Galls develop on plants (often trees) when some types of wasps lay their eggs there.
Spangle galls are found on oak trees, often, as here, many to a leaf. One of the oak trees on the edge of the Barley Field had spangle galls on most of its leaves.
Empty cases which had formed the protective cover for the chrysalis stage of cicadas, showing the hole in the back where the adult insect emerged.
I couldn’t get in close enough to see whether these still had the insect inside.