April, '08] JOURNAL OF ECONOMIC ENTOMOLOGY 



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eggs of insects belonging to four orders, one of its species, T. pretiosa, 

 preying upon no less than 12 species belonging to two orders, the Lepi- 

 doptera and Hymenoptera. 



It has often been customary among hymenopterists to assume that a 

 different host species must ahnost surely have different parasites from 

 those of a related form, even if sharp differential characters could not 

 be observed, but reliance on this physiological character is gradually 

 giving away to a demand for actual structural characters, and recent 

 investigators place little confidence in host relations for the actual sep- 

 aration of species. 



Undoubtedly the explanation for the fixity of habits throughout 

 many of the larger groups, is the common inheritance of specific in- 

 stincts through long periods of time without any, or with but little 

 change, while the varied genera and species of the group have mean- 

 while been evolved. A habit thus formed has been handed down from 

 generation to generation as the groups have passed into a more and 

 more intricate interdependence, through the evolution of new species 

 in each group. Such will be the natural trend of evolution, and we 

 can readily comprehend how the habits of a group like the ScelionidcB, 

 Alysiidm or Aphidiinoe must have originated. The Alysiid are par- 

 ticularly interesting in this respect, as they form a very compact group 

 distinguished from all its relatives with more than usual ease by a 

 single morphological character, which does not allow of the different 

 interpretations to which the characters of most other groups are sub- 

 jected at the hands of the systematist. In cases where groups are 

 more opinionative, habits themselves usually have considerable weight 

 in the segregation of their components. 



Conversely, that the variations from any uniform or related system 

 are due to some sudden change in the nature of a mutation seems prob- 

 able, and if so, the antiquity of the mutation should in some degree be 

 traceable from a knowledge of the extent of the present variations. 



Turning to view this possibility in the light of paleontological evi- 

 dence, we find that several well-marked cases of unusually variable 

 habits within a genus or small group are evidently associated with 

 antiquity. 



Among over one hundred undescribed species of fossil parasitic 

 Hymenoptera, which I am working over from the Miocene shales of 

 Florissant, Colorado, there are several genera that stand out distinctly 

 on account of their abundance. One is the Ichneumonid genus Pimpla. 

 It is represented by four or five species, one of which is the most abun- 

 dant form in the entire collection. Evidently these were as dominant 

 then as our species are at present. We find also that the recorded 

 habits of Pimpla are unusually varied. 



