instar larvae simply scrape off green tissue from both surfaces of the leaf. Once walnut caterpillars molt into the second instar, they will be large enough to make entire leaflets disappear.
The walnut caterpillar has two generations per year in Kansas. The first generation hatches in late June to early July while a second generation appears in August. Walnut caterpillars feed together in large colonies. When it comes time to molt the entire colony moves to the underside of a large limb or the trunk and sheds their exoskeletons together in a large clump. You can see photos of later instars in posts I've made during previous growing seasons (2011 & 2102).