1 1 2 THE NA TURE-S TUD Y RE VIE W [ 2 . 3 _ M a rch, , 9 o6 



twig or leaf for oviposition, and the interesting response or reaction 

 of the plant to the growth-stimulating irritation of the gall-fly larva are 

 subjects which have attracted much attention and study, but concern- 

 ing these much remains to be discovered. In size and shape the galls 

 present amazing variety; some are irregular little swellings on the leaves, 

 others are like small trumpets, others like rosettes, or star-like with 

 radiating points ; on the twigs some are spherical, some elongate, and 

 some large and reniform. In their interior make-up they also differ 

 much ; some have a large hollow central space ; some are filled with 

 open spongy tissue, and some are solid except for the cells and tun- 

 nels of the larvae. In some but a single larva lives ; in others are 

 three or four or a dozen. Externally some are smooth, some rough- 

 ened, some hairy. They occur on leaves, branches, and roots in 

 both oak and rose. Only a few Cynipid galls are known on other 

 plants than these. In the face of the host of species of Cynipidse 

 found in this country — over 200 gall-making kinds are known, 

 besides a score of parasitic species — and their small size and 

 generally similar appearance, we shall not undertake to describe any 

 of the various species. 



Regarding the wonderful instinct of the gall-fly, I quote the follow- 

 ing from Stratton, an English student of galls : 



" It is impossible that intelligence or memory can be of any use in 

 guiding the Cynipidae ; no Cynips ever sees its young, and none ever 

 pricks buds a second season, or lives to know the results that follow 

 the act. Natural selection alone has preserved an impulse which is 

 released by seasonally recurring feelings, sights, or smells, and by 

 the simultaneous ripening of the eggs within the fly. These set the 

 whole physiological apparatus in motion, and secure the insertion of 

 eggs at the right time and in the right place. The number of eggs 

 placed is instinctively proportionate to the space suitable for oviposi- 

 tion, to the size of the fully grown galls, and to the food-supplies 

 available for their nutrition." Certain species will only place from 

 one to six eggs on a leaf which other species would probably prick a 

 hundred times. 



The exact character of the plant's abnormal growth has been 

 recently studied by several investigators. Cook, an American stu- 

 dent, concludes from his studies that in the formation of all leaf-galls 

 (except the cecidomyid or dipterous midge-galls) the normal cell- 

 structure of the leaf is first modified by the formation of a large num- 

 ber of small, compact, irregular-shaped cells. The mesophyll is 



