24 



THE SCOLYTID BEETLES. 



/Precpiskrnal area 

 -Preslcrnal area 



Strnial area 

 Epislernal area 

 . Intercoxal or 

 " stevnellar area 

 'Exocoxal area 



Epimeral area 



''Poitsternellar area 

 Fig. 17. — Dendroctonus valens: Areas of prothorax, ventral 

 aspect, a, Anterior margin ; &, posterior margin; c, an- 

 terior angle; d , posterior angle; e, anterior entotlioracic 

 fold for attach ment of intersegmental membrane; /, an- 

 terior foramen; g, posterior foramen; h, coxal cavity. 

 (Original.) 



divisions are one dorsal, two lateral, and one ventral; the transverse 

 divisions are one anterior, two median, and one posterior. 



Audouin (1824) recognized the four longitudinal divisions and 

 named them sternum, pleurae, and tergum. He also recognized two 

 divisions of the pleura and named them episternum and epimerum, 

 and four transverse divisions of the tergum, which he named pres- 



cutum, scutum, scutellum, 

 and postscutellum. These 

 names have been adopted 

 with but slight modifica- 

 tion by most of the leading 

 writers on insect anatomy, 

 including McLeay, 1830; 

 Newport, 1839; Kolbe, 

 1889; Amans, 1885; Com- 

 stock, 1902; and Voss, 

 1905. The same divisions 

 have been recognized by 

 many other authors, who 

 have designated them by 

 different names. 



The first, second, third, and fourth transverse divisions of the 

 ventral or sternal area were recognized by McLeay in 1830, and 

 named, in order, presternum, sternum, sternellum, and poststernellum. 

 All of the divisions and subdivisions mentioned as having been 

 recognized by Audouin and McLeay are here recognized by the writer, 

 and, in addition, the first and fourth transverse divisions of the pleura; 

 thus four longitudinal and four 

 transverse divisions of each 

 segment in the adult insect 

 have been recognized. The 

 writer has also recognized the 

 same or a similar system of 

 division in the thoracic and 

 abdominal segments of larvae 

 and pupae. Wliile the taxo- 

 nomic significance of the char- 

 acter and modification of these 

 primary and secondary divisions as represented in one or more thoracic 

 segments of the same insect, or in one or more segments in insects of 

 different orders and minor groups, has been recognized, there has been 

 wide difference of opinion as to the origin or homology of these divisions 

 and in their interpretation or definition. This has naturally resulted 

 in much confusion in the adoption and application of the nomencla- 

 ture proposed by different authors, and, more than anything else 



^^-Pre'episiemum 

 —b 



/Episternum 

 -^Epimerum 



^temellar area 

 Poststemellar piece^ 



Fig. 18. — Dendroctonus valens: Mesothorax, ventral 

 aspect, a, Preepisternal process; 6, transverse im- 

 pression; c, coxal cavity. (Original.) 



