Correlation of Structure and Host-Relation 185 



ticularly of the host-relations, with other groups of parasitic 

 Hymenoptera. 



I am perfectly aware that after all I have touched only upon 

 one side of this important subject. The other side is the 

 structural differentiation of forms whose host-relations are 

 identical. Parasites of several genera and even families are 

 parasitic upon the same host-type and even upon the same in- 

 dividual. Comparatively widely different factors must here 

 influence the structure and a wide field of investigation is thus 

 opened. 



Washington, D. C, 

 August 9, 1893. 



