I am building up a my knowledge of organisms that live on trees (not necessarily as pests, but in the respect of the biodiversity that depend on different tree species) and their interactions.
Living on the underside of Fraxinus excelsior, in northern Europe (N. France), I have found these tiny caterpillars (end of May/start of June). They are eating small patches of the leaf, and excreting black frass.
The ash leaf in this photo is 70 x 35 mm, and so the caterpillar shown in approximately 1 mm in length, but there are others up to 2 mm.
The photo above also shows the mines in the leaf.
Under a hand lens, the body is an opal/beige colour and it has two black patches on the body above the head. The body is very shiny and glossy and it looks wet.
From my internet research it looks like a young caterpillar of the Ash Bud Moth (Prays fraxinella)?