ORIGINAL: bugle3
It is correct that the full sized planes used a goodly number of flying wires but they also internally braced the wings with diagonal struts that went from the top of each rib to the bottom of its adjoining ribs. From front to back, their were four of these X pattern braces between each set of ribs; this added a lot of internal stiffness to the wing.
I'm not sure what you mean by "diagonal struts." The original EIII (the ONLY surviving original EIII) in the London Science Museum, absolutely does NOT have internal bracing struts. Perhaps you are referring to the span-wise criss-crossing FABRIC tapes which served to hold the ribs in position as the covering was tightened. These fabric tapes were also found on later Fokker aircraft (such as the DrI) and are still used today of many modern fabric covered aircraft wings. Remember that since the original flew with wing-warping, a flexible wing was an absolute requirement.
As you can see in this photo, there were however, diagonal cable bracings that spanned several rib bays.