Below is the work of a leaf miner, Phyllocnistis unipunctella. The larva feeds on black poplar and related hybrid poplars, which are very widely planted. It leaves a long and meandering shiny trail, like a snail has wandered around on the leaf. Pupation takes place under a strong silk spinning at the leaf edge, which curls conspicuously.

We have about 20 records of this species, which is quite a lot for a leaf miner in Somerset. It's one of many where the dot maps are a bare picture of recorder coverage, not moth distribution. Look for it and help change this!