Pear and cherry slug
They are the soft skinned larvae of a glossy black sawfly (Caliroa cerasi), and will quickly skeletonise cherry and pear leaves if left to their own devices. They also attack plums, apricots, hawthorn, apples and quinces, but are not as problematic on these trees.
The adult sawfly lays eggs on the leaves which hatch out into the slugs in spring. The slugs feed on and skeletonise the leaves, resulting in a typical lacey look. Once the slugs reach full size they drop off the leaves onto the ground where they burrow down into the soil to pupate. The adults emerge, flying to the leaves to repeat the cycle. There are two generations each year. The first slugs pupate and turn into adults again in January so its really important to get the slugs under control in this first generation, as the second generation is much more numerous and can completely strip every leaf on a tree.