A pale, slightly domed swelling on the upper surface, which is induced by mites living and feeding within erineum (hairs), on the lower surface.

In the autumn the mites spend the winter in old cones and bark crevices.

[3] A fungus, Taphrina sadebeckii, can look similar on the upper surface.

[4] Found in Europe from Spain to Russia including the Balkans.

[5] Media related to Acalitus brevitarsus at Wikimedia Commons