My kumquat tree has recently developed shriveled leaves with what looks to be trails of some sort on the leaves also. I am very new to citrus plants so really need some assistance.
My lemon tree is also exhibiting some similar conditions. They are about 20 ft apart in a full sun space. 
You've got a leaf borer/skeletonizer of some kind dining on the leaves. If you would take a few of these leaves, a razor blade you might be able to slice the leaf horizontally and carefully to find an insect. It might look like a whitish or greenish, baby worm? Look under and over leaves, the stems and in the soil around the base to find insects. You should plan to go out at night and with a flash light look for the insect in the act of eating these leaves. Otherwise the insect will have laid its eggs and the culprit would be the larvae already in the leaves.
Sometimes there will be a little damage to leaves no big deal. But this is looking more substantial. Before any treatment at all it is critical to actually find this insect and verify it is the guilty party.
You will probably only need to spray with NEEM. Will not hurt your citrus, just wash when you eat or use these fruits. Please only spray at night focusing on the undersides of the leaves, stems, lightly on top of the soil.
Another excellent treatment that might be indicated after finding the culprit, would be BT. Bacillus thuringiensis, a toxin produced by this bacterium is the chemical sprayed (just on these plants, no others that do not show the same damage). The insect (most likely a larvae or caterpillar) will eat these BT coated leaves and very quickly stop eating. The toxin produced by this bacteria causes the insect to 'feel full' and will eventually starve, causing no more harm to the plants. Again, do not spray during the day while bees are active. Only at night.
I'd cut off as many of the damaged leaves as you are able. Especially if you find little larvae between the top and bottom skins of the leaves. Dispose of them in a separate bag after they were sprayed. This all depends on the insect you find. If you can, get close ups of what you find with the razor blade or your nightly stake out!
