132 The Ohio Naturalist. [Vol. IV, No. 6, 



important protective device. The large intercellular chambers in 

 the parenchyma 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 

 larvae with the ovipositor, The development of these protective 

 devices is probably the result 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 given by the 

 insect. If these variations in character of epidermis, in thickness 

 of parenchyma zone, in the formation of large intercellular spaces, 

 in thickness and density of protective zone, are advantageous to 

 the insect in protecting it from the numerous parasites, these 

 characters ma}^ be perpetuated in succeeding generations and the 

 gall may increase in complexity. 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 itself 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 cause 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 growth of the epidermal 

 cells. The next step in the evolution of the gall maj^ 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 may be represented by a type similar to the more com- 

 plex Phytoptus galls, H. hamamelis, C, ulmicola, the Phylloxera, 

 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 sa}- 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 insect. These galls, as 

 previousl}' demonstrated, show the more comjilex serial line of 

 development of any of the galls, but even the simplest of these is 

 more complex than the most complex gall produced by any other 



