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NO. 8 PARASITES METCALF ^3 



Harrison, as already described, has made use of parasite data from 

 Stratiodrilu's, a genus of archaic annehds, to indicate intimate relation 

 between Australasia, Madagascar and South America. The annehds 

 as a class, however, are poor in parasitic species. 



Among the Crustacea the parasitic copepods may perhaps give light 

 upon some interesting problems, though their host relations and 

 especially the specificity of these relations need further study. The 

 parasitic species of copepods are apparently chiefly ancient and 

 reached for the most part their adaptation to parasitism long ago, 

 having undergone little modification in later geologic periods. Others, 

 however, seem to have adopted parasitism more recently. A thorough 

 analysis of the parasitic copepods from this point of view would be 

 worth while for its own sake and would give added significance to 

 their host-distribution and geographical distribution. 



Among the Arachnoidea (spiders, mites, ticks, etc.) several groups 

 are parasitic, but the parasites are not confined each to one individual 

 host or even to one species of host. They are free to pass from one 

 host to another. This makes them far less useful for host-parasite 

 studies than are more restricted parasites, but, in some instances at 

 least, they present usable data. 



The true insects include many groups among whose members 

 parasitism is more or less well developed. Examples of insect parasites 

 of terrestrial vertebrates and of insect parasites of insects at once 

 come to mind, but with these insects, as with the mites and ticks, 

 specificity of host-infection is in general not highly developed, though 

 there are numerous exceptions in which there is constant relation 

 between kind of host and kind of insect parasite, as, for example, 

 some moths parasitic in bee colonies and some beetles restricted to 



ant nests. . . 



Many insects parasitic upon plants have closely specific host limita- 

 tions being confined each to a single host species or to a related group 

 of species, however freely they may pass from host individual to host 

 individual. One thinks at once of the plant lice (Aphides), but many 

 even of the larger insects have similarly restricted plant prey— ^. g., 

 the potato beetle, the squash bug, the plum curculio, the hessian fly, the 

 cotton boll weevil, grape Phylloxera, some butterflies, some moths, 

 many gall-flies, etc. 



Molluscs, echinoderms, vertebrates and other chordates, show few 

 examples of parasitism, commensalism or obligate association of any 

 kind It is doubtful if the few cases known (shark-Remora, fish hving 

 among the tentacles of jelly-fish, fish living within sea cucumbers, fish 



