THE GENUS DENDROCTONUS. 



21 



lateral movements as in Pterostichus (fig. 7). The relative lengths 

 of the joints vary considerably in the species of the genus, and more 

 or less in the individuals of a species. The first joint is usually 

 longer than the other two together, but is sometimes equal or shorter; 

 the second joint is usually longer than the third, but is sometimes 

 shorter. 



Mandibles (figs. 4, 14). — The mandibles are prominent, stout, 

 triangular, and especially adapted for burrowing in the bark. The 

 inner edges are acute, with a subapical and a median tooth toward 

 the middle and a molar on the basal angle. The lateral area toward 

 the base has a large impression and there is usually a less evident 

 one on the dorsal area, each bearing one or two bristles. The dorsal 



Fig. IZ.—Dendroctonus: Antennae. 18, rufipennis; SO, punctatus; 21, micans; 22, terebrans; 2S, valens (?; 

 2Sa, valens 9; 23b, valens S; 23c, valens 9- (Original.) 



articulation with the epistoma is especially adapted to meet its 

 several requirements. The peculiar trochlear mechanism of the 

 articulating condyles and fossa are illustrated in figure 14; that of 

 the dorsal condyle appears to be common to other rhynchophorous 

 beetles, but apparently not represented in other Coleoptera, including 

 those with similar bark and wood boring habits. The ventral 

 articulation also appears to be different from that in other Coleop- 

 tera, but to a less degree. A detailed comparative study of the 

 mandibles may reveal specific characters, but as a rule such charac- 

 ters are unsatisfactor}^ from the fact that in comparisons the man- 

 dible must be viewed from exactly the same position to avoid error 

 in conclusions. 



