What's been eating my walnut tree?

I have been wondering for the past year what it is that is provoking unpleasant blotchy excrescences on my walnut tree. For the last two years they have been covering many of the leaves. I assumed it was a fungal infection and treated it with Dithane 945 (so much for the organic gardener!) It seemed to go away and this year the new growth in May was completely free of the ugly swellings. Then I spotted a couple of bumps on one of the upper leaves yesterday morning. I plucked the leaf off the tree, thinking that it would limit the spread of the "fungus". I turned it over, and saw - a little caterpillar! I photographed it as a prelude to identification. However, the photo is lousy, but amusing to my sense of humour. If you know what this thing is from my brief description and poor photography, I will be very grateful. To clarify, the bumps are about 1/3 inch in diameter. They swell convexly from the upper leaf surface and leave hollows on the underneath of the leaf, lined with white marks. The caterpillar was sitting right in the middle of one of these hollows so I felt immediate suspicion towards it. It is now dead.