I spotted this in an abandoned quarry in The Lakes. It is only about 2 inches deep and was empty. This was in early May. Is it an abandoned wasps' nest, unfinished, or the nest of a much smaller insect?
A week or two later, back at home, I saw this mayfly which had just emerged from its previous "shell". I understand it is the only insect which does this at this stage in its life, so was pretty pleased to see the two together, given the short life of the fully metamorphosed fly.