AppearanceThe gall growth first appears as a rounded mass of green plant tissue on the leaf buds of the oak, later becoming hard and brown, being up to approximately 25 mm / 1 in in diameter. Although nearly spherical, the galls often have a number of little flattened nodules. The rounded growths are filled with a spongy mass and a single wasp larva is located in a hard seed-like cell in the centre. The word 'marble' derives from the gall's shape, which is a marble-like rounded structure. As stated, although normally distinctive the oak marble gall can, under some growth conditions, be mistaken for the oak apple gall, caused by a number of gall wasps, such as ''Biorhiza pallida''. This may be due to the observer's unfamiliarity with the true oak apple gall which grows to be somewhat larger, has red markings, but does grow on the axillary or terminal buds. The galls sometimes coalesce. The non-parasitised specimens are at the largest end of the size range.
Fused and/or stunted specimens can be confused with A. kollari .
PredatorsMature galls are sometimes broken open by vertebrate predators to recover the larva or pupa. Woodpeckers, such as the lesser spotted woodpecker, as well as other birds or squirrels have been suggested. In territory of former Czechoslovakia, both Bank voles and yellow-necked mice feed on larvae and pupae extracted from Oak marble galls.
A number of insect inquilines live harmlessly within the oak marble gall and some of these, as well as ''Andricus'' itself, are parasitised by insects referred to as parasitoids. The chalcid wasp ''Torymus nitens'' is an example of a parasitoid in oak marble galls. The presence of these inquilines and parasites is often visible on older galls by the presence of fine exit-holes, smaller than that of the gall wasp itself.
A gall can contain the cynipid wasp as the host that made the gall; up to 5 species of inquilines eating the hosts food; as well as up to 13 parasitoid species living on the host, inquilines and each other.
Many old galls bear numerous dark brown excrescences, due to the fungus ''Phoma gallorum''.
UsesThe galls contain large amounts of tannic acid, which was used for making iron gall ink and for dyeing cloth. According to recent research, traces of iron-gall ink have been found on the Dead Sea scrolls and on the 'lost' Gospel of Judas. Iron-gall ink may have been used for 1,800 years, but it does not withstand the test of time well. Over the course of centuries, the ink fades, and discolours and damages the paper. Other water-proof formulae, better suited for writing on paper, became available in the 20th century. Iron gall ink is manufactured chiefly by artists enthusiastic about reviving old methods or possibly forgers of old documents.
A recipe for preparing the ink is as follows: Take one lb. of bruised galls, one gallon of boiling water, 5½ oz of ferrous sulfate in solution, 3 oz of gum arabic previously dissolved, and a few drops of an anti-septic, such as carbolic acid. Macerate the galls for 24 hours, strain the infusion and add the other ingredients.
British galls have too little tannic acid for the best results - Aleppo galls have three times as much.
Powdered galls mixed with hog's lard and applied to the posterior were said to be good for curing piles.
Oak Marble Gall extract is used in deodorants because of tannic acid's anti-bacterial properties.
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