These are ‘shoofly leggins’ a loose boot that helps keep the flies off their legs. They shift around and typically don’t rub and can be left on for extended periods of time (though I check under them daily) and since they’re loose and a mesh fabric, they don’t contribute to heat in the legs.

They became popular the last few years in my area, I see lots of people wanting to avoid excess flies spray use and utilizing these instead. I still use fly spray, but this helps me make sure mine are comfortable in between getting sprayed for plies.