I watched this film in the mid 1990's, but I'm about 90% sure it was a black and white movie, so was likely from the 1950s or 60's.
The film was set in an American town that was sparsely populated. Either an alien invasion or a chemical/nuclear accident caused creatures that looked like woodlice or another Isopod type creatures, to be able to give off excessive heat.
The creatures were large, and would just about fit in a human hand.
When humans pickup the creatures they cause significant burns to the individuals hand. I don't think they burn through a person's hand, and only cause significant burns.
I also feel the following elements may be from the same film, but I don't know if I am confusing two films as one. If that turns out to be the case I will ask the following in a separate question.
I remember a young teen girl looking out of a window to a hill that had a bright light behind it (it turns out later an alien spaceship has crashed behind the hill). There is a pathway with a white fence that goes over the hill, with a single tree on the top of the hill.
The girl sees people from the town going over the hill. The next day the girl meets them around the town and they have changed and become distant and aloof. The viewer is meant to believe that the person has been replaced by an alien.
The girls father dislikes what's going on and says that no-one should go over the hill. One day the girl wakes up to find her father has become distant and aloof.
The viewer is meant to feel that the father has gone over the hill, even though he said he wouldn't.