It is hard to tell from the pictures, but I would guess it is a skeletonizer.

Here is a link that might be useful: Pests in Gardens and Landscapes: Sycamore, London plane tree�

Thank you, Toyon. I will explore the link you posted, it looks like a good one. What's weird though is that there is no sign of any bug, no nests, no silk, no eggs.

That may be partly due to the fact that the damage appears to have occur ed some time ago -- notice the brown edges at the edges of the cut tissue.

Take a sample or a picture to a local large independent garden center to ask, or contact your county's Extension Service office. You can locate the latter with this USDA clickable map.

http://www.csrees.usda.gov/Extension/

Here is a link that might be useful: USDA map to locate Extension Service offices

Sycamores can be hosts for a good number of butterfly species (actually the caterpillars). Swallowtails being one group that utilize these trees; that's why I planted mine...

Here is a link that might be useful: Host plant

Thank you all so much for your input, I do appreciate your comments. I think I will try our extension office, that is a very good idea.

Huachuma, this is the first year for us with this tree, we do see them in this area that are lush and beautiful, so it is possible. I hope it is some form of butterfly, a native butterfly, not something imported like the white cabbage butterfly.