Acer pseudoplatanus, Zeewolde, Harderbos © Hans Jonkman

detail upperside

detail underside: not just the opening, but also the inside of the gall was hairy

Acer pseudoplatanus, Belgium, prov. Liège, Vivegnis © Jean-Yves Baugnée

notes

galls like these are described, but not named, by Buhr (1964b) or Roskam (2019a); according to them several authors associate them with Aceria macrorhyncha.