26 THE SCOLYTID BEETLES. 



In the following discussion of the thoracic segments reference is 

 made to the named parts as represented or not represented, as the case 

 may be, rather than to say that they are present or not present, 

 because in some cases where they are not defined on the external 

 surface they may be indicated by apodemes or lines on the inner 

 surface of the body wall, while in other cases their position or rela- 

 tive areas are indicated only by some character of surface sculpture 

 or vestiture. 



In the adult Dendroctonus there is a wide range of difference in the 

 representation of parts in the prothorax, mesothorax, and meta- 

 thorax. In the pupa there is a similar but not so marked difference 

 between the three thoracic segments, the divisions being less evident 

 in the mesothorax and metathorax than in the adult, but in the 

 abdominal tergites the divisions are quite plainly indicated. In the 

 larvae there is not only less difference in the three thoracic segments, 

 but these are only slightly different from the first to seventh abdomi- 

 nal segments. In the thoracic segments the present al and scutel- 

 lar divisions are clearly represented, with evidence of the scutal divi- 

 sion on the sides. The sternal and sternellar divisions are also 

 clearly represented, with evidences of the presternal and poststernellar 

 divisions in the prosternum, and the latter clearly defined in the 

 mesosternum and metasternum. The pleurites are also represented 

 by pleural lobes. In the abdominal tergites 1 to 6 the prescutal, 

 scutal, and scutellar divisions are clearly represented and the sternal, 

 sternellar, and poststernellar divisions are in like manner repre- 

 sented in abdominal sternites 1 to 8, inclusive. Whether or not these 

 divisions or lobes are homologous with divisions occupying rela- 

 tively the same positions in the pupa and adult may be a subject for 

 difference of opinion, but the names here applied to what appear to 

 be corresponding parts should serve as a reliable guide to their recog- 

 nition and accurate definition and description in comparative studies 

 and identification of species. 



ELEMENTS OP THE ADULT THORAX. 



The primary and secondary elements as represented in the thoracic 

 segments of an adult Dendroctonus beetle are shown in the figures 

 and are interpreted, named, and described as follows : a 



THE PROTHORAX. 



In this genus, as in rhynchophorous beetles generally, the tergal, 

 pleural, and sternal areas are fused into a continuous band. The 



a Notum and tergum. While the names notum and tergum are synonymous, the 

 former has been applied more specifically to the dorsal division of the prothorax, 

 especially in beetles, and is here utilized in that sense. The name tergum is here 

 used to designate the dorsal areas of the mesothorax, metathorax, and abdomen, on 

 account of the use of the term tergite to designate a subdivision. 



