Snail trails.

I found several little folds like this on sapling Populus nigra trees round the car park at College Lake Nature Reserve today. They are the pupation-sites of a small moth called Phyllocnistis unipunctella. The chrysalis inside is 4mm long and looks like this:

The accompanying leaf-mines are difficult to spot, looking as they do like a cross between snail-trails and invisible!

Incidentally, if you collect leaf-mines and want them to emerge as adults, opening up the cocoons, as I have above, reduces your chances of success, as the pupae have very precise procedures for emergence and you may disrupt them; also they may dessicate more easily and die. Also, collect several, as parasitism-rates are often high.
Andy King.