Huge insect going for the horses

I've had a couple of incidences recently where something has driven the horses absolutely mad - and during the latest one, I saw something huge going for them. Whatever it is is really upsetting them - making them gallop round like idiots, and throw themselves on the floor to roll - presumably to try and remove whatever it is.Anyone got any ideas what they could be? I can see them from 20ft away, without my glasses on - so they must be pretty massive! Not prepared to get any closer, as the horses really do lose the plot

أكثر...