Inexplicably, they're not shown for the first 3/4 of the movie. It looks pretty stupid seeing people grabbing at their throats and dying while ostensibly invisible beings slurp their central nervous systems from the backs of their heads. You almost think the director is trying to build suspense by not showing the antagonist, saving the horror of what the beings look like for just the right moment in the movie. If that were the case, there should be some payoff. The creatures should at least be somewhat frightening. Nope. They're human brains with antennae and a tail (spinal cord?) that either slowly squirm disgustedly on the floor or somehow fly through the air. Oh, well. If I wasn't entertained or scared by the proceedings, I at least learned all about the dangers of radioactivity. It can be bad, you know, if in the wrong hands.
Here I am wondering if this would have terrified me in 1958: