Why do ribs have diagonal struts,why not vertical as seen in airfoils of Citabria airplane?
The diagonal struts are best to transfer shear. Imagine an upward pointing load at the nose. In order to get that load transferred to the fuselage, it first needs to be transferred to the wing spar. Imagine now that you hold the rib firmly in your right hand where the spar is and press the nose upward with your left hand. With parallel struts, the rib will have much less stiffness and the nose load will deform the rib easily. With the diagonal struts, the rib is much stiffer when that particular nose load is applied.
This isn't really aviation-specific, it's just good structural engineering practice. Triangles in structures are strong because they don't bend: whichever way you apply load to the triangle, the load is along the members. With a rectangle, any load that's not perfectly aligned to the structure will bend the joints and squash the rectangle into a diamond shape, which will then squash flat easily. You don't want any rectangles in your structure anywhere.
It's a truss. Ideally, all of the beam elements in a truss are in either tension or compression, without major bending loads. This offers the lightest weight for given load.
This same design is commonly seen in the open in steel bridges, construction cranes, and spacecraft. Triangles that a truss consist of cannot be bent without changing the elements' lengths. If all of the struts were vertical, the structure would consist of rectangles, which can be easily deformed by bending the joints.
It's to make the wing stiff and strong in torsion. For torsion stiffness, you want two things. First and foremost, you want as much material on the outermost fibre as possible. The further the material from the neutral axis, the more it contributes to torsion stiffness due to the moment arm from the neutral axis.
Secondly, you want the cross-section to remain undeformed. Consider for example a matchbox 'shell', which is in theory very stiff for torsion. However, if the forces do not enter the matchbox exactly right, the open ends will deform into a diamond shape and will twist easily. This shearing into a diamond shape can be prevented by placing baffles in the matchbox.
This is in fact the very reason wings have ribs: so that the wings maintain their cross-section by transferring shear loads due to torsion from and to the top and bottom skins of the airfoil. In terms of stiffness, thin plates would be ideal, but these are prone to buckling so a strut structure is used instead. The best way to transfer shear loads with struts is to use diagonals.
So, in short: the diagonal struts transfer shear loads to increase torsion strength and stiffness of the wing.
A top picture of number 2 might have been a bit better, but notice the "puny" cross struts run both ways, much like in the wing of an old time biplane. Their function would be help prevent the wing from collapsing BACKWARDS as a result of excess airspeed. (spokes in wheel rims may also seem puny, but the rim cannot come out of round because the spokes, 90 degrees from the load, cannot be pulled apart).
This seems to be a bit of sensibly added insurance, but no, they probably would not add to the torsional stiffness of the wing.
Regarding the design of the ribs themselves, as mentioned, 1 is a classic truss. 2 has the center of the rectangular rib filled in with metal and the center circular portion and edges made as an I beam for strength. 1 of wood, 2 of stamped metal.
As far as making a stiffer wing torsionally, might consider replacing aluminum skin with plywood or some type of composite material. Others may know how better to do it within the wing.