>> How did they manage to train the fly, how? The computer is resting.

I don’t know what happened in that western, but my grandmother (a well-known entomologist in narrow circles) told such a story.
The case was in the middle of the last century, a movie about war was filmed there, and the operator needed flies to crawl on the pillows and the face of the wounded. But in winter, where to get the flies?
They were taken to the grandmother, and she kept these same flies for her needs. They slept in the cold.
She brought them to the set, landed them from the box on the pillow.
“Well, you just take a quick shot while they wake up, they barely crawl as you should, and then fly away!”

We filmed quickly and from one double, and where to go.