Horsefly.

These are large flies which can an do give a painful bite. Over the years myself and most fishermen have probably had at least one Horsefly bite, I myself have had a couple that were very painful, Adult horse flies feed on nectar and sometimes pollen but females require a blood meal for reproduction. Males lack the necessary mouth parts for blood feeding. Most female horse flies feed on mammal blood, but some species are known to feed on birds, amphibians or reptiles. Unlike insects which surreptitiously puncture the skin with needle-like organs, horse flies have tiny, serrated mandibles which they use to rip and/or slice flesh apart. so if you can wear a repellent,  if you don't have one keep a very close eye out for them buzzing around you, sometimes you get lucky and feel them before they bite to deep, but other times you only realise once its all over.