Because the female gall-fly lays her eggs in the buds, which consequently swell, and the excrescences serve for the abode of the larva until it has completed its changes, and is in a state to issue from its prison ; after which the apples often become the residence of various kinds of wasps.
Because the gall-fly has thus diverted the nervures of the leaves, which would have sprung from the bud in which the eggs were inserted, and actually do carry sap-vessels throughout the substance of the gall. - Reaumur.
Blumenbach says, that each egg grows in size after it has been deposited in the plant, and sometimes doubles its size before the larva issues from it.
The excrescences on the leaves of the rose-tree, the oak, the poplar, the willow, and other trees, are also formed by the gall-fly depositing its eggs there; The gall-nut used in making ink is similarly produced. Those on the currant-leaf are produced by aphides.
Because the wild figs are full of gall insects, which becoming winged, quit the same, and penetrate the cultivated ones to lay their eggs: the insects appear to ensure the fructification by dispersing the pollen, and afterwards to hasten the ripening by puncturing the pulp, and causing a dispersion or circulation of nutritious juices. This is called caprification, and in France is imitated by inserting straws dipped in olive oil.