64 THE SCOLYTID BEETLES. 



spiracular tubercles (Plate VIII) are distinct. The ninth segment is 

 without spiracles. 



Chitinous plates. In some species (division I) there are no distinct 

 chitinous plates or tubercles, while in others, section a4 and subdi- 

 vision D, they are present and, excepting Dendroctonus mica us, 

 become more distinct toward and including subdivision D, in which 

 the dorsal plates of the eighth and ninth abdominal segments are 

 distinctly armed. 



DIGESTIVE SYSTEM. 



The peculiar characters of the digestive system of the larva are 

 illustrated in figure 43, showing, at right, a median longitudinal sec- 

 tion through the body from the oral to the anal opening. In every 

 respect the anatomical details of the digestive system are much more 

 simple in the larva than in the adult. The same primary divisions 

 (^ fore, middle, and hind intestine are represented and there is the 

 same number of malpighian tubes, but the fore intestine is very simple 

 as compared with that of the adult, the crop and proventriculus being 

 scarcely different in general details from the oesophagus. 



EGGS. 



The eggs of Dendroctonus have not been studied in detail, but they 

 are short, oval to oblong-oval, pearly white and shining, and appar- 

 ently without distinctive generic or specific characters. 



PHYSIOLOGICAL CHARACTERISTICS. 



In addition to the morphological characters which serve to distin- 

 guish the genus, there are certain physiological characteristics pecul- 

 iar to the species of the genus which serve as additional evidence of 

 distinction. Indeed, it becomes more and more evident that a cor- 

 rect interpretation of natural groups of individuals, termed species, 

 and natural groups of species, termed genera, must be based not alone 

 on a common plan of structure or similarity in one or more anatomical 

 elements, but that, in order to come nearer the truth, the morpho- 

 logic evidence of specific distinction must be supplemented by physio- 

 logic and bionomic evidence. Some of the physiological features 

 common to the species of this genus, and more or less peculiar to them, 

 are found in the character of their brood galleries, in their habit of 

 attacking living trees, in their concentration of effort to overcome the 

 resistance exerted by the tree attacked, and especially by their ability 

 to manipulate and to dispose of the quantities of resin which flow 

 into their burrows in the living bast and cambium; lastly, in their in- 

 timate bionomic relations to definite genera and species of conifers. a 



a See, also, physiological characteristics of the species, as given in the forthcoming 

 Bulletin No. 83, Part I, which deals with the bionomic and economic features, and 

 other characteristics peculiar to the major and minor divisions as denned in the syn- 

 optic tables of galleries, host trees, and distribution in the present paper (pp. 76-79). 



