132 
The Ohio Naturalist. 
[Vol. IV, No. 6 , 
important protective device. The large intercellular chambers in 
the parenchjmia zone place the larvae at a great distance from the 
surface of the gall without increasing the amount of work neces- 
sary for the mature insects to accomplish before reaching the 
outside ; this is undoubtedly a great protection against parasites, 
since it increases the difficulties for the parasite in reaching the 
larv^ae with the ovipositor, The development of these protective 
devices is probably the resi:lt of natural selection. Since the 
character of the gall depends upon the insect, many variations in 
the gall may also depend on variations in the stimuli giv’en by the 
insect. If these variations in character of epidermis, in thickness 
of parench5’ma zone, in the formation of large intercellular spaces, 
in thickness and densit}’ of protective zone, are advantageous to 
the insect in protecting it from the numerous parasites, these 
characters may be perpetuated in succeeding generations and the 
gall may increase in complexit3^ Natural selection is a reasona- 
ble explanation. 
It should be remembered that the plant is making an effort to 
resist a parasite from which it cannot escape. The gall-maker 
derives its nourishment without destroying its host and at the 
same time strives to protect it.self as far as possible from the great 
number of parasitic enemies. The food supply first becomes a 
part of the gall and upon this supply which, in the case of the 
Cynipidae, is stored in the nutritive zone, it feeds. 
Any irritation, such as the cutting or puncturing of plant tis- 
sues, may and usually does cau.se excessive growth. It is proba- 
ble that the primitive galls were of a type similar to the simplest 
of the Phytoptus galls, i. e. , a peculiar grow’th of the epidermal 
cells. The next step in the evolution of the gall may be repre- 
sented by a type similar to Schizoneura americana, in which case 
the stimulus is greater, resulting in a curling of the leaf. The 
next step maj- be represented b}’ a type similar to the more com- 
plex Phytoptus galls, H. hamamelis, C. ulniicola, the Phj’lloxera, 
the Pemphigus and the most complex of the Pachypsylla galls in 
which we find a series of more or less complex folds in the leaf up 
to the increase in amount and differentiation of the tissue as in 
the case of P. p. -mamma. 
In the Cynipidous galls we have the greatest complexity, but 
also a factor somewhat different from that in the forms to which 
we have referred, i. e., the placing of the egg below the surface 
and in those tissues upon which the larva is expected to feed. 
It is impossible to saj* whether this habit of placing the egg below 
the surface was acquired before or after the gall-making habit, 
but it must be a great advantage to the in.sect. These galls, as 
previously demonstrated, show the more complex serial line of 
development of any of the galls, but even the simplest of these is 
more complex than the most complex gall produced b}' ain’ other 
