No. 637] 



PARASITIS3I 



135 



galls of these insects are commonly invaded by fungi and 

 yeast-like organisms. On the other hand, these same 

 gall-wasps exhibit remarkable correlations between struc- 

 ture and habits as showni by many workers, most elabo- 

 rately and clearly by the recent studies of Kinsey ('20). 

 The Cynipida? are at present restricted to a very limited 

 series of plants on which they induce the formation of 

 galls. With the exception of some uiuloubtrdix i.rimi- 

 tive forms, they occur almost exclusively nn iHi-accu' of 

 the genera Rosa and Rubus, and on the unrelated .m iiiis 

 Quercus, the latter harboring a very large number and 

 a far greater variety of forms. In the gall wasps, we 

 see, therefore, a nearly exclusive association with a very 

 few genera of plants and what is still more striking is 

 the fact that the more primitive ones, although few in 

 number, exhibit a wide range of food-plants. This leads 

 to the inevitable conclusion that we can trace the evolu- 

 tion of host relations in this group as now living, from 

 a very generalized condition to a highly specialized one. 



Many other insects, particularly Homoptera, harbor 

 certain probably symbiotic organisms, and recent studies 

 (Brues and Griaser, '21) tend to show that these may be 

 very important factors in the nutrition of the insects. 

 Almost nothing is knowm concerning such organisms in 

 Hymenoptera, but if they are present in some cases, as 

 seems probable, they must be reckoned with in any com- 

 plete studies of food-habits. 



The relation of food-habits to taxonomy in the Hymen- 

 oi)tera becomes particularly interesting in connection 

 with the appearance of parasitism in several forms, 

 which is of widespread occurrence in the order. In fact, 

 a large proportion of the Hymenoptera are parasitic, 

 and with the development of this mode of exisiencc have 

 come such elaborate structural modification.-, asnl spe- 

 cialization in behavior, that species have iiiultipliiMl at 



