NEW YORK STATE MUSEUM 



louse establishes herself on a stein or leaf, drawing nourishment therefrom, 

 and the affected tissues shrink away to some extent, while those adjacent 

 bend over her and eventually form an almost complete inclosure. 



Alternation of generations. Certain gall insects are remarkable on this 

 account. The Cynipids had long been a puzzle to entomologists, and the 

 classic studies of Dr Hermann Adler explained many enigmas. Dr Adler's 

 investigations were largely with the oak gall flies, and prQved that a 

 remarkable alternation of generations occurs in this group. He found that 

 insects, previously assigned to separate genera, were only members of 

 different generations, and in general his conclusions may be stated as fol- 

 lows : that the parthenogenetic bud-inhabiting gall flies are parents of a 

 sexual generation which oviposits on foliage, producing leaf galls, these in 

 turn producing the preceding form. Previous to Dr Adler's investigations, 

 these parthenogenetic forms had been supposed to be asexual, and according 

 to him certain European species are thus restricted. 



The Cynipidae have always attracted more interest, probably because 

 of their higher organization and more perfect galls, yet in complex alterna- 

 tion of generations they are far surpassed by certain plant lice which produce 

 six or seven generations with marked variations in structure and radical 

 changes of food plant, as detailed in the accounts of Hormaphis hama- 

 m e 1 i d i s Fitch and Hamamelistes s p i n o s u s Shim., to which the 

 reader is referred for details. It would not be at all surprising if there 

 were other species of gall-inhabiting plant lice presenting almost as great 

 diversities in their life histories. 



Gall makers. These, as previously stated, belong to very diverse 

 groups and the gall-making habit has undoubtedly arisen independently in 

 each instance. 



The Hymenoptera include some of the best known gall makers, notably 

 the Cynipidae and certain Tenthredinidae. The former are small, dark 

 colored insects with few veins in their four nearly transparent wings. 

 Their galls are always completely closed and though very diverse in char- 

 acter are easily recognized by exclusion, as a rule. They contain, when 



