8o Annals Entomological Society of America [Vol.1, 



complete specialization for boring, but in the flattened form of 

 young and adult show adaptation to the particular portion of the 

 tree which they affect. The bark beetles present a somewhat 

 different condition, but are no less distinctly specialized for their 

 particular habitat. They live for the most part between the 

 bark and hard w^ood and construct intricate tunnels sometimes 

 of most peculiar pattern, and frequently occasion very great 

 injury to the trees infested. The habit in these different groups 

 has very evidently been reached by entirely independent routes 

 and the lines of adaptation must be traced in each group separate- 

 ly. Often the particular steps of adaptation are very beautifully 

 shown in the series of species which illustrate the divergence 

 from external leaf eating forms to those which are most perfectly 

 fitted for living within the plants. 



Of the Hymentopera the horn-tail borers are the most dis- 

 tinctly specialized in this direction, and these show a derivation 

 from the leaf feeding saw-fly forms. The ants and bees which 

 burrow into wood or into the stems of plants show certain other 

 forms in which the habit has been adopted by isolated small 

 groups, but not adopted by the larger divisions. 



THE PARASITIC PATHWAY. 



In the direction of parasitic life some groups of insects have 

 traveled very far and show almost as distinctly as any group 

 of animals the eff'ects of the parasitic life. So numerous are the 

 examples in this line that we must select only a few of those which 

 are most specialized or which illustrate most perfectly the lines of 

 derivation for the habit. In the Alallophaga we have a group in 

 which the parasitism is distinct for all the known species and in 

 which the result of parasitism is shown in the entire absence of 

 wings and in the very perfect adaptation of clasping organs in the 

 feet. In their mouth parts and other structure, however, they 

 show very perfectly their derivation from some psocid ancestor, 

 and by selecting series of genera we can trace quite clearly the 

 different steps in adjustment from forms which are but slightly 

 parasitic to those which are most extremely dependent upon their 

 attachment to a host. In some cases migration from one kind of 

 animal to another is possible and probably frequent, but in other 

 species more strikingly specialized there is a most rigid restriction 

 to a single species and absolute dependence upon the association 

 of individuals in that species for its distribution and survival. 



