invisible enemies?

Did I miss something, or are there supposed to be invisible enemies in Level 5? I keep getting hit when nothing seems to be there, and can sometimes kill the thing after it hits me and alerts me to it's presence. Is this a glitch, or did I miss the part where it says there are invisible enemies on this level (and all sub-levels)?

I've gotten past level 5 and it hasn't happened to me.It's a glitch