No. 637] 



FABASITISM 



151 



tacked. This influence seems to be a rather direct one. 

 On the other hand, I may quote from a previous paper 

 (Brues, '08) the following: "The European Chalcid-fly, 

 Ormyrus tuhulosus, has been minutely studied by Mayr, 

 who has bred it from no less than 27 species of Cynipid 

 galls, and I have from Massachusetts what is apparently 

 the same species, bred from about half as many North 

 American species by the late Dr. M. T. Thompson. The 

 galls formed by the various hosts of this species are 

 many of them entirely dissimilar in form, the only re- 

 semblance between them, aside from their gross gall- 

 like form, being their more or less uniform habitat at- 

 tached to twigs and leaves." Howard ('91) mentions 

 Enrytoma rosce as having over 50 cynipid hosts. A range 

 of hosts of this sort appears to be due not directly to the 

 environment of the host, but to the similar physiological 

 condition of the various Cynpids themselves, which, as 

 we have already said, are closely confined to a very nar- 

 row range of food-plants. 



The great difficulties occasionally imposed upon para- 

 sites in attaining their hosts may be purely a matter of 

 environment, as illustrated by the following considera- 

 tions. 



An interesting series of parasitic Hymenoptera are 

 those which prey upon aquatic insects. In several well- 

 known cases, the behavior of the adult parasites has be- 

 come so profoundly modified that the females not only 

 enter the water in search of their hosts, but they may be, 

 occasionally at least, accompanied by the males. The 

 first observation of this sort was made nearly a century 

 ago by Francis Walker ('36) on Agriotypus, andihe well- 

 known, observer Sir John Lubbock ('63) later gave an 

 account of the habits of two aquatic Chalcis-flies in which 

 he describes the actual process of swimming. One species, 

 the Mymarid {Cataphractus cinctus) makes use of its 

 ciliated, paddle-shaped wings for this purpose, while the 

 other, a Trichogrammid (Prestwichia aquatica) propels 

 itself by means of the legs. Numerous other contribu- 



