THE GENUS DENDEOCTONUS. 67 



PROGRESSIVE MODIFICATIONS. 



The writer has been forcibly impressed with the prevaiUng princi- 

 ple of progressive modification in relative proportions in form and 

 structural details in scolytid and other beetles. Whenever these 

 modifications in relative proportions are available for the statistical 

 method of analysis it is often possible to express in numbers the dif- 

 ference between species and to indicate clearly the lines of modification 

 and rates of departure among the species of a genus or larger group. 



There are some good examples of this principle of progressive modi- 

 fication in the genus Dendroctonus, which is manifested not alone in 

 the adults, but in the pupse, larvae, and character of work, and it is 

 most interesting and significant to note that the modifications are in 

 the same general direction in all cases. When the species are ar- 

 ranged in the order indicated by these modifications and other char- 

 acters, the species of the first division to the last of those of the 

 second division are found to be modified from small to larger size, 

 the extremes being represented by D. frontalis, with the minimum 

 length of 2.5 mm., to D. valens, with the maximum length of 9 

 mm. Naturally we find the same rate of difference in size of the 

 immature stages and galleries. This same tendency toward increased 

 size is manifested within eacli subdivision, section, or minor group of 

 allied forms and appears to be a prevailing principle throughout the 

 Scolytidse, and thus serves, in connection with other lines of modifica- 

 tion, as one of the first guides to a natural arrangement or classifica- 

 tion of the species. In Dendroctonus the progressive modification of 

 characters other than size is shown or indicated as follows: 



Progressive Modification of Characters in the Genus Dendroctonus. 



Primary characters. 

 Body slender to stouter. 

 Head large to smaller. 

 Prothorax long to shorter. 



Pronotum with sides nearly parallel to distinctly narrowed or constricted anteriorly. 

 Pronotum as broad as elytra to narrower. 



(A mean composite ratio of the above gives a number which expresses the relative 

 proportions and serves as a species index.) 



Front grooved and tuberculate to convex and smooth. 



Elytra without long hairs to long hairs over entire surface. ' ■ 



Tibia from slender to broader with a tendency to dilate toward the apex. 

 Funiculus of antenna with second joint long to shorter. 



Secondary sexual cliaracters. 



Front of head with sexual differences to similar or alike in both sexes. 

 Pronotum with sexual differences to alike in both sexes. 

 Elytral declivity without sexual differences to distinct differences. 

 Declivity rugosities small to coarse, smooth in female to coarse in male or reversed. 

 Mandibles alike or similar in both sexes, to much stouter in the male. 

 79980—09 6 



