OTHER INSECTS OF SPECIAL INTEREST 227 
willow twigs, or on the oak (oak apples) are familiar examples. 
They are caused by small four-winged insects called gall- 
flies. These lay their eggs in the tissues of the leaf or stem, 
and when the larva is hatched out and 
begins to eat, the remarkable growth of 
the gall also commences. In many 
species of gallfly the larva changes to 
pupa and then to adult within the galls, 
while in other species the full-grown 
larva leaves the gall and makes these 
changes afterward. 
Plant lice and mites also produce 
many galls, these being much smaller 
than the kind made by flies. These 
smaller galls always have an opening 
through which the residents may pass 
in and out, while those formed by gall- 
flies are closed until the inmates bore (0 eon witn GALL- 
the tunnel to make their escape. NUT 
