Why midge galls aren't leaf mines

Very informative, thank you! If I may summarize to make sure I understand correctly: The extent of a leaf mine is defined by the activity of the larva feeding between the cuticles, but in midge galls the extent is defined by the response of the plant to the presence of the larva, and does not directly change through the feeding action of the larva. Is that correct?

My reading is that these larva never mine the open space they live in, but it forms through a galling action of some sort? That seems like a tough distinction for people to understand. It seems worth including these in a book about leafminers, because people will be using it to learn about insect larva they find living within the walls of a leaf without already knowing it doesn't technically count. Like many gall books include so called 'non-true galls' and use that space to clarify the difference.

Yes, @brnhn, in my leaf mine keys I do try to mention mine-like things that can be confused with mines on particular hosts. But I'd like to discourage people from adding observations to the leafminer project if they know they are galls (which has been happening some, though not as commonly as things like window-feeding and leaftying).

Great stuff, thank you.
Congrats on ferreting out the arrival of the invasive zig zag sawfly incidentally.