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THE AMEBIC AN NATURALIST [Vol. LV 



necessarily veiy incomplete and may be misleading in 

 some details. 



In the Hymenoptera the designation parasitic" has 

 been applied to habits of extremely diverse nature, and 

 this very loose use of the term may easily lead to serious 

 confusion unless we consider the matter carefully before 

 proceeding further. It is most commonly applied to 

 several large and abundant groups whose members live 

 in the larval condition in other insects which they almost 

 invariably destroy after attaining full growth. Such 

 habits are quite similar to those encountered in other 

 orders of insects, although in no other do they attain 

 such a high degree of specialization ; nor do they involve 

 such a series of unrelated smaller groups, with the pos- 

 sible exception of the Diptera. While this is the most 

 abundant and widespread type of parasitism among in- 

 sects, we must not lose sight of the fact that it is a rather 

 unusual condition so far as animals and plants in general 

 are concerned, in that the parasite and host belong to the 

 same class and are thus closely related. A similar rela- 

 tion exists among the Crustacea where certain decapods 

 are parasitized by other members of the same class, and 

 even among Protozoa there are ciliates parasitic in the 

 bodies of other infusorians. Such associations are very 

 rare, however, and the vast majority of parasitic Crus- 

 tacea and Protozoa, as well as other parasitic animals, 

 depend upon animals far removed from themselves for 

 hosts, although there is very generally a close correla- 

 tion between the host and parasite in that related para- 

 sites depend upon related hosts. Another peculiarity of 

 this type of insect parasitism lies in the prompt death of 

 the host, which does not usually ensue as a result of other 

 animal parasites, although it is a common result of pres- 

 ence of some protozoan parasites of the higher animals. 

 In others, again, like the well-known nematode, Trichina, 

 the ultimate death of the host is necessary for the con- 

 tinued propagation of the parasite, but actual death nor- 

 mally results from other causes. Still another charac- 



