24 BRITISH GALLS 



The only British gall-causing species are Isosoma 

 hyalipenne and /. depressum. The former causes thickening of 

 the haulm in the Sea Mat-grass ; the intemodes are short- 

 ened, the imbricated leaves are frequently not larger than 

 the enlarged sheath, and the gall is a fusiform mass. There 

 is an excellent illustration of it in Connold's " Vegetable 

 Galls," Plate XLI. Isosoma depressum gives rise to irregular 

 swellings of a yellowish -green hue on the Sheep's Fescue- 

 grass. 



The galls caused by the Cynipidae have received more 

 attention than those of any other group of gall-causers, 

 partly because of their frequency, but chiefly through the 

 great interest attached to the life-histories of the insects. 

 Theodore Hartig was the earliest scientific investigator of 

 them ; he was ably supported by Schenk, Mayr, and 

 others, but it remained for Dr. Hermann Adler to discover 

 the alternation of parthenogenesis and sexual reproduction 

 in these insects. He published his discovery in 1877. His 

 famous book on Alternating Generations was translated 

 into French by Lichtenstein (1881), and into English by 

 Dr. Charles Straton (1894). To the latter work and to 

 Cameron's "British Phytophagous Hymenoptera" the 

 reader is referred for detailed information concerning the 

 gall-wasps. 



The eggs are stalked. Adler suggested that the peduncle 

 is used for respiration, for in those species in which the 

 eggs are so placed that they cannot receive oxygen from 

 the plant the peduncle is very long ; in those which place 

 their eggs in leaves it is short. As a rule, the peduncle 

 is long in winter generations and short in those of spring. 

 Some species deposit a large number of eggs ; the ovaries 

 may contain more than 600. It is obvious that oviposition 

 is easiest in those species which deposit their eggs in leaves. 

 Cameron observed that the egg swells after introduction 

 into the plant. 



The larva is footless, white, and of sluggish habits. In 

 the spring brood the larval life is short ; in the winter 



