The tree isn't full grown but it's a good size hickory tree.We have them growing all over our property and we really do enjoy them and the wild life they attract.

This one i have noticed has been attacked.I don't even know how to describe it.So here's my picture show of leaves i removed from the tree.

I think i have them in order from beginning to end of the "bug's" life on the leaves.

Here is a leaf that seems to be the beginning.When i broke it open i don't see where there is a bug inside.

the underside of the leaf start's with the line through the leaf and then busts through the front of the leaf.

then other leaves looked like this

They are insect galls. The insect that causes it was either too tiny for you to see or it's already completed that part of its life cycle and left the gall. Gall forming insects are extremely difficult to control. They also generally don't cause much harm, other than being unsightly.