As far as wing strength goes, it is primarily on the surface. Remember the pattern planes of the 70's/80's with foam wings? No strength at all in the foam, but the wing sheeting 1/16" balsa took it all. And at high speeds and G forces. Even then, you could cut huge sections of balsa out of the middle of the wing, and still have a structurally sound wing.

Look at my Perigee I just finished, the spars (full depth) are soft balsa 1/16 and primarily serve to hold the ribs in place, but the strength is in the sheeting, with lots of open bay area. There are no square spars just under the skin.