horse flies on the attack

When the first settlers ventured out onto the prairies around here, they noticed that in the late summer a type of huge blood sucking fly would attack in swarms with such fierceness that it could kill a horse. They called the predators "Green Heads". Eventually the "horse guard fly" was seen on the prairies, and it soon eliminated the green heads in insect to insect combat. The settlers considered the "horse guards" a sort of devine intervention. Ive always wondered just what these legendary prairie flies actually were, and where they went...they sort of reminded me of giant horse flies and cicada killers respectively by the descriptions, but thats only as guess...
Well, last night Bonnie and I went out back to look at a newly tilled strip of land behing the rail fence just about twighlight. I was too dark to see much detail, but light enough to need no flash light...the gloaming I believe my Scottish ancestors callled we looked across the fence, we were suddenly both attacked from behind by one or more huge, black flying insects. Their drone was similar to but much deeper than any wasp, implying that an even bigger bug was agressively buzzing us. Something about cicada size or just a bit smaller based on the sound of it and on the few quick glimpses of it that we got. Niether of us got a good look at the thing or things, Bonnie did say the one she saw attacking the back of my head was black...and needless to say we both ducked for the cover of the back porch door about 20 yards off. The insects only followed us for a few steps it seems, but as it was now nearlr dark and they were black, we couldnt be sure, but we didnt hear anymore of their unmistakable dive bomber like buzz.
I suspect that we were being attacked by the "giant black horse fly".
The females of that species are agressive bloodsuckers, and are especially active on hot summer nights. Weve never had these flies that we can remember. The first one I ever saw was one I captured in broad daylight by the water hose out back of the house last summer.
It was huge, and in retrospect I suppose it was a harbinger of last nights encounter as I am still assuming that it was giant black horse flies that were after us...but it got me to wondering...were green heads still around after all...was that what attacked us ?...probably not, and we were only attacked by one or two of the things, but either way, it made me wonder even more about the past and some old settler, ridding his tired saddle horse out half way across the prairies when they were suddenly beset with a swarm of these flies just like the old stories tell often happened. Having the back door with in easy reach is one thing, but being alone and miles from shelter in the middle of nowhere is something completely different.