363 



The Development of Insect Galls as Illustrated by the 

 Genus Amphibolips. 



Mel T. Cook. 



The study of the development of insect galls involves more complicat- 

 ing factors than most problems of evolution, since the host plant is forced 

 to give both nourishment and protection to its enemy. The result of this 

 enforced action is the formation of a structure which is normal for the 

 parasite and pathological for the host. The histology of these gall struc- 

 tures presents some very interesting questions involving the point of stimu- 

 lation, the character of the stimulation and the evolutionary lines along 

 which the various species of galls have developed. For some time we have 

 recognized that the point of stimulation is in the meristomatic tissues, and 

 that in most cases the stimulation is not due to a glandular secretion from 

 the parent insect. 1 However, there appears to be abundant evidence that 

 in most cases the stimulation comes from the larva, but whether mechani- 

 cal or chemical, or both, or the former in some species and the latter in 

 others, is a practically untouched problem. 



In 1902 the writer 2 advanced the opinion that "the morphological 

 character of the gall depends upon the genus of the insect producing it, 

 rather than upon the plant upon which it is produced, i. e., galls produced 

 by insects of a particular genus show great similarity of structure, even 

 though on plants widely separated ; while galls on a particular genus of 

 plants and produced by insects of different genera show great difference." 

 Further studies along this line have convinced the writer of the correct- 

 ness of this view, and have also led to efforts to work out a system of 

 classification based on the histological character of the galls which would 

 be correlated with the classification of the insects. However, the comple- 

 tion of such a series of studies is largely dependent upon a more satis- 

 factory knowledge of the taxonomic relations. 



While it is true that the histological characters of the galls depend 

 upon the insects rather than upon the host plants, it is also true that we 

 find certain characters common to all groups. The first step in the forma.-- 



1 Adler & Straton. Oak Galls and Gall Flies. 1894. 



2 Galls and Insects Producing Them. Ohio Naturalist, JI :7, p. 270. 1907, 



