Callirhytis erythrocephala (Giraud, 1859)
on Quercus, agamous generation

Quercus cerris, Denmark © Simon Haarder: the acorn is not notably modified …

… but contains several young gall chambers
gall
The interior of an acorn has become hard and is partly or entirely transformed into a mass of gall chambers; exteriorly the acorns hardly show any change. The development of the wasps in the gall takes at 3-8 years.
hostplants
Fagaceae, monophagous
Querus cerris.
on Quercus, sexual generation
gall
Gall chambers under the bark on unchanged one-year twigs; only the eventual appearance of exit holes betrays their presence. See Callirhytis spec..
hostplants
Fagaceae, monophagous
Quercus robur.
synonyms
Callirhytis hartigi Förster, 1869; C. erythrostoma Dettmer (1933) A third species, viz. C. erythrosoma Dettmer (1933) also generally is taken as a synonym of erythrocephala, but this is doubted by Pujade-Villar ao (2005a).
references
Bellmann (2012a) Dauphin & Aniotsbehere (1997a), Eady & Quinlan (1963a), Groom (2011a), Melika (2006a), Nieves-Aldrey (1992a), Pujade-Villar, Barbotin, Folliot & Melika (2005a), Redfern & Shirley (2011a), Roskam (2009a), Williams (2010a).