Frist how do they train the flies and nats to land and swarm all over dead bodies and stuff, secondly how do they get horses to trip and fall over without hurting themselves?
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For the flies I would say they'd put some kind of meat or food on the dead bodies, concealed most likely, so the flies go there anyway. You can't train a fly lol. And for horses, I'm not sure. They either train them to do something that looks like tripping but isn't, or they use CGI.