notum and the latter as broad as the elytra, with the sides nearly 

 parallel, to a head much narrower than the pronotum, the latter 

 slightly narrower than the elytra, with the sides narrowed and con- 

 stricted toward the head; also, from a slender, subelongate, to a 

 stout body. 



Color. — The color ranges from pale yellowish-red to brown and 

 deep black, but is fairly constant in the matured individuals of a 

 species. The immature individuals are always lighter, and some of 

 those of the black species are reddish. In some species the head, 

 prothorax, and ventral surface of the body are darker than the 

 elytra, while in others little or no difference is noticeable. 



Vestiture. — The body is more or less clothed with short to long 

 hairs, the presence or absence of which on different areas is of far 

 more taxonomic significance than was at first recognized. Except 

 in old rubbed specimens, the vestiture serves as one of the important 

 characters distmguishing the major, as well as some of the minor, divis- 

 ions. See synopsis. Divisions I and II, sections al and a2 (PI. I) . 



Sculpture. — Within the genus and also within each species there 

 is considerable variation in the sculpture of the front, pronotum, 

 and elytra. Nevertheless, such characters as the presence or absence 



