Have you seen this fly?

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i have these horse flies running my horses to the ground.. I have bug spray on them but nothing seems to work to get them off the horses. they fly in sworms around the horses and they are fast. they keep up with the horses when they are running aroung trying to get away from them. if you have any idea what they are called.. i have looked them up on the net and cant find a thing on them...
any ideas