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 Pachyps3’lla 
{ Figs. 59, 60) are the most liighl}' 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 maj' 
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 entirel}' mechanical. 
3. DIPTERA. 
As previousl}' stated, the Cecidomjdd 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. HYMENOPTER.V. 
As previous!}- stated, the Cynipidous galls are the most highly 
developed and show a greater number of morphological structures 
peculiar to themselves than ain- other group (Part I, Con. 2 ; 
Part V, Con 3). 
Since the gall does not begin to develop until after the hatching 
of the larvae, oviposition cannot be an important factor except in 
so far as it is necessar}- to have the egg placed in certain ti.ssues. 
lixamination of the mouthparts show few, small and insignifi- 
cant gland-like structures the character of which is doubtful. It 
is therefore prol)able that the stimulus is purely mechanical except 
in the Nematus. But how are we to account for the great num- 
