Do mono spar wings need compression strut(or ribs)?
The wing in your picture has a tubular spar. In addition, smaller tubes are used at the leading and trailing edge, and one is running diagonally through the inner half to add in-plane bending stiffness.

Tubes have the same stiffness in all directions, so using a tube gives already some in-plane and torsional stiffness. For the outer part of the wing in your picture, this stiffness seems to be sufficient to not use drag or anti-drag wires. Since loads accumulate from tip to root, the root has the highest loading, and here a bracing tube is added in the inner wing so in-plane stiffness and strength are sufficient. I expect that this wing also uses bracing for the out-of-plane loads, and I checked with the manufacturer's home page: Yes, it does.



Kolb Fire Star II side view, from the manufacturer's home page.

The ribs are supplemented by half ribs placed between the full ribs, so the forward part of the airfoil, where more aero loads and more curvature require more support, is adequately shaped and strengthened. The ribs themselves seem also to be made from rather thin tubes, and this being a rectangular wing, need only a single jig to be manufactured. An N-type truss allows the ribs to transmit shear to the spar. 

In all, this is a very efficient and pragmatic wing design. There is nothing missing (well, except for the fabric covering, of course)