PEELIMINAKY CLASSIFICATIOlSr OF SCOLYTOIDEA. 199 



lacinia of the maxilla, but this is evidently clue to parallel adaptation 

 to similar uses and not to common origin or phylogenetic descent 

 from a common ambrosia-feeding ancestor. 



Food Habits of the Adults. 



As a rule the adults obtain their food from the substance in which 

 they excavate their egg galleries or from the fungi growmg on the 

 walls of the galleries, but there are numerous examples of special 

 food habit such as that found in Scolytus, Pteleohius, Phlceosinus, 

 and Tomicus (see p. 220), which excavate food burrows in the living 

 twigs of their host trees. 



Food Habits of the Larv^. 



There is a wide range of variation in the food habits of larvse of dif- 

 ferent species, especially in the character of their food burrows or 

 larval mines. Each species of a group of closely allied species may 

 have similar habits, but, as shown in the genus Dendroctonus, there 

 may also be a wide range of variation and some striking examples of 

 progressive modification m this habit within a genus in which there 

 is a restricted range in structural characters in the adults. In the 

 genus Dendroctonus there is a tendency throughout for the larval 

 mines to occur m groups of increasing numbers from the simple, isolated 

 mine of Dendroctonus hrevicomis to closely placed groups in Dendroc- 

 tonus simplex and D. jnceaperda and to the large social chamber of 

 micans, valens, and terebrans. (See figs. 73, 75, 79, 88, and 91, of 

 Part I.) Thus the stage in the modification of the larval mine of a 

 given species may indicate, in connection with stages in the modifi.ca- 

 tion of structural characters, the natural position of the species. 



In the ambrosia beetles the larvse of some species and groups of 

 genera, as Xyleborus, StepJianoderes, and Crossotarsus, live in the 

 primary galleries in direct association with the eggs, larvse, pupae, 

 young adults, and parent adults, while in the subfamily Corthylmse, 

 the genus Scolytoplatypus, and at least some of the species of Platypus, 

 the larvse occupy separate chambers in the sides of the gallery, these 

 chambers not extending beyond a size sufficient for the accommoda- 

 tion of the body. 



Pupal Habits. 



Considerable variation exists in the habits of the pupse and in the 

 cells occupied by them in transformation from larvse to adults. In 

 perhaps the majority of species the transformation takes place at the 

 end of the food burrow with or without a definite cell. In some ambro- 

 sia beetles the pupation takes place in the social galleries occuped by 

 different stages of the brood, in others it is in the lateral larval cell, 

 and in StepJianoderes the transformation takes place in closely jomed 



