Answer: Black slug-like things are usually pear slugs, the larvae of flying insect. Generally they're found on cherry and plum tree leaves, but it's possible you'll find them on apple tree leaves, as well. These little guys feed on the surface of the leaf and they appear late in the season. Since the leaves are going to fall off soon, why not just rake the leaves and destroy them? This practice will reduce, if not eliminate, next year's population of adults. By the way, they're called pear slugs because of their pear-like appearance, not because they're fond of pears.
If this doesn't sound like your pest, call your county agricultural extension office (in the White Pages under U.S. Gov't; USDA) and find out where you can send a sample for identification. Best of luck!
Q&A Library Searching Tips