Horrible things. I've had a few bites that have ended up with a big bruise that is really red and itchy for weeks.
Must get my "smidge" spray looked out.
I remember being bitten by 4 horseflies at the same time, 2 on each leg out with the mountain bike club a good few years ago. I was wearing training tights as it was autumn but still they had me. They caused dizziness and immense amount of pain.
At the time, we were on a rack going round a field of cows out on the marshes.