KEY TO AMERICAN INSECT GALLS 9 



are to be found species, the young or the maggots of which live 

 exposed upon the foHage, some of these evidently absorbing nourish- 

 ment from the leaf tissues and producing a characteristic modifica- 

 tion, such for example as the very common leaf spot caused by 

 Cecidomyia ocellaris on soft maple leaves ; others sub- 

 sist upon fungi, some are predaceous, and the young of a number 

 may be found in decaying wood or in subcortical tissues of the host 

 and cause little or no disturbance of the plant functions. Such 

 species are very likely to be reared by the student of insect galls 

 and we have therefore included in the key a number of these forms 

 which have been definitely associated with various food plants. 



A comparison of the food habits of the gall midges or Itonididae 

 and the gall wasps or Cynipidae is of more than passing interest. 

 In the first place, the 682 galls produced by the midges occur on plants 

 belonging to 69 botanical families and 202 genera. The larvae of 

 66 species live at the expense of the willows or Salicaceae (52 of these 

 are found on Salix); 29 species occur upon the Juglandaceae, all but 

 one infesting hickory or Carya; 43 attack the Fagaceae (36 of these 

 being upon the oaks) ; 56 species produce galls on the Rosaceae, 24 on 

 the Leguminosae, 22 upon the Vitaceae, and 151 on the Compositae. 

 The most obvious concentration of species, aside from those men- 

 tioned above, is the 44 midges reared from Solidago and the 22 found 

 upon aster. These figures are approximate, yet taken in connection 

 with the great diversity in the structure of these small insects, 

 indicate that this group has been able to maintain itself upon a 

 great many different plants through a considerable physiological 

 adaptability and that the distinctness of the species has been estab- 

 lished by relatively small modifications in structure. 



The gall wasps or Cynipidae present an entirely different con- 

 dition so far as relation to the flora is concerned. They attack 

 plants referable to only 6 botanical families and assignable to but 

 17 plant genera.^ There is, however, a most striking concentration 

 in food habits, since a very large proportion of the 445 gall makers 

 subsist at the expense of the Fagaceae which, for this group, means 

 the oaks, the exact number in our list being 359 though this figure, 

 like the above, is an approximation; 38 species have been reared 

 from members of the rose family or Rosaceae, 28 (Rhodites) living 

 at the expense of the genus Rosa. The other species referable to 

 the Cynipidae are scattered in their food habits, the most evident 

 concentration, and this far from marked, being the 12 species reared 



' Mr L. H. Weld has called our attention to two undescribed gall wasps which would change the 

 above fi;;ures to 7 botanical families and 19 plant genera. There is also a South American species 

 on Acacia. 



