Thanks Charley! I hadn't thought of any of your ideas! I will try to do some more post processing of the RAW image to see if I can bring out more detail. Almost every sagittaria leaf in the bog bore marks, although many seemed like criss-crossing, meandering trails versus these lovely curves. Could there be that much damage from the damseflies and dragonflies? I thought they oviposited in water, but do some do so on plants above water? Perhaps these leaves were submerged at the time of oviposition - although I doubt the bog had standing water for quite some time given the weather. I wish I had thought to feel the leaf surface texture.

See in the info and photo on pages 4-5 of my book, here. Damselflies and dragonflies can be very abundant, and I have seen situations where oviposition marks were as abundant as you describe, though I mostly see them on leaves right at the water surface. I can't remember now if I've actually seen them on emergent vegetation in person, but it does happen.

What a wealth of fabulous information your book seems to contain! This is definitely going onto my birthday wish list! Thanks so much for your ID help. I can see there is just soooo much more to learn about insects.