lO NEW YORK STATE MUSEUM 



from various Compositae, the genera Silphitim and Lactuca support- 

 ing 4 and 3, respectively. This well-defined limitation in food 

 habits is accompanied, as might be expected, by a high degree of 

 specialization in structure. 



The plant lice or Aphididae live on a great variety of plants, 

 though the gall-making forms occur upon the members of relatively 

 few plant families and genera, the most evident food preference 

 being in the gall aphids belonging to the genus Phylloxera, with its 

 29 species producing deformations on hickories. The nearly allied 

 jimiping plant lice or Psyllidae present a similar condition in the 

 genus Pachypsylla and its relation to the numerous galls upon 

 Celtis. 



It will be seen by referring to the key that the galls are grouped 

 by food plants and that "in the case of those infested by numerous 

 gall insects, additional divisions are made according to the location 

 on the plant and the structure of the galls. The difference between 

 the leaf and twig gall is not sharply defined in nature, since the same 

 species may produce a gall diverting only a portion of the nourish- 

 ment and a part of the leaf is therefore clearly present, while in 

 other cases the deformation may develop to such an extent that 

 there is no hint of a leaf and the gall is apparently, at least, a twig 

 deformation; consequently some galls are entered more than once 

 because of an apparently different origin and the same is true to a 

 certain extent in regard to variations in form. 



A few special terms used in the key are explained below : 



Blister galls. This is applied primarily to the blisterlike, apparently 

 fungous-filled swelHngs occurring upon the leaves of Solidago and 

 aster and usually inhabited by species of Asteromyia. 



Bud galls. Deformations which evidently originate from buds. 

 They may vary from a plainly aborted bud to a great enlargement 

 developing from such parts. 



Bullet galls. A term which has become somewhat general and 

 refers in particular to the nearly solid, monothalamous bullet galls 

 produced by certain species of Disholcaspis upon oak twigs. 



Cecidomyia. A term used here in a broad sense and appHed 

 to any species referable to the family Itonididae and which can not 

 be readily assigned to more closely defined genera. 



Erineum is applied in a general way to the hairy growths upon 

 leaf surfaces inhabited by species of plant mites or Eriophyidae. 



Flower galls. Deformed flowers or masses of flowers, frequently 

 more or less variable in shape. 



