I confess I didn't think they would work. They are open at the top, thought flies could get in. The flies not only don't get in, they don't even land around the hole. I've been using them for the second year now. My horses' hooves are in better shape because they don't stamp, my period is in better shape because I don't have to apply fly spray all day. I actually think I'm saving money by using a lot less fly spray. I would say this is the last fly season, 4 or 5 months. I leave them on 24/7 and undress them every few days to check their feet. Please note that the boots on the hind legs are more worn. The plastic frames seem to constantly flex onto their hind legs. The size chart is accurate. I have a donkey the size of a pony, a smaller mustang the size of a small horse, a quarter horse the size of a horse, and a large mustang and a warmblood the size of a large.