Okay, this is going to be a tough one. Back in the seventies (max early eighties) I watched a TV production - a low cost one with basic (not so special) effects - probably only painted backgrounds. It looked like a British production (for the style).
What I remember of the story: strange happenings all over the world; people and cattle disappear, body parts are found as if fallen from the sky; possibly a blood rain over some cities. Special effects were almost nonexistent - mostly backdrops. It could have been a radio show adaptation of a H.G. Wells short story, transposed for television. If I remember correctly it was set at the end of the XIX century or, at most, the beginning of the XX century.
I remember that in the end the mystery is solved:
It was aliens that were experimenting with the lifeforms of this planet. The aliens were small spiderlike creatures, transparent and lighter than the air. They died for some reason I cannot remember (somehow like in The War of the Worlds). At the end, one of the protagonists has a dead alien specimen in a jar, paints it in some color and let it float upwards to show how they were lighter than air.
This was not a movie, it was at most half an hour, forty minutes long, possibly part of a series of two or three similar productions. Probably shot in a single studio, with backgrounds.
I had seen it on a Christmas eve of particular significance to me, and it would be nice to find it to try to recover part of that lost atmosphere. Monsieur Proust willing.