that is the effect of a leaf miner. They are moths that lay eggs on the underside and then the caterpillars burrow in and eat away the inside of the leaf. what you see is the remains of their tunnels.
personally if there are only a few affected leaves i'd pick them off.

I'm not one for spraying insecticides as many of these small moths do more good than harm.

Thanks SBG for your knowlege ,yes its only on a few leafs so ive taken those off.Same as yourself i dont use insecticides of any kind ,:)