Thanks H, I had no idea that Leaf miners even existed! Though saying that I have seen pictures of similar (but different) looking "sacks" that some moths produce -so the understanding is slowly ticking into place!
It would be interesting to find out which sort of beetle produces these. I will go through my beetle images from the garden, though I know some are unknown (lack of mandible detail), I don't recall having seen that many types though.
Thanks again
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