One of the first movies to leverage the idea of radiation- it came out the same year the Godzilla debuted in Japan, and two years before Godzilla came to the US theaters.
The effects are rather impressive when you think about them. Full sized bug puppets, which is more effort than many films which just super imposed footage of real spiders or bugs against actors. The giant puppet ants were able to grab and attack actors, which brings to mind the bug animatronics of Starship Troopers.
The plot and execution is pretty good, and the early mystery is actually quite creepy as the investigators uncover ransacked isolated buildings and vehicles with no sensical explanation for the destruction.
Damn commie ants.
Great poster though.
https://www.sothebys.com/en/buy/collectibles/_them-movie-poster-44a2