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Indiana University Studies 



times the larval cell entirely separable at maturity. On both black and 

 white oaks, the agamic generation in larger, more woody stem galls, and 

 the bisexual generation in smaller galls on veins, petioles, flower stems. 



RANGE. — Western Asia, Europe, northern Africa, and North 

 America. Probably wherever oaks occur. 



TYPE. — Cynips ilicis Fabricius. Designated by Ashmead, 1903, 

 Psyche, X, p. 151. 



This description largely agrees with the Das Tierreich de- 

 scription, and is somewhat broadened to include both Euro- 

 pean and American species showing close relationships. I 

 have specimens of P. ilicis and some other European species 

 kindly sent me by Prof. A. Trotter, of Portici, Italy, and have 

 compared all other species with ilicis. This genus has not 

 been recognized in the North American fauna previously. It 

 is known from western Asia, Europe, and northern Africa 

 from only three or possibly four species. The insects are 

 characterized by the key characters at the beginning of this 

 description. The genus as I recognize it is a very compact 

 group of species, distinguished by insect morphology, very 

 uniform gall characters, and probably uniform life histories. 

 Here is a good instance of the correlation of morphologic and 

 physiologic characters. I repeat that I question the quality 

 of any "genus" where the two sorts of characters are not 

 correlated! All of the American species previously described 

 have been placed in the heterogeneous mixtures called An- 

 dricus or Callirhytis. Plagiotrichus is closely related to 

 Dryocosmus Giraud, but the two groups should not be united. 

 Dryocosnius shows some specialization in structure, particu- 

 larly of the abdomen, and the galls are more highly specialized, 

 being aggregations of monothalamous stem galls. Plagio- 

 trichus is distinctly a more primitive group, probably 

 ''ancestral" to Dryocosnius and to a group including some of 

 the other North American stem gall producers not yet assigned 

 to exact genera. 



The relatively smaller second abdominal segment, the 

 simple tarsal claws, the simple nature of the galls (polythala- 

 mous, inseparable, without special structures, the larval cell 

 often inseparable), and the fundamental similarity of the 

 alternate generations are the primitive characters. The open 

 radial cell, the reduced thoracic sculpture, and the occurrence 

 of heterogeny mark some degree of specialization, greater 

 than occurs in the genus Neuroterus, for instance. 



