I held leaves up to the light, and sure enough, I could see tiny wiggly things under the"skin" of the leaf. They seemed quite snug in there. Well not for much longer, haha!! I dusted off my macro lens (Canon 100mm f/2.8), which has been sitting in the corner for far too long. Then set about gently peeling the leaf apart, to reveal the offending leaf miners. The larvae tried their best to squeeze into the pieces of leaf that remained, but they had no place to hide. Clearly, they did not like being in the sun, or having their photo taken.
I may have to prune more leaves. It is interesting that the mama egg layer chose only large leaves. It makes sense - the leaf miner larvae is going to need a lot of food. Laying on a small leaves makes leaf miner larvae life harder. That means mama egg layers can assess leaf size.
I don't like to share my chard with them, but insects sure are interesting!