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TRE AMERICAN NATURALIST [Vol. LY 



mary parasitism, althougli it may depend upon a general 

 similarity in the tissues of all entomophagous parasites, 

 or a less aggressive condition of the tissue in parasites 

 due to their generally secluded and protected environ- 

 ment. As the latter condition seems not unlikely, it prob- 

 ably acts regularly to make hyperparasitism an easily 

 acquired characteristic. 



Striking divergencies, like the following, noted by 

 Swezey ('08), are of interest in this connection. In his 

 studios of Dryinid leaf-hopper parasites, he found a 

 Ceraphronid parasitic on a species of the related Dryi- 

 nidsB, although the group normally and abundantly para- 

 sitizes entirely ditferent types of insects. 



The adaptation of animals and plants in conformity 

 with the demands of diverse environmental conditions 

 is now an axiom among biologists. From its manifesta- 

 tions it is evidently a physiological adjustment which 

 leads secondarily to structural changes, and many con- 

 vergences in form and function are traceable to it. On 

 account of the close interdependence of plants and in- 

 sects it appears, in some instances at least, to exert an 

 indirect influence upon phytophagous insects (Brues, 

 '20), whereby a species may feed rather indiscriminately 

 on herbs, and another on woody plants, but not upon the 

 two in combination. 



In the case of parasitic Hymenoptera there are many 

 instances which might be cited where environment ap- 

 pears to have exerted a direct influence upon the acqui- 

 sition of host relations and others where we must, I 

 think, believe the influence to be indirectly related to the 

 environment through a second insect, the host. This 

 rather obscure statement may be clarified by a few ex- 

 amples. From what we have said in connection with 

 hyperparasitism, it seems quite clear that a species which 

 may assume the role of either a primary or secondary 

 parasite, responds quite directly to the environment, in 

 this case the primary host, which may be either sound 

 or already infested by a parasite which is in turn at- 



