This is a mite which produces tiny red pustule galls on the upper surface of Sycamore (Acer pseudoplatanus) tree leaves. They grow up to 6mm high.
The females overwinter in crevices in the bark of the Sycamore tree, emerging in spring to feed on the new leaves, which is what causes the red raised spots. On the underside of the leaf and beneath each gall are tiny holes where the mites have laid their eggs in spring. The resulting larvae feed on the soft tissue of the leaves, which does not appear harmful to the tree.
Seen summer until autumn. Common and widespread.
Photographs taken June 2013, local wood, Staffordshire.