24 
BRITISH GALLS 
The only British gall-causing species are Isosoma 
hyalipenne and I. depvessum. The former causes thickening of 
the haulm in the Sea Mat-grass ; the internodes 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 depvessum 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 
