THE GENUS DENDKOCTONUS. 



23 



beyond the ventral margin of the same segments, while the posterior 

 dorsal margin of the pronotum and the anterior dorsal margin of the 

 mesonotum are not produced beyond 

 the corresponding sternal margin. 



czy 



Divisions of the Thoracic Segment. 



The divisions and other characters 

 peculiar to the thoracic segments of a 

 scolytid beetle are shown in figures 16, 

 17, 18, 19, and 20. 



It will be noted that while the usual 

 divisions or sclerites are quite clearly de- 

 fined in the metathorax, corresponding 

 divisions are less distinct in the mesotho- 

 rax, and are obsolete or completely fused 

 in the prothorax. The taxonomic signifi- 

 cance of this wide range in the modifica- 

 tion of similar parts or areas in the three 

 thoracic segments of the same insect is 

 realized when we compare these parts 

 with corresponding segments in repre- 

 sentative species of other families, sub- 

 orders, and orders of insects. It will be seen that each segment has 

 characters peculiar to the order or minor group to which the species 



belongs, and that in like 



.^d^ 



Fig. 15.—Dendroctonus: Eyes. 1, brevi- 

 comis; 2, barberi; 5, arizonicus; 8, ap- 

 proximatus; 10, 10a, ponderosse; 12, 

 simplex; lS,pseudotsugie; H, piceaperda; 

 15, 15a, engelmanni; 20, punctatus; 22, 

 22a, b, c, terebrans; 23, 23a, b, c, d, e,f, 

 g, h, i, i, k, I, valens. (Original.) 



aa 



combined 

 any two 



manner the 

 characters of 

 or all three in the same 

 insect present many fea- 

 tures peculiar to the groups , 

 the suborder, family, genus, 

 or species represented. 



It is also significant of 

 the influence of a domi- 

 nant principle or plan of 

 structure and order of mod- 

 ification that one or more 

 thoracic segments of prac- 

 tically any insect examined 

 will show certain divisions 

 more or less clearly defined, 

 which are common to all 

 other insects, and that 

 when we compare the segments of different stages of insects of all 

 orders, we find that a composite segment would represent a system 

 of four longitudinal and four transverse divisions. The longitudinal 



D 



Fig. 16. — Dendroctonus valens: Areas of pronotum. A, an- 

 terior area; B, median area; C, jposterior area; D, lateral 

 area; E, dorsal area; a, anterior angle; aa, anterior margin; 

 6, posterior angle; d, basal margin; e, posterior declivity; 

 /, anterior section of lateral area; g, anterior section of dorsal 

 area; h, median section of lateral area; i, median section of 

 dorsal area; j, posterior section of lateral area; k, posterior 

 section of dorsal area; I, posterior margin or vertex. 

 (Original.) 



