THE GENUS DENDEOCTONUS. 11 



of frontal grooves and tubercles serve to distinguish some of the 

 minor divisions of the genus, while the presence or absence of a 

 posterior median impression in those species without a frontal groove 

 is of considerable importance in distinguishing some of the minor 

 divisions. The relative size, density, and arrangement of the punc- 

 tures of the pronotum, while variable within the species, is of con- 

 siderable taxonomic value. The character of the rugosities of the 

 interspaces and the punctures of the strige are also variable within 

 the species and are of secondary value in distinguishing minor divi- 

 sions. The sculpture of the elytral declivity is of special specific 

 and sexual importance, and in some cases the characters are of value 

 in distinguishing minor divisions. 



THE HEAD. 



The general characters and details of the external skeleton and 

 appendages of the head are shown in figures 4, 5, and 6.' It will be 

 noted that the elements which in some other Coleoptera and other 

 insects are more or less clearly defined are quite completely fused and 

 obscured in this genus as in other rhynchophorous beetles. The 

 labrum and clypeus are obsolete. The epistoma, or ^'post-clypeus," 

 or ''pre-front," as recognized by difi^erent authors, is not separated 

 from the front by a line or suture, but is quite clearly defined, and the 

 epistomal process is far more prominent than in other allied genera. 

 The front is completely fused with the epicranium, which in turn is 

 fused with the gense, the latter joined beneath with a single gular 

 suture. Anterior to the gular suture there are three quite clearly 

 defined sclerites, which may be designated as pregula, pregena, and 

 hypostoma (fig. 5, E). By comparing the head of Dendroctonus 

 with that of a carabid beetle, Pterostichus (fig. 7), and a typical curcu- 

 lionid beetle, Pissodes strohi (figs. 8, 9), the striking difference in struc- 

 ture and relative position of the corresponding elements and their 

 extreme modification are at once apparent. 



Labrum. — The labrum is not present as a distinct element, but may 

 be represented by a part of the anterior margin of the epipharynx 

 beneath the anterior median section of the epistoma (fig. 6, A). 



Clypeus. — The clypeus is not represented unless it is by the pro- 

 duced anterior margin of the epistoma, and by the long epistomal 

 bristles. 



Epistoma (figs. 2, 3, 4, 5, 6, 7, 10, 40, B, D, E".)— The epistoma is 

 apparently represented in both the larvae and adults of all true man- 

 dibulate insects, but is more distinctly defined in some than in others. 

 In some insects it is separated from the clypeus by a suture, line, or 

 articulating membrane, while in others there is no evidence of sepa- 

 ration or the clypeus is not represented. Its separation from the 

 front is often defined by a line, impression, elevation, or otherwise, 

 although sometimes it is so completely fused that the line of junction 



