1912] Morphology and Biology of Insect Galls 303 



The types of the galls constitute a clear phylogenetic series from the 

 simple erineum to the well developed pouch-gall. The distinction be- 

 tween the different members of this series is largely a difference in degree 

 rather than in kind, and seems to be explainable on the assumption of a 

 gradually increasing intensity of stimulus from the lowest to the highest 

 member of the series. 



The literature of this group of galls contains a number of state- 

 ments, to the effect that the host plant under the influence of the gall 

 stimulus can originate entirely new types of hairs. While it is true, as a 

 general rule, that hairs produced by an organ under such a stimulus differ 

 from those originated by that organ under normal conditions, yet in such 

 cases I have found that the abnormal type of hair is being produced on 

 another part of the host plant. Thus the curiously convoluted, glandular 

 hairs, originating in the dimple galls on the leaves of Acer negundo L., 

 are duplicated exactly in the hairs occurring on the reproductive axis 

 of the same plant, and the long acicular hairs composing the brown pubes- 

 cence that fills the concavities on the leaves of Quercus macrocarpa 

 Michx have their exact counterparts in those produced on the flowering 

 axis of that plant. In the former example the normal hairs on the leaves 

 of the host are straight and acicular, in the latter they are of the stellate 

 type. 



Order Hemiptera. 



The following Hemipterous galls have been studied : — 

 Fam. Aphididse. 



Gall on Populus balsamifera L. (Unclass.). 



Hormaphis hamamelidis Fitch. 



Hamamelistes spinosus Shimer. 



Aphid Corrugations on Birch. 



Pemphigus vagahundus Walsh. 



Pemphigus rhois Fitch. 



Chermes ahietis Choi. 



Chermes floccus Patch. 

 Fam. Psyllidae. 



Pachypsylla celtidis-mamma Riley. 



Aphid Gall (Unclassified). 

 Host Populus balsamifera L. 



A pouch-like gall on the under surface of the leaf, produced by a 

 fold in the blade near the base of the midrib. One edge of the fold is 

 attached along this midrib. The slit-like opening, which is on the upper 

 surface of the leaf, extends the full length of the gall. This species re- 

 sembles very closely the gall produced by Cecidomyia majalis Bass. The 

 general structure is shown in Fig. 7. 



