The levers should all operate by pressing Up Arrow. I thought they had an icon indicating this, but maybe only one does. I should maybe adjust that if so...
There is no icon, and pressing the up arrow only changes the angle of the flashlight, not the position of the lever. We're talking about the one that's on the floor just up the ladder after the screen with the dark tunnel, yes?
Ah, if I understand which one you meant, you probably already turned it; it doesn't open the bars right from the lever, but rather a gate elsewhere in the same screen.
Then I guess I'm stuck. I have no idea what I missed, there's just not much to do here.
Hah. I found it, missed the other cavern that you get to by jumping over the two pylons earlier. Now I see the lever you were talking about, and I found the spider, or it found me.