This is a mite which produces felt-like galls on the surface of Sycamore (Acer pseudoplatanus) tree leaves. On the top side of the leaf the gall is like a blister which is usually yellowish to begin which then turns brown, and underside it is cream or yellowish when fresh, browning as it ages.
The mites leave the gall in which they shelter and feed on the leaf in the autumn and overwinter in the twigs and branches.
Seen summer to autumn, a common and widespread species. Native to Britain.