The oak tree is like an enormous hotel, for it houses many hundred of tiny creatures,and many insect rear their families within its shelter. These insect make what is know as "gall"on the end of the twigs which is really the nursery where the grubs are hatched out.
The foundation stone of this "gall"or Oak-Apple is laid by an insect who visit the "hotel" about Christmas. She lay her eggs in the heart of the small leaves within the buds. In springtime Oak-Apple develop and the young grub crawl out and feed on the sap which is in tree. By June they have developed into full grown flies