198 THE SCOLYTID BEETLES. 



PHYSIOLOGICAL CHARACTERISTICS. 



As pointed out in Part I (p. 64), the physiological characteristics 

 are of special taxononiic importance when correlated with morpho- 

 logical characters. The food, social, and sexual habits, character 

 of the brood galleries, choice of host plants, and distribution of 

 genera and species are all more or less rich in facts of taxonomic 

 importance. 



GENERAL HABITS. 



The scolytoid beetles are distinguished from nearly all other 

 Rhynchophora by their habit of excavating characteristic egg gal- 

 leries in the living or dead plant tissue. The few notable exceptions 

 are found in the genus Stenoscelis, of the Calandridse, the adults of 

 which excavate a primary egg burrow, but as a rule this habit within 

 the suborder is peculiar to the Scolytoidea. The egg galleries of 

 Scolytoidea are excavated in the bark or wood of trees and shrubs, the 

 roots, stems, and leaves of herbaceous plants, the fruits or seeds of 

 palms and other plants, young pine cones, the wood of barrels or 

 casks containing water or spirituous liquors, etc. Some of the species 

 excavate their galleries in decaying bark or wood or even in the 

 fruiting bodies of fungi, while others confine their work to the bark 

 or wood of weakened, dying, or recently dead plants, and still others 

 prefer to enter the living and sound tissues. 



The food of the adults and larvae consists of the sugars, starches, 

 and other nutritive elements of their host plants, or of fungi which 

 grow in their brood galleries. 



CLASSIFICATION ACCORDING TO HABITS. 



Any classification of the families or subfamilies based on food 

 habits alone would not indicate a natural arrangement, as is plainly 

 indicated by the parallel habits of groups of species in widely sepa- 

 rated families, subfamilies, and genera. It is true that there are 

 several well marked classes according to habits, such as bark beetles, 

 twig beetles, seed beetles, cone beetles, and ambrosia beetles. It is 

 evident, however, that food habits, like many other characteristics 

 and structural characters, have evolved along parallel lines in allied 

 as well as in widely separated groups. There are many examples 

 illustrating this principle. The genera Xyleborus, Corthylus, Scoly- 

 toijlatypus, and Platypus are, according to fundamental morpho- 

 logical elements as well as groups of correlated characters, so widely 

 separated that they each represent a different family or subfamily; 

 yet the habit of excavating their galleries in wood and feeding on 

 ambrosial fungi is common to them all. Between some of these genera 

 there is also a more or less constant resemblance in certain morpho- 

 logical characters, especially in the hairs and slender teeth of the 



