302 OUR WOODLAND TREES. 
take flight. Sometimes an ichneumon fly lays its 
eggs in the bodies of the enclosed grubs, and 
when these eggs are hatched, the resulting worms 
feed on the bodies of their predecessors, and 
themselves in turn becoming flies, make their 
escape from their vegetable house. The round 
Oak ‘ galls,’ familiar to everybody, are formed in a 
similar manner by the action of insects, which 
perforate the Oak twigs. These galls, at first 
green, soon become brown, and the holes which 
are usually found in them indicate the passages 
by which the enclosed insects have escaped to 
light and air. The currant-shaped galls, known 
as ‘ flower-galls,’ are made by the action of another 
fly, which selects, for the deposit of its eggs, the 
stamen-bearing stalks of the Oak catkins. When 
there are several of these little galls on one 
catkin the latter has very much the appearance 
of a bunch of currants; and it is a very curious 
fact that the existence of these galls prolongs 
the existence of the catkin stem, which, ordi- 
narily, drops from the Tree so soon as it has 
shed its pollen. When, however, it is covered by 
flower-galls the catkin remains attached to the 
