13° The Ohio Naturalist. [Vol. IV, No. 6, 



anomalum (Part V, Figs. 47, 48); (4) The witchbroom forma- 

 tion, as in the case of the Phytoptus sp , and Sphaerotheca 



phytoptophila Kell. and Sw. on Celtis occidentalis. 



The author has studied only the second and third types. The 

 difference between these two may be accounted for by the fact that 

 the Phytoptus attacks the blade while the Erineum attacks the 

 petiole, mid-rib or larger vein. The part affected undergoes a 

 curvature in each case in the direction of the least resistance. 



2. HEMIPTERA. 



The method of attack by the Hemiptera is practically the same 

 as in Arachnida, i. e., by sucking mouthparts. The galls present 

 a complete serial line of development, the lowest form being a 

 simple curling of the leaf as in the case of Schizoneura americana, 

 the next higher, a simple folding of the leaf, as in the case of 

 Colopha ulmicola, the next higher is a more complex structure, 

 such as the Phylloxera galls and H. hamamelis, the next higher, 

 the slightly more complex, as in the case of the Pemphigus galls 

 (Figs. 12 to 21, and 49 to 58). The galls of the Pachypsylla 

 (Figs. 59, 60) are the most highl}' developed of the entire series. 



Although in this case we have a complete series, it is difficult 

 to understand how this development has been produced. It may 

 be that the different forms are due to the attack being made upon 

 different tissues in each case, or to the degree in which the tissues 

 are injured. Upon this point we have no direct proof. However, 

 there is ver}' little doubt that the stimulus is entirely mechanical. 



3. DIPTERA. 



As previously stated, the Cecidomyid galls are far more varied 

 in location and in morphological structure than any other group 

 of galls and show less number of characters peculiar to them- 

 selves alone. There is not sufficient data to draw even theoretical 

 conclusions concerning the influencing causes in their devel- 

 opment. 



4. HYMENOPTERA. 



As previously stated, the Cynipidous galls are the most highly 

 developed and show a greater number of morphological structures 

 peculiar to themselves than any other group (Part I, Con. 2 ; 

 Party, Con 3). 



vSince the gall does not begin to develop until after the hatching 

 of the larvae, oviposition cannot be an important factor excei)t in 

 so far as it is necessary to have the i^i:^^ j^laced in certain tissues. 



Examination of the mouthparts show few, small and insignifi- 

 cant gland-like structures the character of which is doubtful. It 

 is therefore probable tliat the stinudus is purely mechanical except 

 in the Nematus. But how are we to account for the great num- 



